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| 1<html> 2<head> 3<title>PLY (Python Lex-Yacc)</title> 4</head> 5<body bgcolor="#ffffff"> 6 7<h1>PLY (Python Lex-Yacc)</h1> | 1<html> 2<head> 3<title>PLY (Python Lex-Yacc)</title> 4</head> 5<body bgcolor="#ffffff"> 6 7<h1>PLY (Python Lex-Yacc)</h1> |
| 8 | 8 |
| 9<b> 10David M. Beazley <br> | 9<b> 10David M. Beazley <br> |
| 11Department of Computer Science <br> 12University of Chicago <br> 13Chicago, IL 60637 <br> 14beazley@cs.uchicago.edu <br> | 11dave@dabeaz.com<br> |
| 15</b> 16 17<p> | 12</b> 13 14<p> |
| 18Documentation version: $Header: /home/stever/bk/newmem2/ext/ply/doc/ply.html 1.1 03/06/06 14:53:34-00:00 stever@ $ | 15<b>PLY Version: 2.3</b> 16<p> |
| 19 | 17 |
| 20<h2>Introduction</h2> | 18<!-- INDEX --> 19<div class="sectiontoc"> 20<ul> 21<li><a href="#ply_nn1">Introduction</a> 22<li><a href="#ply_nn2">PLY Overview</a> 23<li><a href="#ply_nn3">Lex</a> 24<ul> 25<li><a href="#ply_nn4">Lex Example</a> 26<li><a href="#ply_nn5">The tokens list</a> 27<li><a href="#ply_nn6">Specification of tokens</a> 28<li><a href="#ply_nn7">Token values</a> 29<li><a href="#ply_nn8">Discarded tokens</a> 30<li><a href="#ply_nn9">Line numbers and positional information</a> 31<li><a href="#ply_nn10">Ignored characters</a> 32<li><a href="#ply_nn11">Literal characters</a> 33<li><a href="#ply_nn12">Error handling</a> 34<li><a href="#ply_nn13">Building and using the lexer</a> 35<li><a href="#ply_nn14">The @TOKEN decorator</a> 36<li><a href="#ply_nn15">Optimized mode</a> 37<li><a href="#ply_nn16">Debugging</a> 38<li><a href="#ply_nn17">Alternative specification of lexers</a> 39<li><a href="#ply_nn18">Maintaining state</a> 40<li><a href="#ply_nn19">Duplicating lexers</a> 41<li><a href="#ply_nn20">Internal lexer state</a> 42<li><a href="#ply_nn21">Conditional lexing and start conditions</a> 43<li><a href="#ply_nn21">Miscellaneous Issues</a> 44</ul> 45<li><a href="#ply_nn22">Parsing basics</a> 46<li><a href="#ply_nn23">Yacc reference</a> 47<ul> 48<li><a href="#ply_nn24">An example</a> 49<li><a href="#ply_nn25">Combining Grammar Rule Functions</a> 50<li><a href="#ply_nn26">Character Literals</a> 51<li><a href="#ply_nn26">Empty Productions</a> 52<li><a href="#ply_nn28">Changing the starting symbol</a> 53<li><a href="#ply_nn27">Dealing With Ambiguous Grammars</a> 54<li><a href="#ply_nn28">The parser.out file</a> 55<li><a href="#ply_nn29">Syntax Error Handling</a> 56<ul> 57<li><a href="#ply_nn30">Recovery and resynchronization with error rules</a> 58<li><a href="#ply_nn31">Panic mode recovery</a> 59<li><a href="#ply_nn32">General comments on error handling</a> 60</ul> 61<li><a href="#ply_nn33">Line Number and Position Tracking</a> 62<li><a href="#ply_nn34">AST Construction</a> 63<li><a href="#ply_nn35">Embedded Actions</a> 64<li><a href="#ply_nn36">Yacc implementation notes</a> 65</ul> 66<li><a href="#ply_nn37">Parser and Lexer State Management</a> 67<li><a href="#ply_nn38">Using Python's Optimized Mode</a> 68<li><a href="#ply_nn39">Where to go from here?</a> 69</ul> 70</div> 71<!-- INDEX --> |
| 21 | 72 |
| 22PLY is a Python-only implementation of the popular compiler 23construction tools lex and yacc. The implementation borrows ideas 24from a number of previous efforts; most notably John Aycock's SPARK 25toolkit. However, the overall flavor of the implementation is more 26closely modeled after the C version of lex and yacc. The other 27significant feature of PLY is that it provides extensive input 28validation and error reporting--much more so than other Python parsing 29tools. | |
| 30 | 73 |
| 74 75 76 77 78<H2><a name="ply_nn1"></a>1. Introduction</H2> 79 80 81PLY is a pure-Python implementation of the popular compiler 82construction tools lex and yacc. The main goal of PLY is to stay 83fairly faithful to the way in which traditional lex/yacc tools work. 84This includes supporting LALR(1) parsing as well as providing 85extensive input validation, error reporting, and diagnostics. Thus, 86if you've used yacc in another programming language, it should be 87relatively straightforward to use PLY. 88 |
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| 31<p> | 89<p> |
| 32Early versions of PLY were developed to support the Introduction to 33Compilers Course at the University of Chicago. In this course, | 90Early versions of PLY were developed to support an Introduction to 91Compilers Course I taught in 2001 at the University of Chicago. In this course, |
| 34students built a fully functional compiler for a simple Pascal-like 35language. Their compiler, implemented entirely in Python, had to 36include lexical analysis, parsing, type checking, type inference, 37nested scoping, and code generation for the SPARC processor. 38Approximately 30 different compiler implementations were completed in | 92students built a fully functional compiler for a simple Pascal-like 93language. Their compiler, implemented entirely in Python, had to 94include lexical analysis, parsing, type checking, type inference, 95nested scoping, and code generation for the SPARC processor. 96Approximately 30 different compiler implementations were completed in |
| 39this course. Most of PLY's interface and operation has been motivated by common | 97this course. Most of PLY's interface and operation has been influenced by common |
| 40usability problems encountered by students. 41 42<p> | 98usability problems encountered by students. 99 100<p> |
| 43Because PLY was primarily developed as an instructional tool, you will 44find it to be <em>MUCH</em> more picky about token and grammar rule 45specification than most other Python parsing tools. In part, this | 101Since PLY was primarily developed as an instructional tool, you will 102find it to be fairly picky about token and grammar rule 103specification. In part, this |
| 46added formality is meant to catch common programming mistakes made by 47novice users. However, advanced users will also find such features to 48be useful when building complicated grammars for real programming | 104added formality is meant to catch common programming mistakes made by 105novice users. However, advanced users will also find such features to 106be useful when building complicated grammars for real programming |
| 49languages. It should also be noted that PLY does not provide much in the way 50of bells and whistles (e.g., automatic construction of abstract syntax trees, 51tree traversal, etc.). Instead, you will find a bare-bones, yet | 107languages. It should also be noted that PLY does not provide much in 108the way of bells and whistles (e.g., automatic construction of 109abstract syntax trees, tree traversal, etc.). Nor would I consider it 110to be a parsing framework. Instead, you will find a bare-bones, yet |
| 52fully capable lex/yacc implementation written entirely in Python. 53 54<p> 55The rest of this document assumes that you are somewhat familar with | 111fully capable lex/yacc implementation written entirely in Python. 112 113<p> 114The rest of this document assumes that you are somewhat familar with |
| 56parsing theory, syntax directed translation, and automatic tools such 57as lex and yacc. If you are unfamilar with these topics, you will 58probably want to consult an introductory text such as "Compilers: 59Principles, Techniques, and Tools", by Aho, Sethi, and Ullman. "Lex 60and Yacc" by John Levine may also be handy. | 115parsing theory, syntax directed translation, and the use of compiler 116construction tools such as lex and yacc in other programming 117languages. If you are unfamilar with these topics, you will probably 118want to consult an introductory text such as "Compilers: Principles, 119Techniques, and Tools", by Aho, Sethi, and Ullman. O'Reilly's "Lex 120and Yacc" by John Levine may also be handy. In fact, the O'Reilly book can be 121used as a reference for PLY as the concepts are virtually identical. |
| 61 | 122 |
| 62<h2>PLY Overview</h2> | 123<H2><a name="ply_nn2"></a>2. PLY Overview</H2> |
| 63 | 124 |
| 64PLY consists of two separate tools; <tt>lex.py</tt> and 65<tt>yacc.py</tt>. <tt>lex.py</tt> is used to break input text into a | 125 126PLY consists of two separate modules; <tt>lex.py</tt> and 127<tt>yacc.py</tt>, both of which are found in a Python package 128called <tt>ply</tt>. The <tt>lex.py</tt> module is used to break input text into a |
| 66collection of tokens specified by a collection of regular expression 67rules. <tt>yacc.py</tt> is used to recognize language syntax that has | 129collection of tokens specified by a collection of regular expression 130rules. <tt>yacc.py</tt> is used to recognize language syntax that has |
| 68been specified in the form of a context free grammar. Currently, 69<tt>yacc.py</tt> uses LR parsing and generates its parsing tables 70using the SLR algorithm. LALR(1) parsing may be supported in a future 71release. | 131been specified in the form of a context free grammar. <tt>yacc.py</tt> uses LR parsing and generates its parsing tables 132using either the LALR(1) (the default) or SLR table generation algorithms. |
| 72 73<p> 74The two tools are meant to work together. Specifically, 75<tt>lex.py</tt> provides an external interface in the form of a 76<tt>token()</tt> function that returns the next valid token on the 77input stream. <tt>yacc.py</tt> calls this repeatedly to retrieve 78tokens and invoke grammar rules. The output of <tt>yacc.py</tt> is 79often an Abstract Syntax Tree (AST). However, this is entirely up to 80the user. If desired, <tt>yacc.py</tt> can also be used to implement | 133 134<p> 135The two tools are meant to work together. Specifically, 136<tt>lex.py</tt> provides an external interface in the form of a 137<tt>token()</tt> function that returns the next valid token on the 138input stream. <tt>yacc.py</tt> calls this repeatedly to retrieve 139tokens and invoke grammar rules. The output of <tt>yacc.py</tt> is 140often an Abstract Syntax Tree (AST). However, this is entirely up to 141the user. If desired, <tt>yacc.py</tt> can also be used to implement |
| 81simple one-pass compilers. | 142simple one-pass compilers. |
| 82 83<p> 84Like its Unix counterpart, <tt>yacc.py</tt> provides most of the 85features you expect including extensive error checking, grammar 86validation, support for empty productions, error tokens, and ambiguity | 143 144<p> 145Like its Unix counterpart, <tt>yacc.py</tt> provides most of the 146features you expect including extensive error checking, grammar 147validation, support for empty productions, error tokens, and ambiguity |
| 87resolution via precedence rules. The primary difference between 88<tt>yacc.py</tt> and <tt>yacc</tt> is the use of SLR parsing instead 89of LALR(1). Although this slightly restricts the types of grammars 90than can be successfully parsed, it is sufficiently powerful to handle most 91kinds of normal programming language constructs. | 148resolution via precedence rules. In fact, everything that is possible in traditional yacc 149should be supported in PLY. |
| 92 93<p> | 150 151<p> |
| 94Finally, it is important to note that PLY relies on reflection 95(introspection) to build its lexers and parsers. Unlike traditional 96lex/yacc which require a special input file that is converted into a 97separate source file, the specifications given to PLY <em>are</em> 98valid Python programs. This means that there are no extra source 99files nor is there a special compiler construction step (e.g., running 100yacc to generate Python code for the compiler). | 152The primary difference between 153<tt>yacc.py</tt> and Unix <tt>yacc</tt> is that <tt>yacc.py</tt> 154doesn't involve a separate code-generation process. 155Instead, PLY relies on reflection (introspection) 156to build its lexers and parsers. Unlike traditional lex/yacc which 157require a special input file that is converted into a separate source 158file, the specifications given to PLY <em>are</em> valid Python 159programs. This means that there are no extra source files nor is 160there a special compiler construction step (e.g., running yacc to 161generate Python code for the compiler). Since the generation of the 162parsing tables is relatively expensive, PLY caches the results and 163saves them to a file. If no changes are detected in the input source, 164the tables are read from the cache. Otherwise, they are regenerated. |
| 101 | 165 |
| 102<h2>Lex Example</h2> | 166<H2><a name="ply_nn3"></a>3. Lex</H2> |
| 103 | 167 |
| 104<tt>lex.py</tt> is used to write tokenizers. To do this, each token 105must be defined by a regular expression rule. The following file 106implements a very simple lexer for tokenizing simple integer expressions: | |
| 107 | 168 |
| 169<tt>lex.py</tt> is used to tokenize an input string. For example, suppose 170you're writing a programming language and a user supplied the following input string: 171 |
|
| 108<blockquote> 109<pre> | 172<blockquote> 173<pre> |
| 174x = 3 + 42 * (s - t) 175</pre> 176</blockquote> 177 178A tokenizer splits the string into individual tokens 179 180<blockquote> 181<pre> 182'x','=', '3', '+', '42', '*', '(', 's', '-', 't', ')' 183</pre> 184</blockquote> 185 186Tokens are usually given names to indicate what they are. For example: 187 188<blockquote> 189<pre> 190'ID','EQUALS','NUMBER','PLUS','NUMBER','TIMES', 191'LPAREN','ID','MINUS','ID','RPAREN' 192</pre> 193</blockquote> 194 195More specifically, the input is broken into pairs of token types and values. For example: 196 197<blockquote> 198<pre> 199('ID','x'), ('EQUALS','='), ('NUMBER','3'), 200('PLUS','+'), ('NUMBER','42), ('TIMES','*'), 201('LPAREN','('), ('ID','s'), ('MINUS','-'), 202('ID','t'), ('RPAREN',')' 203</pre> 204</blockquote> 205 206The identification of tokens is typically done by writing a series of regular expression 207rules. The next section shows how this is done using <tt>lex.py</tt>. 208 209<H3><a name="ply_nn4"></a>3.1 Lex Example</H3> 210 211 212The following example shows how <tt>lex.py</tt> is used to write a simple tokenizer. 213 214<blockquote> 215<pre> |
|
| 110# ------------------------------------------------------------ 111# calclex.py 112# 113# tokenizer for a simple expression evaluator for 114# numbers and +,-,*,/ 115# ------------------------------------------------------------ | 216# ------------------------------------------------------------ 217# calclex.py 218# 219# tokenizer for a simple expression evaluator for 220# numbers and +,-,*,/ 221# ------------------------------------------------------------ |
| 116import lex | 222import ply.lex as lex |
| 117 118# List of token names. This is always required 119tokens = ( 120 'NUMBER', 121 'PLUS', 122 'MINUS', 123 'TIMES', 124 'DIVIDE', --- 17 unchanged lines hidden (view full) --- 142 except ValueError: 143 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 144 t.value = 0 145 return t 146 147# Define a rule so we can track line numbers 148def t_newline(t): 149 r'\n+' | 223 224# List of token names. This is always required 225tokens = ( 226 'NUMBER', 227 'PLUS', 228 'MINUS', 229 'TIMES', 230 'DIVIDE', --- 17 unchanged lines hidden (view full) --- 248 except ValueError: 249 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 250 t.value = 0 251 return t 252 253# Define a rule so we can track line numbers 254def t_newline(t): 255 r'\n+' |
| 150 t.lineno += len(t.value) | 256 t.lexer.lineno += len(t.value) |
| 151 152# A string containing ignored characters (spaces and tabs) 153t_ignore = ' \t' 154 155# Error handling rule 156def t_error(t): 157 print "Illegal character '%s'" % t.value[0] | 257 258# A string containing ignored characters (spaces and tabs) 259t_ignore = ' \t' 260 261# Error handling rule 262def t_error(t): 263 print "Illegal character '%s'" % t.value[0] |
| 158 t.skip(1) | 264 t.lexer.skip(1) |
| 159 160# Build the lexer 161lex.lex() 162 | 265 266# Build the lexer 267lex.lex() 268 |
| 269</pre> 270</blockquote> 271To use the lexer, you first need to feed it some input text using its <tt>input()</tt> method. After that, repeated calls to <tt>token()</tt> produce tokens. The following code shows how this works: 272 273<blockquote> 274<pre> 275 |
|
| 163# Test it out 164data = ''' 1653 + 4 * 10 166 + -20 *2 167''' 168 169# Give the lexer some input 170lex.input(data) 171 172# Tokenize 173while 1: 174 tok = lex.token() 175 if not tok: break # No more input 176 print tok 177</pre> 178</blockquote> 179 | 276# Test it out 277data = ''' 2783 + 4 * 10 279 + -20 *2 280''' 281 282# Give the lexer some input 283lex.input(data) 284 285# Tokenize 286while 1: 287 tok = lex.token() 288 if not tok: break # No more input 289 print tok 290</pre> 291</blockquote> 292 |
| 180In the example, the <tt>tokens</tt> list defines all of the possible 181token names that can be produced by the lexer. This list is always required 182and is used to perform a variety of validation checks. Following the <tt>tokens</tt> 183list, regular expressions are written for each token. Each of these 184rules are defined by making declarations with a special prefix <tt>t_</tt> to indicate that it | 293When executed, the example will produce the following output: 294 295<blockquote> 296<pre> 297$ python example.py 298LexToken(NUMBER,3,2,1) 299LexToken(PLUS,'+',2,3) 300LexToken(NUMBER,4,2,5) 301LexToken(TIMES,'*',2,7) 302LexToken(NUMBER,10,2,10) 303LexToken(PLUS,'+',3,14) 304LexToken(MINUS,'-',3,16) 305LexToken(NUMBER,20,3,18) 306LexToken(TIMES,'*',3,20) 307LexToken(NUMBER,2,3,21) 308</pre> 309</blockquote> 310 311The tokens returned by <tt>lex.token()</tt> are instances 312of <tt>LexToken</tt>. This object has 313attributes <tt>tok.type</tt>, <tt>tok.value</tt>, 314<tt>tok.lineno</tt>, and <tt>tok.lexpos</tt>. The following code shows an example of 315accessing these attributes: 316 317<blockquote> 318<pre> 319# Tokenize 320while 1: 321 tok = lex.token() 322 if not tok: break # No more input 323 print tok.type, tok.value, tok.line, tok.lexpos 324</pre> 325</blockquote> 326 327The <tt>tok.type</tt> and <tt>tok.value</tt> attributes contain the 328type and value of the token itself. 329<tt>tok.line</tt> and <tt>tok.lexpos</tt> contain information about 330the location of the token. <tt>tok.lexpos</tt> is the index of the 331token relative to the start of the input text. 332 333<H3><a name="ply_nn5"></a>3.2 The tokens list</H3> 334 335 336All lexers must provide a list <tt>tokens</tt> that defines all of the possible token 337names that can be produced by the lexer. This list is always required 338and is used to perform a variety of validation checks. The tokens list is also used by the 339<tt>yacc.py</tt> module to identify terminals. 340 341<p> 342In the example, the following code specified the token names: 343 344<blockquote> 345<pre> 346tokens = ( 347 'NUMBER', 348 'PLUS', 349 'MINUS', 350 'TIMES', 351 'DIVIDE', 352 'LPAREN', 353 'RPAREN', 354) 355</pre> 356</blockquote> 357 358<H3><a name="ply_nn6"></a>3.3 Specification of tokens</H3> 359 360 361Each token is specified by writing a regular expression rule. Each of these rules are 362are defined by making declarations with a special prefix <tt>t_</tt> to indicate that it |
| 185defines a token. For simple tokens, the regular expression can 186be specified as strings such as this (note: Python raw strings are used since they are the 187most convenient way to write regular expression strings): 188 189<blockquote> 190<pre> 191t_PLUS = r'\+' 192</pre> 193</blockquote> 194 195In this case, the name following the <tt>t_</tt> must exactly match one of the 196names supplied in <tt>tokens</tt>. If some kind of action needs to be performed, | 363defines a token. For simple tokens, the regular expression can 364be specified as strings such as this (note: Python raw strings are used since they are the 365most convenient way to write regular expression strings): 366 367<blockquote> 368<pre> 369t_PLUS = r'\+' 370</pre> 371</blockquote> 372 373In this case, the name following the <tt>t_</tt> must exactly match one of the 374names supplied in <tt>tokens</tt>. If some kind of action needs to be performed, |
| 197a token rule can be specified as a function. For example: | 375a token rule can be specified as a function. For example, this rule matches numbers and 376converts the string into a Python integer. |
| 198 199<blockquote> 200<pre> 201def t_NUMBER(t): 202 r'\d+' 203 try: 204 t.value = int(t.value) 205 except ValueError: 206 print "Number %s is too large!" % t.value 207 t.value = 0 208 return t 209</pre> 210</blockquote> 211 | 377 378<blockquote> 379<pre> 380def t_NUMBER(t): 381 r'\d+' 382 try: 383 t.value = int(t.value) 384 except ValueError: 385 print "Number %s is too large!" % t.value 386 t.value = 0 387 return t 388</pre> 389</blockquote> 390 |
| 212In this case, the regular expression rule is specified in the function documentation string. | 391When a function is used, the regular expression rule is specified in the function documentation string. |
| 213The function always takes a single argument which is an instance of | 392The function always takes a single argument which is an instance of |
| 214LexToken. This object has attributes of t.type which is the token type, 215<tt>t.value</tt> which is the lexeme, and <tt>t.lineno</tt> which is the current line number. | 393<tt>LexToken</tt>. This object has attributes of <tt>t.type</tt> which is the token type (as a string), 394<tt>t.value</tt> which is the lexeme (the actual text matched), <tt>t.lineno</tt> which is the current line number, and <tt>t.lexpos</tt> which 395is the position of the token relative to the beginning of the input text. |
| 216By default, <tt>t.type</tt> is set to the name following the <tt>t_</tt> prefix. The action 217function can modify the contents of the <tt>LexToken</tt> object as appropriate. However, 218when it is done, the resulting token should be returned. If no value is returned by the action 219function, the token is simply discarded and the next token read. 220 221<p> | 396By default, <tt>t.type</tt> is set to the name following the <tt>t_</tt> prefix. The action 397function can modify the contents of the <tt>LexToken</tt> object as appropriate. However, 398when it is done, the resulting token should be returned. If no value is returned by the action 399function, the token is simply discarded and the next token read. 400 401<p> |
| 222The rule <tt>t_newline()</tt> illustrates a regular expression rule 223for a discarded token. In this case, a rule is written to match 224newlines so that proper line number tracking can be performed. 225By returning no value, the function causes the newline character to be 226discarded. | 402Internally, <tt>lex.py</tt> uses the <tt>re</tt> module to do its patten matching. When building the master regular expression, 403rules are added in the following order: 404<p> 405<ol> 406<li>All tokens defined by functions are added in the same order as they appear in the lexer file. 407<li>Tokens defined by strings are added next by sorting them in order of decreasing regular expression length (longer expressions 408are added first). 409</ol> 410<p> 411Without this ordering, it can be difficult to correctly match certain types of tokens. For example, if you 412wanted to have separate tokens for "=" and "==", you need to make sure that "==" is checked first. By sorting regular 413expressions in order of decreasing length, this problem is solved for rules defined as strings. For functions, 414the order can be explicitly controlled since rules appearing first are checked first. |
| 227 228<p> | 415 416<p> |
| 417To handle reserved words, it is usually easier to just match an identifier and do a special name lookup in a function 418like this: 419 420<blockquote> 421<pre> 422reserved = { 423 'if' : 'IF', 424 'then' : 'THEN', 425 'else' : 'ELSE', 426 'while' : 'WHILE', 427 ... 428} 429 430def t_ID(t): 431 r'[a-zA-Z_][a-zA-Z_0-9]*' 432 t.type = reserved.get(t.value,'ID') # Check for reserved words 433 return t 434</pre> 435</blockquote> 436 437This approach greatly reduces the number of regular expression rules and is likely to make things a little faster. 438 439<p> 440<b>Note:</b> You should avoid writing individual rules for reserved words. For example, if you write rules like this, 441 442<blockquote> 443<pre> 444t_FOR = r'for' 445t_PRINT = r'print' 446</pre> 447</blockquote> 448 449those rules will be triggered for identifiers that include those words as a prefix such as "forget" or "printed". This is probably not 450what you want. 451 452<H3><a name="ply_nn7"></a>3.4 Token values</H3> 453 454 455When tokens are returned by lex, they have a value that is stored in the <tt>value</tt> attribute. Normally, the value is the text 456that was matched. However, the value can be assigned to any Python object. For instance, when lexing identifiers, you may 457want to return both the identifier name and information from some sort of symbol table. To do this, you might write a rule like this: 458 459<blockquote> 460<pre> 461def t_ID(t): 462 ... 463 # Look up symbol table information and return a tuple 464 t.value = (t.value, symbol_lookup(t.value)) 465 ... 466 return t 467</pre> 468</blockquote> 469 470It is important to note that storing data in other attribute names is <em>not</em> recommended. The <tt>yacc.py</tt> module only exposes the 471contents of the <tt>value</tt> attribute. Thus, accessing other attributes may be unnecessarily awkward. 472 473<H3><a name="ply_nn8"></a>3.5 Discarded tokens</H3> 474 475 476To discard a token, such as a comment, simply define a token rule that returns no value. For example: 477 478<blockquote> 479<pre> 480def t_COMMENT(t): 481 r'\#.*' 482 pass 483 # No return value. Token discarded 484</pre> 485</blockquote> 486 487Alternatively, you can include the prefix "ignore_" in the token declaration to force a token to be ignored. For example: 488 489<blockquote> 490<pre> 491t_ignore_COMMENT = r'\#.*' 492</pre> 493</blockquote> 494 495Be advised that if you are ignoring many different kinds of text, you may still want to use functions since these provide more precise 496control over the order in which regular expressions are matched (i.e., functions are matched in order of specification whereas strings are 497sorted by regular expression length). 498 499<H3><a name="ply_nn9"></a>3.6 Line numbers and positional information</H3> 500 501 502<p>By default, <tt>lex.py</tt> knows nothing about line numbers. This is because <tt>lex.py</tt> doesn't know anything 503about what constitutes a "line" of input (e.g., the newline character or even if the input is textual data). 504To update this information, you need to write a special rule. In the example, the <tt>t_newline()</tt> rule shows how to do this. 505 506<blockquote> 507<pre> 508# Define a rule so we can track line numbers 509def t_newline(t): 510 r'\n+' 511 t.lexer.lineno += len(t.value) 512</pre> 513</blockquote> 514Within the rule, the <tt>lineno</tt> attribute of the underlying lexer <tt>t.lexer</tt> is updated. 515After the line number is updated, the token is simply discarded since nothing is returned. 516 517<p> 518<tt>lex.py</tt> does not perform and kind of automatic column tracking. However, it does record positional 519information related to each token in the <tt>lexpos</tt> attribute. Using this, it is usually possible to compute 520column information as a separate step. For instance, just count backwards until you reach a newline. 521 522<blockquote> 523<pre> 524# Compute column. 525# input is the input text string 526# token is a token instance 527def find_column(input,token): 528 i = token.lexpos 529 while i > 0: 530 if input[i] == '\n': break 531 i -= 1 532 column = (token.lexpos - i)+1 533 return column 534</pre> 535</blockquote> 536 537Since column information is often only useful in the context of error handling, calculating the column 538position can be performed when needed as opposed to doing it for each token. 539 540<H3><a name="ply_nn10"></a>3.7 Ignored characters</H3> 541 542 543<p> |
|
| 229The special <tt>t_ignore</tt> rule is reserved by <tt>lex.py</tt> for characters 230that should be completely ignored in the input stream. 231Usually this is used to skip over whitespace and other non-essential characters. 232Although it is possible to define a regular expression rule for whitespace in a manner 233similar to <tt>t_newline()</tt>, the use of <tt>t_ignore</tt> provides substantially better 234lexing performance because it is handled as a special case and is checked in a much 235more efficient manner than the normal regular expression rules. 236 | 544The special <tt>t_ignore</tt> rule is reserved by <tt>lex.py</tt> for characters 545that should be completely ignored in the input stream. 546Usually this is used to skip over whitespace and other non-essential characters. 547Although it is possible to define a regular expression rule for whitespace in a manner 548similar to <tt>t_newline()</tt>, the use of <tt>t_ignore</tt> provides substantially better 549lexing performance because it is handled as a special case and is checked in a much 550more efficient manner than the normal regular expression rules. 551 |
| 552<H3><a name="ply_nn11"></a>3.8 Literal characters</H3> 553 554 |
|
| 237<p> | 555<p> |
| 556Literal characters can be specified by defining a variable <tt>literals</tt> in your lexing module. For example: 557 558<blockquote> 559<pre> 560literals = [ '+','-','*','/' ] 561</pre> 562</blockquote> 563 564or alternatively 565 566<blockquote> 567<pre> 568literals = "+-*/" 569</pre> 570</blockquote> 571 572A literal character is simply a single character that is returned "as is" when encountered by the lexer. Literals are checked 573after all of the defined regular expression rules. Thus, if a rule starts with one of the literal characters, it will always 574take precedence. 575<p> 576When a literal token is returned, both its <tt>type</tt> and <tt>value</tt> attributes are set to the character itself. For example, <tt>'+'</tt>. 577 578<H3><a name="ply_nn12"></a>3.9 Error handling</H3> 579 580 581<p> |
|
| 238Finally, the <tt>t_error()</tt> 239function is used to handle lexing errors that occur when illegal 240characters are detected. In this case, the <tt>t.value</tt> attribute contains the | 582Finally, the <tt>t_error()</tt> 583function is used to handle lexing errors that occur when illegal 584characters are detected. In this case, the <tt>t.value</tt> attribute contains the |
| 241rest of the input string that has not been tokenized. In the example, we simply print 242the offending character and skip ahead one character by calling <tt>t.skip(1)</tt>. | 585rest of the input string that has not been tokenized. In the example, the error function 586was defined as follows: |
| 243 | 587 |
| 588<blockquote> 589<pre> 590# Error handling rule 591def t_error(t): 592 print "Illegal character '%s'" % t.value[0] 593 t.lexer.skip(1) 594</pre> 595</blockquote> 596 597In this case, we simply print the offending character and skip ahead one character by calling <tt>t.lexer.skip(1)</tt>. 598 599<H3><a name="ply_nn13"></a>3.10 Building and using the lexer</H3> 600 601 |
|
| 244<p> 245To build the lexer, the function <tt>lex.lex()</tt> is used. This function 246uses Python reflection (or introspection) to read the the regular expression rules 247out of the calling context and build the lexer. Once the lexer has been built, two functions can 248be used to control the lexer. 249 250<ul> 251<li><tt>lex.input(data)</tt>. Reset the lexer and store a new input string. 252<li><tt>lex.token()</tt>. Return the next token. Returns a special <tt>LexToken</tt> instance on success or 253None if the end of the input text has been reached. 254</ul> 255 | 602<p> 603To build the lexer, the function <tt>lex.lex()</tt> is used. This function 604uses Python reflection (or introspection) to read the the regular expression rules 605out of the calling context and build the lexer. Once the lexer has been built, two functions can 606be used to control the lexer. 607 608<ul> 609<li><tt>lex.input(data)</tt>. Reset the lexer and store a new input string. 610<li><tt>lex.token()</tt>. Return the next token. Returns a special <tt>LexToken</tt> instance on success or 611None if the end of the input text has been reached. 612</ul> 613 |
| 256The code at the bottom of the example shows how the lexer is actually used. When executed, 257the following output will be produced: | 614If desired, the lexer can also be used as an object. The <tt>lex()</tt> returns a <tt>Lexer</tt> object that 615can be used for this purpose. For example: |
| 258 259<blockquote> 260<pre> | 616 617<blockquote> 618<pre> |
| 261$ python example.py 262LexToken(NUMBER,3,2) 263LexToken(PLUS,'+',2) 264LexToken(NUMBER,4,2) 265LexToken(TIMES,'*',2) 266LexToken(NUMBER,10,2) 267LexToken(PLUS,'+',3) 268LexToken(MINUS,'-',3) 269LexToken(NUMBER,20,3) 270LexToken(TIMES,'*',3) 271LexToken(NUMBER,2,3) | 619lexer = lex.lex() 620lexer.input(sometext) 621while 1: 622 tok = lexer.token() 623 if not tok: break 624 print tok |
| 272</pre> 273</blockquote> 274 | 625</pre> 626</blockquote> 627 |
| 275<h2>Lex Implementation Notes</h2> | 628<p> 629This latter technique should be used if you intend to use multiple lexers in your application. Simply define each 630lexer in its own module and use the object returned by <tt>lex()</tt> as appropriate. |
| 276 | 631 |
| 277<ul> 278<li><tt>lex.py</tt> uses the <tt>re</tt> module to do its patten matching. When building the master regular expression, 279rules are added in the following order: | |
| 280<p> | 632<p> |
| 281<ol> 282<li>All tokens defined by functions are added in the same order as they appear in the lexer file. 283<li>Tokens defined by strings are added by sorting them in order of decreasing regular expression length (longer expressions 284are added first). 285</ol> 286<p> 287Without this ordering, it can be difficult to correctly match certain types of tokens. For example, if you 288wanted to have separate tokens for "=" and "==", you need to make sure that "==" is checked first. By sorting regular 289expressions in order of decreasing length, this problem is solved for rules defined as strings. For functions, 290the order can be explicitly controlled since rules appearing first are checked first. | 633Note: The global functions <tt>lex.input()</tt> and <tt>lex.token()</tt> are bound to the <tt>input()</tt> 634and <tt>token()</tt> methods of the last lexer created by the lex module. |
| 291 | 635 |
| 292<P> 293<li>The lexer requires input to be supplied as a single input string. Since most machines have more than enough memory, this 294rarely presents a performance concern. However, it means that the lexer currently can't be used with streaming data 295such as open files or sockets. This limitation is primarily a side-effect of using the <tt>re</tt> module. | 636<H3><a name="ply_nn14"></a>3.11 The @TOKEN decorator</H3> |
| 296 | 637 |
| 297<p> 298<li> 299To handle reserved words, it is usually easier to just match an identifier and do a special name lookup in a function 300like this: | |
| 301 | 638 |
| 639In some applications, you may want to define build tokens from as a series of 640more complex regular expression rules. For example: 641 |
|
| 302<blockquote> 303<pre> | 642<blockquote> 643<pre> |
| 304reserved = { 305 'if' : 'IF', 306 'then' : 'THEN', 307 'else' : 'ELSE', 308 'while' : 'WHILE', 309 ... 310} | 644digit = r'([0-9])' 645nondigit = r'([_A-Za-z])' 646identifier = r'(' + nondigit + r'(' + digit + r'|' + nondigit + r')*)' |
| 311 312def t_ID(t): | 647 648def t_ID(t): |
| 313 r'[a-zA-Z_][a-zA-Z_0-9]*' 314 t.type = reserved.get(t.value,'ID') # Check for reserved words 315 return t | 649 # want docstring to be identifier above. ????? 650 ... |
| 316</pre> 317</blockquote> 318 | 651</pre> 652</blockquote> 653 |
| 319<p> 320<li>The lexer requires tokens to be defined as class instances with <tt>t.type</tt>, <tt>t.value</tt>, and <tt>t.lineno</tt> 321attributes. By default, tokens are created as instances of the <tt>LexToken</tt> class defined internally to <tt>lex.py</tt>. 322If desired, you can create new kinds of tokens provided that they have the three required attributes. However, 323in practice, it is probably safer to stick with the default. | 654In this case, we want the regular expression rule for <tt>ID</tt> to be one of the variables above. However, there is no 655way to directly specify this using a normal documentation string. To solve this problem, you can use the <tt>@TOKEN</tt> 656decorator. For example: |
| 324 | 657 |
| 325<p> 326<li>The only safe attribute for assigning token properties is <tt>t.value</tt>. In some cases, you may want to attach 327a number of different properties to a token (e.g., symbol table entries for identifiers). To do this, replace <tt>t.value</tt> 328with a tuple or class instance. For example: 329 | |
| 330<blockquote> 331<pre> | 658<blockquote> 659<pre> |
| 660from ply.lex import TOKEN 661 662@TOKEN(identifier) |
|
| 332def t_ID(t): 333 ... | 663def t_ID(t): 664 ... |
| 334 # For identifiers, create a (lexeme, symtab) tuple 335 t.value = (t.value, symbol_lookup(t.value)) 336 ... 337 return t | |
| 338</pre> 339</blockquote> 340 | 665</pre> 666</blockquote> 667 |
| 341Although allowed, do NOT assign additional attributes to the token object. For example, | 668This will attach <tt>identifier</tt> to the docstring for <tt>t_ID()</tt> allowing <tt>lex.py</tt> to work normally. An alternative 669approach this problem is to set the docstring directly like this: 670 |
| 342<blockquote> 343<pre> 344def t_ID(t): 345 ... | 671<blockquote> 672<pre> 673def t_ID(t): 674 ... |
| 346 # Bad implementation of above 347 t.symtab = symbol_lookup(t.value) 348 ... | 675 676t_ID.__doc__ = identifier |
| 349</pre> 350</blockquote> 351 | 677</pre> 678</blockquote> 679 |
| 352The reason you don't want to do this is that the <tt>yacc.py</tt> 353module only provides public access to the <tt>t.value</tt> attribute of each token. 354Therefore, any other attributes you assign are inaccessible (if you are familiar 355with the internals of C lex/yacc, <tt>t.value</tt> is the same as <tt>yylval.tok</tt>). | 680<b>NOTE:</b> Use of <tt>@TOKEN</tt> requires Python-2.4 or newer. If you're concerned about backwards compatibility with older 681versions of Python, use the alternative approach of setting the docstring directly. |
| 356 | 682 |
| 683<H3><a name="ply_nn15"></a>3.12 Optimized mode</H3> 684 685 686For improved performance, it may be desirable to use Python's 687optimized mode (e.g., running Python with the <tt>-O</tt> 688option). However, doing so causes Python to ignore documentation 689strings. This presents special problems for <tt>lex.py</tt>. To 690handle this case, you can create your lexer using 691the <tt>optimize</tt> option as follows: 692 693<blockquote> 694<pre> 695lexer = lex.lex(optimize=1) 696</pre> 697</blockquote> 698 699Next, run Python in its normal operating mode. When you do 700this, <tt>lex.py</tt> will write a file called <tt>lextab.py</tt> to 701the current directory. This file contains all of the regular 702expression rules and tables used during lexing. On subsequent 703executions, 704<tt>lextab.py</tt> will simply be imported to build the lexer. This 705approach substantially improves the startup time of the lexer and it 706works in Python's optimized mode. 707 |
|
| 357<p> | 708<p> |
| 358<li>To track line numbers, the lexer internally maintains a line 359number variable. Each token automatically gets the value of the 360current line number in the <tt>t.lineno</tt> attribute. To modify the 361current line number, simply change the <tt>t.lineno</tt> attribute 362in a function rule (as previously shown for 363<tt>t_newline()</tt>). Even if the resulting token is discarded, 364changes to the line number remain in effect for subsequent tokens. | 709To change the name of the lexer-generated file, use the <tt>lextab</tt> keyword argument. For example: |
| 365 | 710 |
| 711<blockquote> 712<pre> 713lexer = lex.lex(optimize=1,lextab="footab") 714</pre> 715</blockquote> 716 717When running in optimized mode, it is important to note that lex disables most error checking. Thus, this is really only recommended 718if you're sure everything is working correctly and you're ready to start releasing production code. 719 720<H3><a name="ply_nn16"></a>3.13 Debugging</H3> 721 722 723For the purpose of debugging, you can run <tt>lex()</tt> in a debugging mode as follows: 724 725<blockquote> 726<pre> 727lexer = lex.lex(debug=1) 728</pre> 729</blockquote> 730 731This will result in a large amount of debugging information to be printed including all of the added rules and the master 732regular expressions. 733 734In addition, <tt>lex.py</tt> comes with a simple main function which 735will either tokenize input read from standard input or from a file specified 736on the command line. To use it, simply put this in your lexer: 737 738<blockquote> 739<pre> 740if __name__ == '__main__': 741 lex.runmain() 742</pre> 743</blockquote> 744 745<H3><a name="ply_nn17"></a>3.14 Alternative specification of lexers</H3> 746 747 748As shown in the example, lexers are specified all within one Python module. If you want to 749put token rules in a different module from the one in which you invoke <tt>lex()</tt>, use the 750<tt>module</tt> keyword argument. 751 |
|
| 366<p> | 752<p> |
| 367<li>To support multiple scanners in the same application, the <tt>lex.lex()</tt> function 368actually returns a special <tt>Lexer</tt> object. This object has two methods 369<tt>input()</tt> and <tt>token()</tt> that can be used to supply input and get tokens. For example: | 753For example, you might have a dedicated module that just contains 754the token rules: |
| 370 371<blockquote> 372<pre> | 755 756<blockquote> 757<pre> |
| 758# module: tokrules.py 759# This module just contains the lexing rules 760 761# List of token names. This is always required 762tokens = ( 763 'NUMBER', 764 'PLUS', 765 'MINUS', 766 'TIMES', 767 'DIVIDE', 768 'LPAREN', 769 'RPAREN', 770) 771 772# Regular expression rules for simple tokens 773t_PLUS = r'\+' 774t_MINUS = r'-' 775t_TIMES = r'\*' 776t_DIVIDE = r'/' 777t_LPAREN = r'\(' 778t_RPAREN = r'\)' 779 780# A regular expression rule with some action code 781def t_NUMBER(t): 782 r'\d+' 783 try: 784 t.value = int(t.value) 785 except ValueError: 786 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 787 t.value = 0 788 return t 789 790# Define a rule so we can track line numbers 791def t_newline(t): 792 r'\n+' 793 t.lexer.lineno += len(t.value) 794 795# A string containing ignored characters (spaces and tabs) 796t_ignore = ' \t' 797 798# Error handling rule 799def t_error(t): 800 print "Illegal character '%s'" % t.value[0] 801 t.lexer.skip(1) 802</pre> 803</blockquote> 804 805Now, if you wanted to build a tokenizer from these rules from within a different module, you would do the following (shown for Python interactive mode): 806 807<blockquote> 808<pre> 809>>> import tokrules 810>>> <b>lexer = lex.lex(module=tokrules)</b> 811>>> lexer.input("3 + 4") 812>>> lexer.token() 813LexToken(NUMBER,3,1,1,0) 814>>> lexer.token() 815LexToken(PLUS,'+',1,2) 816>>> lexer.token() 817LexToken(NUMBER,4,1,4) 818>>> lexer.token() 819None 820>>> 821</pre> 822</blockquote> 823 824The <tt>object</tt> option can be used to define lexers as a class instead of a module. For example: 825 826<blockquote> 827<pre> 828import ply.lex as lex 829 830class MyLexer: 831 # List of token names. This is always required 832 tokens = ( 833 'NUMBER', 834 'PLUS', 835 'MINUS', 836 'TIMES', 837 'DIVIDE', 838 'LPAREN', 839 'RPAREN', 840 ) 841 842 # Regular expression rules for simple tokens 843 t_PLUS = r'\+' 844 t_MINUS = r'-' 845 t_TIMES = r'\*' 846 t_DIVIDE = r'/' 847 t_LPAREN = r'\(' 848 t_RPAREN = r'\)' 849 850 # A regular expression rule with some action code 851 # Note addition of self parameter since we're in a class 852 def t_NUMBER(self,t): 853 r'\d+' 854 try: 855 t.value = int(t.value) 856 except ValueError: 857 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 858 t.value = 0 859 return t 860 861 # Define a rule so we can track line numbers 862 def t_newline(self,t): 863 r'\n+' 864 t.lexer.lineno += len(t.value) 865 866 # A string containing ignored characters (spaces and tabs) 867 t_ignore = ' \t' 868 869 # Error handling rule 870 def t_error(self,t): 871 print "Illegal character '%s'" % t.value[0] 872 t.lexer.skip(1) 873 874 <b># Build the lexer 875 def build(self,**kwargs): 876 self.lexer = lex.lex(object=self, **kwargs)</b> 877 878 # Test it output 879 def test(self,data): 880 self.lexer.input(data) 881 while 1: 882 tok = lexer.token() 883 if not tok: break 884 print tok 885 886# Build the lexer and try it out 887m = MyLexer() 888m.build() # Build the lexer 889m.test("3 + 4") # Test it 890</pre> 891</blockquote> 892 893For reasons that are subtle, you should <em>NOT</em> invoke <tt>lex.lex()</tt> inside the <tt>__init__()</tt> method of your class. If you 894do, it may cause bizarre behavior if someone tries to duplicate a lexer object. Keep reading. 895 896<H3><a name="ply_nn18"></a>3.15 Maintaining state</H3> 897 898 899In your lexer, you may want to maintain a variety of state information. This might include mode settings, symbol tables, and other details. There are a few 900different ways to handle this situation. First, you could just keep some global variables: 901 902<blockquote> 903<pre> 904num_count = 0 905def t_NUMBER(t): 906 r'\d+' 907 global num_count 908 num_count += 1 909 try: 910 t.value = int(t.value) 911 except ValueError: 912 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 913 t.value = 0 914 return t 915</pre> 916</blockquote> 917 918Alternatively, you can store this information inside the Lexer object created by <tt>lex()</tt>. To this, you can use the <tt>lexer</tt> attribute 919of tokens passed to the various rules. For example: 920 921<blockquote> 922<pre> 923def t_NUMBER(t): 924 r'\d+' 925 t.lexer.num_count += 1 # Note use of lexer attribute 926 try: 927 t.value = int(t.value) 928 except ValueError: 929 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 930 t.value = 0 931 return t 932 |
|
| 373lexer = lex.lex() | 933lexer = lex.lex() |
| 374lexer.input(sometext) 375while 1: 376 tok = lexer.token() 377 if not tok: break 378 print tok | 934lexer.num_count = 0 # Set the initial count |
| 379</pre> 380</blockquote> 381 | 935</pre> 936</blockquote> 937 |
| 382The functions <tt>lex.input()</tt> and <tt>lex.token()</tt> are bound to the <tt>input()</tt> 383and <tt>token()</tt> methods of the last lexer created by the lex module. | 938This latter approach has the advantage of storing information inside 939the lexer itself---something that may be useful if multiple instances 940of the same lexer have been created. However, it may also feel kind 941of "hacky" to the purists. Just to put their mind at some ease, all 942internal attributes of the lexer (with the exception of <tt>lineno</tt>) have names that are prefixed 943by <tt>lex</tt> (e.g., <tt>lexdata</tt>,<tt>lexpos</tt>, etc.). Thus, 944it should be perfectly safe to store attributes in the lexer that 945don't have names starting with that prefix. |
| 384 | 946 |
| 947<p> 948A third approach is to define the lexer as a class as shown in the previous example: |
|
| 385 | 949 |
| 950<blockquote> 951<pre> 952class MyLexer: 953 ... 954 def t_NUMBER(self,t): 955 r'\d+' 956 self.num_count += 1 957 try: 958 t.value = int(t.value) 959 except ValueError: 960 print "Line %d: Number %s is too large!" % (t.lineno,t.value) 961 t.value = 0 962 return t 963 964 def build(self, **kwargs): 965 self.lexer = lex.lex(object=self,**kwargs) 966 967 def __init__(self): 968 self.num_count = 0 969 970# Create a lexer 971m = MyLexer() 972lexer = lex.lex(object=m) 973</pre> 974</blockquote> 975 976The class approach may be the easiest to manage if your application is going to be creating multiple instances of the same lexer and 977you need to manage a lot of state. 978 979<H3><a name="ply_nn19"></a>3.16 Duplicating lexers</H3> 980 981 982<b>NOTE: I am thinking about deprecating this feature. Post comments on <a href="http://groups.google.com/group/ply-hack">ply-hack@googlegroups.com</a> or send me a private email at dave@dabeaz.com.</b> 983 |
|
| 386<p> | 984<p> |
| 387<li>To reduce compiler startup time and improve performance, the lexer can be built in optimized mode as follows: | 985If necessary, a lexer object can be quickly duplicated by invoking its <tt>clone()</tt> method. For example: |
| 388 389<blockquote> 390<pre> | 986 987<blockquote> 988<pre> |
| 391lex.lex(optimize=1) | 989lexer = lex.lex() 990... 991newlexer = lexer.clone() |
| 392</pre> 393</blockquote> 394 | 992</pre> 993</blockquote> 994 |
| 395When used, most error checking and validation is disabled. This provides a slight performance 396gain while tokenizing and tends to chop a few tenths of a second off startup time. Since it disables 397error checking, this mode is not the default and is not recommended during development. However, once 398you have your compiler fully working, it is usually safe to disable the error checks. | 995When a lexer is cloned, the copy is identical to the original lexer, 996including any input text. However, once created, different text can be 997fed to the clone which can be used independently. This capability may 998be useful in situations when you are writing a parser/compiler that 999involves recursive or reentrant processing. For instance, if you 1000needed to scan ahead in the input for some reason, you could create a 1001clone and use it to look ahead. |
| 399 400<p> | 1002 1003<p> |
| 401<li>You can enable some additional debugging by building the lexer like this: | 1004The advantage of using <tt>clone()</tt> instead of reinvoking <tt>lex()</tt> is 1005that it is significantly faster. Namely, it is not necessary to re-examine all of the 1006token rules, build a regular expression, and construct internal tables. All of this 1007information can simply be reused in the new lexer. |
| 402 | 1008 |
| 1009<p> 1010Special considerations need to be made when cloning a lexer that is defined as a class. Previous sections 1011showed an example of a class <tt>MyLexer</tt>. If you have the following code: 1012 |
|
| 403<blockquote> 404<pre> | 1013<blockquote> 1014<pre> |
| 405lex.lex(debug=1) | 1015m = MyLexer() 1016a = lex.lex(object=m) # Create a lexer 1017 1018b = a.clone() # Clone the lexer |
| 406</pre> 407</blockquote> 408 | 1019</pre> 1020</blockquote> 1021 |
| 1022Then both <tt>a</tt> and <tt>b</tt> are going to be bound to the same 1023object <tt>m</tt>. If the object <tt>m</tt> contains internal state 1024related to lexing, this sharing may lead to quite a bit of confusion. To fix this, 1025the <tt>clone()</tt> method accepts an optional argument that can be used to supply a new object. This 1026can be used to clone the lexer and bind it to a new instance. For example: 1027 1028<blockquote> 1029<pre> 1030m = MyLexer() # Create a lexer 1031a = lex.lex(object=m) 1032 1033# Create a clone 1034n = MyLexer() # New instance of MyLexer 1035b = a.clone(n) # New lexer bound to n 1036</pre> 1037</blockquote> 1038 1039It may make sense to encapsulate all of this inside a method: 1040 1041<blockquote> 1042<pre> 1043class MyLexer: 1044 ... 1045 def clone(self): 1046 c = MyLexer() # Create a new instance of myself 1047 # Copy attributes from self to c as appropriate 1048 ... 1049 # Clone the lexer 1050 c.lexer = self.lexer.clone(c) 1051 return c 1052</pre> 1053</blockquote> 1054 1055The fact that a new instance of <tt>MyLexer</tt> may be created while cloning a lexer is the reason why you should never 1056invoke <tt>lex.lex()</tt> inside <tt>__init__()</tt>. If you do, the lexer will be rebuilt from scratch and you lose 1057all of the performance benefits of using <tt>clone()</tt> in the first place. 1058 1059<H3><a name="ply_nn20"></a>3.17 Internal lexer state</H3> 1060 1061 1062A Lexer object <tt>lexer</tt> has a number of internal attributes that may be useful in certain 1063situations. 1064 |
|
| 409<p> | 1065<p> |
| 410<li>To help you debug your lexer, <tt>lex.py</tt> comes with a simple main program which will either 411tokenize input read from standard input or from a file. To use it, simply put this in your lexer: | 1066<tt>lexer.lexpos</tt> 1067<blockquote> 1068This attribute is an integer that contains the current position within the input text. If you modify 1069the value, it will change the result of the next call to <tt>token()</tt>. Within token rule functions, this points 1070to the first character <em>after</em> the matched text. If the value is modified within a rule, the next returned token will be 1071matched at the new position. 1072</blockquote> |
| 412 | 1073 |
| 1074<p> 1075<tt>lexer.lineno</tt> |
|
| 413<blockquote> | 1076<blockquote> |
| 1077The current value of the line number attribute stored in the lexer. This can be modified as needed to 1078change the line number. 1079</blockquote> 1080 1081<p> 1082<tt>lexer.lexdata</tt> 1083<blockquote> 1084The current input text stored in the lexer. This is the string passed with the <tt>input()</tt> method. It 1085would probably be a bad idea to modify this unless you really know what you're doing. 1086</blockquote> 1087 1088<P> 1089<tt>lexer.lexmatch</tt> 1090<blockquote> 1091This is the raw <tt>Match</tt> object returned by the Python <tt>re.match()</tt> function (used internally by PLY) for the 1092current token. If you have written a regular expression that contains named groups, you can use this to retrieve those values. 1093</blockquote> 1094 1095<H3><a name="ply_nn21"></a>3.18 Conditional lexing and start conditions</H3> 1096 1097 1098In advanced parsing applications, it may be useful to have different 1099lexing states. For instance, you may want the occurrence of a certain 1100token or syntactic construct to trigger a different kind of lexing. 1101PLY supports a feature that allows the underlying lexer to be put into 1102a series of different states. Each state can have its own tokens, 1103lexing rules, and so forth. The implementation is based largely on 1104the "start condition" feature of GNU flex. Details of this can be found 1105at <a 1106href="http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html">http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html.</a>. 1107 1108<p> 1109To define a new lexing state, it must first be declared. This is done by including a "states" declaration in your 1110lex file. For example: 1111 1112<blockquote> |
|
| 414<pre> | 1113<pre> |
| 415if __name__ == '__main__': 416 lex.runmain() | 1114states = ( 1115 ('foo','exclusive'), 1116 ('bar','inclusive'), 1117) |
| 417</pre> 418</blockquote> 419 | 1118</pre> 1119</blockquote> 1120 |
| 420Then, run you lexer as a main program such as <tt>python mylex.py</tt> | 1121This declaration declares two states, <tt>'foo'</tt> 1122and <tt>'bar'</tt>. States may be of two types; <tt>'exclusive'</tt> 1123and <tt>'inclusive'</tt>. An exclusive state completely overrides the 1124default behavior of the lexer. That is, lex will only return tokens 1125and apply rules defined specifically for that state. An inclusive 1126state adds additional tokens and rules to the default set of rules. 1127Thus, lex will return both the tokens defined by default in addition 1128to those defined for the inclusive state. |
| 421 422<p> | 1129 1130<p> |
| 1131Once a state has been declared, tokens and rules are declared by including the 1132state name in token/rule declaration. For example: 1133 1134<blockquote> 1135<pre> 1136t_foo_NUMBER = r'\d+' # Token 'NUMBER' in state 'foo' 1137t_bar_ID = r'[a-zA-Z_][a-zA-Z0-9_]*' # Token 'ID' in state 'bar' 1138 1139def t_foo_newline(t): 1140 r'\n' 1141 t.lexer.lineno += 1 1142</pre> 1143</blockquote> 1144 1145A token can be declared in multiple states by including multiple state names in the declaration. For example: 1146 1147<blockquote> 1148<pre> 1149t_foo_bar_NUMBER = r'\d+' # Defines token 'NUMBER' in both state 'foo' and 'bar' 1150</pre> 1151</blockquote> 1152 1153Alternative, a token can be declared in all states using the 'ANY' in the name. 1154 1155<blockquote> 1156<pre> 1157t_ANY_NUMBER = r'\d+' # Defines a token 'NUMBER' in all states 1158</pre> 1159</blockquote> 1160 1161If no state name is supplied, as is normally the case, the token is associated with a special state <tt>'INITIAL'</tt>. For example, 1162these two declarations are identical: 1163 1164<blockquote> 1165<pre> 1166t_NUMBER = r'\d+' 1167t_INITIAL_NUMBER = r'\d+' 1168</pre> 1169</blockquote> 1170 1171<p> 1172States are also associated with the special <tt>t_ignore</tt> and <tt>t_error()</tt> declarations. For example, if a state treats 1173these differently, you can declare: 1174 1175<blockquote> 1176<pre> 1177t_foo_ignore = " \t\n" # Ignored characters for state 'foo' 1178 1179def t_bar_error(t): # Special error handler for state 'bar' 1180 pass 1181</pre> 1182</blockquote> 1183 1184By default, lexing operates in the <tt>'INITIAL'</tt> state. This state includes all of the normally defined tokens. 1185For users who aren't using different states, this fact is completely transparent. If, during lexing or parsing, you want to change 1186the lexing state, use the <tt>begin()</tt> method. For example: 1187 1188<blockquote> 1189<pre> 1190def t_begin_foo(t): 1191 r'start_foo' 1192 t.lexer.begin('foo') # Starts 'foo' state 1193</pre> 1194</blockquote> 1195 1196To get out of a state, you use <tt>begin()</tt> to switch back to the initial state. For example: 1197 1198<blockquote> 1199<pre> 1200def t_foo_end(t): 1201 r'end_foo' 1202 t.lexer.begin('INITIAL') # Back to the initial state 1203</pre> 1204</blockquote> 1205 1206The management of states can also be done with a stack. For example: 1207 1208<blockquote> 1209<pre> 1210def t_begin_foo(t): 1211 r'start_foo' 1212 t.lexer.push_state('foo') # Starts 'foo' state 1213 1214def t_foo_end(t): 1215 r'end_foo' 1216 t.lexer.pop_state() # Back to the previous state 1217</pre> 1218</blockquote> 1219 1220<p> 1221The use of a stack would be useful in situations where there are many ways of entering a new lexing state and you merely want to go back 1222to the previous state afterwards. 1223 1224<P> 1225An example might help clarify. Suppose you were writing a parser and you wanted to grab sections of arbitrary C code enclosed by 1226curly braces. That is, whenever you encounter a starting brace '{', you want to read all of the enclosed code up to the ending brace '}' 1227and return it as a string. Doing this with a normal regular expression rule is nearly (if not actually) impossible. This is because braces can 1228be nested and can be included in comments and strings. Thus, simply matching up to the first matching '}' character isn't good enough. Here is how 1229you might use lexer states to do this: 1230 1231<blockquote> 1232<pre> 1233# Declare the state 1234states = ( 1235 ('ccode','exclusive'), 1236) 1237 1238# Match the first {. Enter ccode state. 1239def t_ccode(t): 1240 r'\{' 1241 t.lexer.code_start = t.lexer.lexpos # Record the starting position 1242 t.lexer.level = 1 # Initial brace level 1243 t.lexer.begin('ccode') # Enter 'ccode' state 1244 1245# Rules for the ccode state 1246def t_ccode_lbrace(t): 1247 r'\{' 1248 t.lexer.level +=1 1249 1250def t_ccode_rbrace(t): 1251 r'\}' 1252 t.lexer.level -=1 1253 1254 # If closing brace, return the code fragment 1255 if t.lexer.level == 0: 1256 t.value = t.lexer.lexdata[t.lexer.code_start:t.lexer.lexpos+1] 1257 t.type = "CCODE" 1258 t.lexer.lineno += t.value.count('\n') 1259 t.lexer.begin('INITIAL') 1260 return t 1261 1262# C or C++ comment (ignore) 1263def t_ccode_comment(t): 1264 r'(/\*(.|\n)*?*/)|(//.*)' 1265 pass 1266 1267# C string 1268def t_ccode_string(t): 1269 r'\"([^\\\n]|(\\.))*?\"' 1270 1271# C character literal 1272def t_ccode_char(t): 1273 r'\'([^\\\n]|(\\.))*?\'' 1274 1275# Any sequence of non-whitespace characters (not braces, strings) 1276def t_ccode_nonspace(t): 1277 r'[^\s\{\}\'\"]+' 1278 1279# Ignored characters (whitespace) 1280t_ccode_ignore = " \t\n" 1281 1282# For bad characters, we just skip over it 1283def t_ccode_error(t): 1284 t.lexer.skip(1) 1285</pre> 1286</blockquote> 1287 1288In this example, the occurrence of the first '{' causes the lexer to record the starting position and enter a new state <tt>'ccode'</tt>. A collection of rules then match 1289various parts of the input that follow (comments, strings, etc.). All of these rules merely discard the token (by not returning a value). 1290However, if the closing right brace is encountered, the rule <tt>t_ccode_rbrace</tt> collects all of the code (using the earlier recorded starting 1291position), stores it, and returns a token 'CCODE' containing all of that text. When returning the token, the lexing state is restored back to its 1292initial state. 1293 1294<H3><a name="ply_nn21"></a>3.19 Miscellaneous Issues</H3> 1295 1296 1297<P> 1298<li>The lexer requires input to be supplied as a single input string. Since most machines have more than enough memory, this 1299rarely presents a performance concern. However, it means that the lexer currently can't be used with streaming data 1300such as open files or sockets. This limitation is primarily a side-effect of using the <tt>re</tt> module. 1301 1302<p> 1303<li>The lexer should work properly with both Unicode strings given as token and pattern matching rules as 1304well as for input text. 1305 1306<p> 1307<li>If you need to supply optional flags to the re.compile() function, use the reflags option to lex. For example: 1308 1309<blockquote> 1310<pre> 1311lex.lex(reflags=re.UNICODE) 1312</pre> 1313</blockquote> 1314 1315<p> |
|
| 423<li>Since the lexer is written entirely in Python, its performance is 424largely determined by that of the Python <tt>re</tt> module. Although 425the lexer has been written to be as efficient as possible, it's not | 1316<li>Since the lexer is written entirely in Python, its performance is 1317largely determined by that of the Python <tt>re</tt> module. Although 1318the lexer has been written to be as efficient as possible, it's not |
| 426blazingly fast when used on very large input files. Sorry. If | 1319blazingly fast when used on very large input files. If |
| 427performance is concern, you might consider upgrading to the most 428recent version of Python, creating a hand-written lexer, or offloading | 1320performance is concern, you might consider upgrading to the most 1321recent version of Python, creating a hand-written lexer, or offloading |
| 429the lexer into a C extension module. In defense of <tt>lex.py</tt>, 430it's performance is not <em>that</em> bad when used on reasonably 431sized input files. For instance, lexing a 4700 line C program with 43232000 input tokens takes about 20 seconds on a 200 Mhz PC. Obviously, 433it will run much faster on a more speedy machine. | 1322the lexer into a C extension module. |
| 434 | 1323 |
| 1324<p> 1325If you are going to create a hand-written lexer and you plan to use it with <tt>yacc.py</tt>, 1326it only needs to conform to the following requirements: 1327 1328<ul> 1329<li>It must provide a <tt>token()</tt> method that returns the next token or <tt>None</tt> if no more 1330tokens are available. 1331<li>The <tt>token()</tt> method must return an object <tt>tok</tt> that has <tt>type</tt> and <tt>value</tt> attributes. |
|
| 435</ul> 436 | 1332</ul> 1333 |
| 437<h2>Parsing basics</h2> | 1334<H2><a name="ply_nn22"></a>4. Parsing basics</H2> |
| 438 | 1335 |
| 1336 |
|
| 439<tt>yacc.py</tt> is used to parse language syntax. Before showing an 440example, there are a few important bits of background that must be | 1337<tt>yacc.py</tt> is used to parse language syntax. Before showing an 1338example, there are a few important bits of background that must be |
| 441mentioned. First, <tt>syntax</tt> is usually specified in terms of a 442context free grammar (CFG). For example, if you wanted to parse | 1339mentioned. First, <em>syntax</em> is usually specified in terms of a BNF grammar. 1340For example, if you wanted to parse |
| 443simple arithmetic expressions, you might first write an unambiguous 444grammar specification like this: 445 446<blockquote> 447<pre> 448expression : expression + term 449 | expression - term 450 | term 451 452term : term * factor 453 | term / factor 454 | factor 455 456factor : NUMBER 457 | ( expression ) 458</pre> 459</blockquote> 460 | 1341simple arithmetic expressions, you might first write an unambiguous 1342grammar specification like this: 1343 1344<blockquote> 1345<pre> 1346expression : expression + term 1347 | expression - term 1348 | term 1349 1350term : term * factor 1351 | term / factor 1352 | factor 1353 1354factor : NUMBER 1355 | ( expression ) 1356</pre> 1357</blockquote> 1358 |
| 461Next, the semantic behavior of a language is often specified using a | 1359In the grammar, symbols such as <tt>NUMBER</tt>, <tt>+</tt>, <tt>-</tt>, <tt>*</tt>, and <tt>/</tt> are known 1360as <em>terminals</em> and correspond to raw input tokens. Identifiers such as <tt>term</tt> and <tt>factor</tt> refer to more 1361complex rules, typically comprised of a collection of tokens. These identifiers are known as <em>non-terminals</em>. 1362<P> 1363The semantic behavior of a language is often specified using a |
| 462technique known as syntax directed translation. In syntax directed 463translation, attributes are attached to each symbol in a given grammar 464rule along with an action. Whenever a particular grammar rule is 465recognized, the action describes what to do. For example, given the 466expression grammar above, you might write the specification for a 467simple calculator like this: 468 469<blockquote> --- 8 unchanged lines hidden (view full) --- 478 | term1 / factor term0.val = term1.val / factor.val 479 | factor term0.val = factor.val 480 481factor : NUMBER factor.val = int(NUMBER.lexval) 482 | ( expression ) factor.val = expression.val 483</pre> 484</blockquote> 485 | 1364technique known as syntax directed translation. In syntax directed 1365translation, attributes are attached to each symbol in a given grammar 1366rule along with an action. Whenever a particular grammar rule is 1367recognized, the action describes what to do. For example, given the 1368expression grammar above, you might write the specification for a 1369simple calculator like this: 1370 1371<blockquote> --- 8 unchanged lines hidden (view full) --- 1380 | term1 / factor term0.val = term1.val / factor.val 1381 | factor term0.val = factor.val 1382 1383factor : NUMBER factor.val = int(NUMBER.lexval) 1384 | ( expression ) factor.val = expression.val 1385</pre> 1386</blockquote> 1387 |
| 486Finally, Yacc uses a parsing technique known as LR-parsing or shift-reduce parsing. LR parsing is a | 1388A good way to think about syntax directed translation is to simply think of each symbol in the grammar as some 1389kind of object. The semantics of the language are then expressed as a collection of methods/operations on these 1390objects. 1391 1392<p> 1393Yacc uses a parsing technique known as LR-parsing or shift-reduce parsing. LR parsing is a |
| 487bottom up technique that tries to recognize the right-hand-side of various grammar rules. 488Whenever a valid right-hand-side is found in the input, the appropriate action code is triggered and the 489grammar symbols are replaced by the grammar symbol on the left-hand-side. 490 491<p> 492LR parsing is commonly implemented by shifting grammar symbols onto a stack and looking at the stack and the next 493input token for patterns. The details of the algorithm can be found in a compiler text, but the 494following example illustrates the steps that are performed if you wanted to parse the expression --- 34 unchanged lines hidden (view full) --- 529When parsing the expression, an underlying state machine and the current input token determine what to do next. 530If the next token looks like part of a valid grammar rule (based on other items on the stack), it is generally shifted 531onto the stack. If the top of the stack contains a valid right-hand-side of a grammar rule, it is 532usually "reduced" and the symbols replaced with the symbol on the left-hand-side. When this reduction occurs, the 533appropriate action is triggered (if defined). If the input token can't be shifted and the top of stack doesn't match 534any grammar rules, a syntax error has occurred and the parser must take some kind of recovery step (or bail out). 535 536<p> | 1394bottom up technique that tries to recognize the right-hand-side of various grammar rules. 1395Whenever a valid right-hand-side is found in the input, the appropriate action code is triggered and the 1396grammar symbols are replaced by the grammar symbol on the left-hand-side. 1397 1398<p> 1399LR parsing is commonly implemented by shifting grammar symbols onto a stack and looking at the stack and the next 1400input token for patterns. The details of the algorithm can be found in a compiler text, but the 1401following example illustrates the steps that are performed if you wanted to parse the expression --- 34 unchanged lines hidden (view full) --- 1436When parsing the expression, an underlying state machine and the current input token determine what to do next. 1437If the next token looks like part of a valid grammar rule (based on other items on the stack), it is generally shifted 1438onto the stack. If the top of the stack contains a valid right-hand-side of a grammar rule, it is 1439usually "reduced" and the symbols replaced with the symbol on the left-hand-side. When this reduction occurs, the 1440appropriate action is triggered (if defined). If the input token can't be shifted and the top of stack doesn't match 1441any grammar rules, a syntax error has occurred and the parser must take some kind of recovery step (or bail out). 1442 1443<p> |
| 537It is important to note that the underlying implementation is actually built around a large finite-state machine 538and some tables. The construction of these tables is quite complicated and beyond the scope of this discussion. | 1444It is important to note that the underlying implementation is built around a large finite-state machine that is encoded 1445in a collection of tables. The construction of these tables is quite complicated and beyond the scope of this discussion. |
| 539However, subtle details of this process explain why, in the example above, the parser chooses to shift a token 540onto the stack in step 9 rather than reducing the rule <tt>expr : expr + term</tt>. 541 | 1446However, subtle details of this process explain why, in the example above, the parser chooses to shift a token 1447onto the stack in step 9 rather than reducing the rule <tt>expr : expr + term</tt>. 1448 |
| 542<h2>Yacc example</h2> | 1449<H2><a name="ply_nn23"></a>5. Yacc reference</H2> |
| 543 | 1450 |
| 1451 1452This section describes how to use write parsers in PLY. 1453 1454<H3><a name="ply_nn24"></a>5.1 An example</H3> 1455 1456 |
|
| 544Suppose you wanted to make a grammar for simple arithmetic expressions as previously described. Here is 545how you would do it with <tt>yacc.py</tt>: 546 547<blockquote> 548<pre> 549# Yacc example 550 | 1457Suppose you wanted to make a grammar for simple arithmetic expressions as previously described. Here is 1458how you would do it with <tt>yacc.py</tt>: 1459 1460<blockquote> 1461<pre> 1462# Yacc example 1463 |
| 551import yacc | 1464import ply.yacc as yacc |
| 552 553# Get the token map from the lexer. This is required. 554from calclex import tokens 555 | 1465 1466# Get the token map from the lexer. This is required. 1467from calclex import tokens 1468 |
| 556def p_expression_plus(t): | 1469def p_expression_plus(p): |
| 557 'expression : expression PLUS term' | 1470 'expression : expression PLUS term' |
| 558 t[0] = t[1] + t[3] | 1471 p[0] = p[1] + p[3] |
| 559 | 1472 |
| 560def p_expression_minus(t): | 1473def p_expression_minus(p): |
| 561 'expression : expression MINUS term' | 1474 'expression : expression MINUS term' |
| 562 t[0] = t[1] - t[3] | 1475 p[0] = p[1] - p[3] |
| 563 | 1476 |
| 564def p_expression_term(t): | 1477def p_expression_term(p): |
| 565 'expression : term' | 1478 'expression : term' |
| 566 t[0] = t[1] | 1479 p[0] = p[1] |
| 567 | 1480 |
| 568def p_term_times(t): | 1481def p_term_times(p): |
| 569 'term : term TIMES factor' | 1482 'term : term TIMES factor' |
| 570 t[0] = t[1] * t[3] | 1483 p[0] = p[1] * p[3] |
| 571 | 1484 |
| 572def p_term_div(t): | 1485def p_term_div(p): |
| 573 'term : term DIVIDE factor' | 1486 'term : term DIVIDE factor' |
| 574 t[0] = t[1] / t[3] | 1487 p[0] = p[1] / p[3] |
| 575 | 1488 |
| 576def p_term_factor(t): | 1489def p_term_factor(p): |
| 577 'term : factor' | 1490 'term : factor' |
| 578 t[0] = t[1] | 1491 p[0] = p[1] |
| 579 | 1492 |
| 580def p_factor_num(t): | 1493def p_factor_num(p): |
| 581 'factor : NUMBER' | 1494 'factor : NUMBER' |
| 582 t[0] = t[1] | 1495 p[0] = p[1] |
| 583 | 1496 |
| 584def p_factor_expr(t): | 1497def p_factor_expr(p): |
| 585 'factor : LPAREN expression RPAREN' | 1498 'factor : LPAREN expression RPAREN' |
| 586 t[0] = t[2] | 1499 p[0] = p[2] |
| 587 588# Error rule for syntax errors | 1500 1501# Error rule for syntax errors |
| 589def p_error(t): | 1502def p_error(p): |
| 590 print "Syntax error in input!" 591 592# Build the parser 593yacc.yacc() 594 | 1503 print "Syntax error in input!" 1504 1505# Build the parser 1506yacc.yacc() 1507 |
| 1508# Use this if you want to build the parser using SLR instead of LALR 1509# yacc.yacc(method="SLR") 1510 |
|
| 595while 1: 596 try: 597 s = raw_input('calc > ') 598 except EOFError: 599 break 600 if not s: continue 601 result = yacc.parse(s) 602 print result 603</pre> 604</blockquote> 605 606In this example, each grammar rule is defined by a Python function where the docstring to that function contains the | 1511while 1: 1512 try: 1513 s = raw_input('calc > ') 1514 except EOFError: 1515 break 1516 if not s: continue 1517 result = yacc.parse(s) 1518 print result 1519</pre> 1520</blockquote> 1521 1522In this example, each grammar rule is defined by a Python function where the docstring to that function contains the |
| 607appropriate context-free grammar specification (an idea borrowed from John Aycock's SPARK toolkit). Each function accepts a single 608argument <tt>t</tt> that is a sequence containing the values of each grammar symbol in the corresponding rule. The values of 609<tt>t[i]</tt> are mapped to grammar symbols as shown here: | 1523appropriate context-free grammar specification. Each function accepts a single 1524argument <tt>p</tt> that is a sequence containing the values of each grammar symbol in the corresponding rule. The values of 1525<tt>p[i]</tt> are mapped to grammar symbols as shown here: |
| 610 611<blockquote> 612<pre> | 1526 1527<blockquote> 1528<pre> |
| 613def p_expression_plus(t): | 1529def p_expression_plus(p): |
| 614 'expression : expression PLUS term' 615 # ^ ^ ^ ^ | 1530 'expression : expression PLUS term' 1531 # ^ ^ ^ ^ |
| 616 # t[0] t[1] t[2] t[3] | 1532 # p[0] p[1] p[2] p[3] |
| 617 | 1533 |
| 618 t[0] = t[1] + t[3] | 1534 p[0] = p[1] + p[3] |
| 619</pre> 620</blockquote> 621 | 1535</pre> 1536</blockquote> 1537 |
| 622For tokens, the "value" in the corresponding <tt>t[i]</tt> is the 623<em>same</em> as the value of the <tt>t.value</tt> attribute assigned | 1538For tokens, the "value" of the corresponding <tt>p[i]</tt> is the 1539<em>same</em> as the <tt>p.value</tt> attribute assigned |
| 624in the lexer module. For non-terminals, the value is determined by | 1540in the lexer module. For non-terminals, the value is determined by |
| 625whatever is placed in <tt>t[0]</tt> when rules are reduced. This | 1541whatever is placed in <tt>p[0]</tt> when rules are reduced. This |
| 626value can be anything at all. However, it probably most common for 627the value to be a simple Python type, a tuple, or an instance. In this example, we 628are relying on the fact that the <tt>NUMBER</tt> token stores an integer value in its value 629field. All of the other rules simply perform various types of integer operations and store 630the result. 631 | 1542value can be anything at all. However, it probably most common for 1543the value to be a simple Python type, a tuple, or an instance. In this example, we 1544are relying on the fact that the <tt>NUMBER</tt> token stores an integer value in its value 1545field. All of the other rules simply perform various types of integer operations and store 1546the result. 1547 |
| 1548<P> 1549Note: The use of negative indices have a special meaning in yacc---specially <tt>p[-1]</tt> does 1550not have the same value as <tt>p[3]</tt> in this example. Please see the section on "Embedded Actions" for further 1551details. 1552 |
|
| 632<p> 633The first rule defined in the yacc specification determines the starting grammar 634symbol (in this case, a rule for <tt>expression</tt> appears first). Whenever 635the starting rule is reduced by the parser and no more input is available, parsing 636stops and the final value is returned (this value will be whatever the top-most rule | 1553<p> 1554The first rule defined in the yacc specification determines the starting grammar 1555symbol (in this case, a rule for <tt>expression</tt> appears first). Whenever 1556the starting rule is reduced by the parser and no more input is available, parsing 1557stops and the final value is returned (this value will be whatever the top-most rule |
| 637placed in <tt>t[0]</tt>). | 1558placed in <tt>p[0]</tt>). Note: an alternative starting symbol can be specified using the <tt>start</tt> keyword argument to 1559<tt>yacc()</tt>. |
| 638 | 1560 |
| 639<p>The <tt>p_error(t)</tt> rule is defined to catch syntax errors. See the error handling section | 1561<p>The <tt>p_error(p)</tt> rule is defined to catch syntax errors. See the error handling section |
| 640below for more detail. 641 642<p> 643To build the parser, call the <tt>yacc.yacc()</tt> function. This function 644looks at the module and attempts to construct all of the LR parsing tables for the grammar 645you have specified. The first time <tt>yacc.yacc()</tt> is invoked, you will get a message 646such as this: 647 648<blockquote> 649<pre> 650$ python calcparse.py | 1562below for more detail. 1563 1564<p> 1565To build the parser, call the <tt>yacc.yacc()</tt> function. This function 1566looks at the module and attempts to construct all of the LR parsing tables for the grammar 1567you have specified. The first time <tt>yacc.yacc()</tt> is invoked, you will get a message 1568such as this: 1569 1570<blockquote> 1571<pre> 1572$ python calcparse.py |
| 651yacc: Generating SLR parsing table... | 1573yacc: Generating LALR parsing table... |
| 652calc > 653</pre> 654</blockquote> 655 656Since table construction is relatively expensive (especially for large 657grammars), the resulting parsing table is written to the current 658directory in a file called <tt>parsetab.py</tt>. In addition, a 659debugging file called <tt>parser.out</tt> is created. On subsequent 660executions, <tt>yacc</tt> will reload the table from 661<tt>parsetab.py</tt> unless it has detected a change in the underlying 662grammar (in which case the tables and <tt>parsetab.py</tt> file are | 1574calc > 1575</pre> 1576</blockquote> 1577 1578Since table construction is relatively expensive (especially for large 1579grammars), the resulting parsing table is written to the current 1580directory in a file called <tt>parsetab.py</tt>. In addition, a 1581debugging file called <tt>parser.out</tt> is created. On subsequent 1582executions, <tt>yacc</tt> will reload the table from 1583<tt>parsetab.py</tt> unless it has detected a change in the underlying 1584grammar (in which case the tables and <tt>parsetab.py</tt> file are |
| 663regenerated). | 1585regenerated). Note: The names of parser output files can be changed if necessary. See the notes that follow later. |
| 664 665<p> 666If any errors are detected in your grammar specification, <tt>yacc.py</tt> will produce 667diagnostic messages and possibly raise an exception. Some of the errors that can be detected include: 668 669<ul> 670<li>Duplicated function names (if more than one rule function have the same name in the grammar file). 671<li>Shift/reduce and reduce/reduce conflicts generated by ambiguous grammars. 672<li>Badly specified grammar rules. 673<li>Infinite recursion (rules that can never terminate). 674<li>Unused rules and tokens 675<li>Undefined rules and tokens 676</ul> 677 678The next few sections now discuss a few finer points of grammar construction. 679 | 1586 1587<p> 1588If any errors are detected in your grammar specification, <tt>yacc.py</tt> will produce 1589diagnostic messages and possibly raise an exception. Some of the errors that can be detected include: 1590 1591<ul> 1592<li>Duplicated function names (if more than one rule function have the same name in the grammar file). 1593<li>Shift/reduce and reduce/reduce conflicts generated by ambiguous grammars. 1594<li>Badly specified grammar rules. 1595<li>Infinite recursion (rules that can never terminate). 1596<li>Unused rules and tokens 1597<li>Undefined rules and tokens 1598</ul> 1599 1600The next few sections now discuss a few finer points of grammar construction. 1601 |
| 680<h2>Combining Grammar Rule Functions</h2> | 1602<H3><a name="ply_nn25"></a>5.2 Combining Grammar Rule Functions</H3> |
| 681 | 1603 |
| 1604 |
|
| 682When grammar rules are similar, they can be combined into a single function. 683For example, consider the two rules in our earlier example: 684 685<blockquote> 686<pre> | 1605When grammar rules are similar, they can be combined into a single function. 1606For example, consider the two rules in our earlier example: 1607 1608<blockquote> 1609<pre> |
| 687def p_expression_plus(t): | 1610def p_expression_plus(p): |
| 688 'expression : expression PLUS term' | 1611 'expression : expression PLUS term' |
| 689 t[0] = t[1] + t[3] | 1612 p[0] = p[1] + p[3] |
| 690 691def p_expression_minus(t): 692 'expression : expression MINUS term' | 1613 1614def p_expression_minus(t): 1615 'expression : expression MINUS term' |
| 693 t[0] = t[1] - t[3] | 1616 p[0] = p[1] - p[3] |
| 694</pre> 695</blockquote> 696 697Instead of writing two functions, you might write a single function like this: 698 699<blockquote> 700<pre> | 1617</pre> 1618</blockquote> 1619 1620Instead of writing two functions, you might write a single function like this: 1621 1622<blockquote> 1623<pre> |
| 701def p_expression(t): | 1624def p_expression(p): |
| 702 '''expression : expression PLUS term 703 | expression MINUS term''' | 1625 '''expression : expression PLUS term 1626 | expression MINUS term''' |
| 704 if t[2] == '+': 705 t[0] = t[1] + t[3] 706 elif t[2] == '-': 707 t[0] = t[1] - t[3] | 1627 if p[2] == '+': 1628 p[0] = p[1] + p[3] 1629 elif p[2] == '-': 1630 p[0] = p[1] - p[3] |
| 708</pre> 709</blockquote> 710 711In general, the doc string for any given function can contain multiple grammar rules. So, it would 712have also been legal (although possibly confusing) to write this: 713 714<blockquote> 715<pre> | 1631</pre> 1632</blockquote> 1633 1634In general, the doc string for any given function can contain multiple grammar rules. So, it would 1635have also been legal (although possibly confusing) to write this: 1636 1637<blockquote> 1638<pre> |
| 716def p_binary_operators(t): | 1639def p_binary_operators(p): |
| 717 '''expression : expression PLUS term 718 | expression MINUS term 719 term : term TIMES factor 720 | term DIVIDE factor''' | 1640 '''expression : expression PLUS term 1641 | expression MINUS term 1642 term : term TIMES factor 1643 | term DIVIDE factor''' |
| 721 if t[2] == '+': 722 t[0] = t[1] + t[3] 723 elif t[2] == '-': 724 t[0] = t[1] - t[3] 725 elif t[2] == '*': 726 t[0] = t[1] * t[3] 727 elif t[2] == '/': 728 t[0] = t[1] / t[3] | 1644 if p[2] == '+': 1645 p[0] = p[1] + p[3] 1646 elif p[2] == '-': 1647 p[0] = p[1] - p[3] 1648 elif p[2] == '*': 1649 p[0] = p[1] * p[3] 1650 elif p[2] == '/': 1651 p[0] = p[1] / p[3] |
| 729</pre> 730</blockquote> 731 732When combining grammar rules into a single function, it is usually a good idea for all of the rules to have 733a similar structure (e.g., the same number of terms). Otherwise, the corresponding action code may be more | 1652</pre> 1653</blockquote> 1654 1655When combining grammar rules into a single function, it is usually a good idea for all of the rules to have 1656a similar structure (e.g., the same number of terms). Otherwise, the corresponding action code may be more |
| 734complicated than necessary. | 1657complicated than necessary. However, it is possible to handle simple cases using len(). For example: |
| 735 | 1658 |
| 736<h2>Empty Productions</h2> | 1659<blockquote> 1660<pre> 1661def p_expressions(p): 1662 '''expression : expression MINUS expression 1663 | MINUS expression''' 1664 if (len(p) == 4): 1665 p[0] = p[1] - p[3] 1666 elif (len(p) == 3): 1667 p[0] = -p[2] 1668</pre> 1669</blockquote> |
| 737 | 1670 |
| 1671<H3><a name="ply_nn26"></a>5.3 Character Literals</H3> 1672 1673 1674If desired, a grammar may contain tokens defined as single character literals. For example: 1675 1676<blockquote> 1677<pre> 1678def p_binary_operators(p): 1679 '''expression : expression '+' term 1680 | expression '-' term 1681 term : term '*' factor 1682 | term '/' factor''' 1683 if p[2] == '+': 1684 p[0] = p[1] + p[3] 1685 elif p[2] == '-': 1686 p[0] = p[1] - p[3] 1687 elif p[2] == '*': 1688 p[0] = p[1] * p[3] 1689 elif p[2] == '/': 1690 p[0] = p[1] / p[3] 1691</pre> 1692</blockquote> 1693 1694A character literal must be enclosed in quotes such as <tt>'+'</tt>. In addition, if literals are used, they must be declared in the 1695corresponding <tt>lex</tt> file through the use of a special <tt>literals</tt> declaration. 1696 1697<blockquote> 1698<pre> 1699# Literals. Should be placed in module given to lex() 1700literals = ['+','-','*','/' ] 1701</pre> 1702</blockquote> 1703 1704<b>Character literals are limited to a single character</b>. Thus, it is not legal to specify literals such as <tt>'<='</tt> or <tt>'=='</tt>. For this, use 1705the normal lexing rules (e.g., define a rule such as <tt>t_EQ = r'=='</tt>). 1706 1707<H3><a name="ply_nn26"></a>5.4 Empty Productions</H3> 1708 1709 |
|
| 738<tt>yacc.py</tt> can handle empty productions by defining a rule like this: 739 740<blockquote> 741<pre> | 1710<tt>yacc.py</tt> can handle empty productions by defining a rule like this: 1711 1712<blockquote> 1713<pre> |
| 742def p_empty(t): | 1714def p_empty(p): |
| 743 'empty :' 744 pass 745</pre> 746</blockquote> 747 748Now to use the empty production, simply use 'empty' as a symbol. For example: 749 750<blockquote> 751<pre> | 1715 'empty :' 1716 pass 1717</pre> 1718</blockquote> 1719 1720Now to use the empty production, simply use 'empty' as a symbol. For example: 1721 1722<blockquote> 1723<pre> |
| 752def p_optitem(t): | 1724def p_optitem(p): |
| 753 'optitem : item' 754 ' | empty' 755 ... 756</pre> 757</blockquote> 758 | 1725 'optitem : item' 1726 ' | empty' 1727 ... 1728</pre> 1729</blockquote> 1730 |
| 759<h2>Dealing With Ambiguous Grammars</h2> | 1731Note: You can write empty rules anywhere by simply specifying an empty right hand side. However, I personally find that 1732writing an "empty" rule and using "empty" to denote an empty production is easier to read. |
| 760 | 1733 |
| 1734<H3><a name="ply_nn28"></a>5.5 Changing the starting symbol</H3> 1735 1736 1737Normally, the first rule found in a yacc specification defines the starting grammar rule (top level rule). To change this, simply 1738supply a <tt>start</tt> specifier in your file. For example: 1739 1740<blockquote> 1741<pre> 1742start = 'foo' 1743 1744def p_bar(p): 1745 'bar : A B' 1746 1747# This is the starting rule due to the start specifier above 1748def p_foo(p): 1749 'foo : bar X' 1750... 1751</pre> 1752</blockquote> 1753 1754The use of a <tt>start</tt> specifier may be useful during debugging since you can use it to have yacc build a subset of 1755a larger grammar. For this purpose, it is also possible to specify a starting symbol as an argument to <tt>yacc()</tt>. For example: 1756 1757<blockquote> 1758<pre> 1759yacc.yacc(start='foo') 1760</pre> 1761</blockquote> 1762 1763<H3><a name="ply_nn27"></a>5.6 Dealing With Ambiguous Grammars</H3> 1764 1765 |
|
| 761The expression grammar given in the earlier example has been written in a special format to eliminate ambiguity. 762However, in many situations, it is extremely difficult or awkward to write grammars in this format. A 763much more natural way to express the grammar is in a more compact form like this: 764 765<blockquote> 766<pre> 767expression : expression PLUS expression 768 | expression MINUS expression 769 | expression TIMES expression 770 | expression DIVIDE expression 771 | LPAREN expression RPAREN 772 | NUMBER 773</pre> 774</blockquote> 775 776Unfortunately, this grammar specification is ambiguous. For example, if you are parsing the string 777"3 * 4 + 5", there is no way to tell how the operators are supposed to be grouped. | 1766The expression grammar given in the earlier example has been written in a special format to eliminate ambiguity. 1767However, in many situations, it is extremely difficult or awkward to write grammars in this format. A 1768much more natural way to express the grammar is in a more compact form like this: 1769 1770<blockquote> 1771<pre> 1772expression : expression PLUS expression 1773 | expression MINUS expression 1774 | expression TIMES expression 1775 | expression DIVIDE expression 1776 | LPAREN expression RPAREN 1777 | NUMBER 1778</pre> 1779</blockquote> 1780 1781Unfortunately, this grammar specification is ambiguous. For example, if you are parsing the string 1782"3 * 4 + 5", there is no way to tell how the operators are supposed to be grouped. |
| 778For example, does this expression mean "(3 * 4) + 5" or is it "3 * (4+5)"? | 1783For example, does the expression mean "(3 * 4) + 5" or is it "3 * (4+5)"? |
| 779 780<p> 781When an ambiguous grammar is given to <tt>yacc.py</tt> it will print messages about "shift/reduce conflicts" 782or a "reduce/reduce conflicts". A shift/reduce conflict is caused when the parser generator can't decide 783whether or not to reduce a rule or shift a symbol on the parsing stack. For example, consider 784the string "3 * 4 + 5" and the internal parsing stack: 785 786<blockquote> --- 4 unchanged lines hidden (view full) --- 7912 $ 3 * 4 + 5$ Reduce : expression : NUMBER 7923 $ expr * 4 + 5$ Shift * 7934 $ expr * 4 + 5$ Shift 4 7945 $ expr * 4 + 5$ Reduce: expression : NUMBER 7956 $ expr * expr + 5$ SHIFT/REDUCE CONFLICT ???? 796</pre> 797</blockquote> 798 | 1784 1785<p> 1786When an ambiguous grammar is given to <tt>yacc.py</tt> it will print messages about "shift/reduce conflicts" 1787or a "reduce/reduce conflicts". A shift/reduce conflict is caused when the parser generator can't decide 1788whether or not to reduce a rule or shift a symbol on the parsing stack. For example, consider 1789the string "3 * 4 + 5" and the internal parsing stack: 1790 1791<blockquote> --- 4 unchanged lines hidden (view full) --- 17962 $ 3 * 4 + 5$ Reduce : expression : NUMBER 17973 $ expr * 4 + 5$ Shift * 17984 $ expr * 4 + 5$ Shift 4 17995 $ expr * 4 + 5$ Reduce: expression : NUMBER 18006 $ expr * expr + 5$ SHIFT/REDUCE CONFLICT ???? 1801</pre> 1802</blockquote> 1803 |
| 799In this case, when the parser reaches step 6, it has two options. One is the reduce the | 1804In this case, when the parser reaches step 6, it has two options. One is to reduce the |
| 800rule <tt>expr : expr * expr</tt> on the stack. The other option is to shift the 801token <tt>+</tt> on the stack. Both options are perfectly legal from the rules 802of the context-free-grammar. 803 804<p> 805By default, all shift/reduce conflicts are resolved in favor of shifting. Therefore, in the above 806example, the parser will always shift the <tt>+</tt> instead of reducing. Although this 807strategy works in many cases (including the ambiguous if-then-else), it is not enough for arithmetic 808expressions. In fact, in the above example, the decision to shift <tt>+</tt> is completely wrong---we should have | 1805rule <tt>expr : expr * expr</tt> on the stack. The other option is to shift the 1806token <tt>+</tt> on the stack. Both options are perfectly legal from the rules 1807of the context-free-grammar. 1808 1809<p> 1810By default, all shift/reduce conflicts are resolved in favor of shifting. Therefore, in the above 1811example, the parser will always shift the <tt>+</tt> instead of reducing. Although this 1812strategy works in many cases (including the ambiguous if-then-else), it is not enough for arithmetic 1813expressions. In fact, in the above example, the decision to shift <tt>+</tt> is completely wrong---we should have |
| 809reduced expr * expr since multiplication has higher precedence than addition. | 1814reduced <tt>expr * expr</tt> since multiplication has higher mathematical precedence than addition. |
| 810 811<p>To resolve ambiguity, especially in expression grammars, <tt>yacc.py</tt> allows individual 812tokens to be assigned a precedence level and associativity. This is done by adding a variable 813<tt>precedence</tt> to the grammar file like this: 814 815<blockquote> 816<pre> 817precedence = ( 818 ('left', 'PLUS', 'MINUS'), 819 ('left', 'TIMES', 'DIVIDE'), 820) 821</pre> 822</blockquote> 823 824This declaration specifies that <tt>PLUS</tt>/<tt>MINUS</tt> have 825the same precedence level and are left-associative and that | 1815 1816<p>To resolve ambiguity, especially in expression grammars, <tt>yacc.py</tt> allows individual 1817tokens to be assigned a precedence level and associativity. This is done by adding a variable 1818<tt>precedence</tt> to the grammar file like this: 1819 1820<blockquote> 1821<pre> 1822precedence = ( 1823 ('left', 'PLUS', 'MINUS'), 1824 ('left', 'TIMES', 'DIVIDE'), 1825) 1826</pre> 1827</blockquote> 1828 1829This declaration specifies that <tt>PLUS</tt>/<tt>MINUS</tt> have 1830the same precedence level and are left-associative and that |
| 826TIMES/DIVIDE have the same precedence and are left-associative. 827Furthermore, the declaration specifies that <tt>TIMES</tt>/<tt>DIVIDE</tt> have higher | 1831<tt>TIMES</tt>/<tt>DIVIDE</tt> have the same precedence and are left-associative. 1832Within the <tt>precedence</tt> declaration, tokens are ordered from lowest to highest precedence. Thus, 1833this declaration specifies that <tt>TIMES</tt>/<tt>DIVIDE</tt> have higher |
| 828precedence than <tt>PLUS</tt>/<tt>MINUS</tt> (since they appear later in the 829precedence specification). 830 831<p> | 1834precedence than <tt>PLUS</tt>/<tt>MINUS</tt> (since they appear later in the 1835precedence specification). 1836 1837<p> |
| 832The precedence specification is used to attach a numerical precedence value and associativity direction 833to each grammar rule. This is always determined by the precedence of the right-most terminal symbol. Therefore, 834if PLUS/MINUS had a precedence of 1 and TIMES/DIVIDE had a precedence of 2, the grammar rules 835would have precedence values as follows: | 1838The precedence specification works by associating a numerical precedence level value and associativity direction to 1839the listed tokens. For example, in the above example you get: |
| 836 837<blockquote> 838<pre> | 1840 1841<blockquote> 1842<pre> |
| 839expression : expression PLUS expression # prec = 1, left 840 | expression MINUS expression # prec = 1, left 841 | expression TIMES expression # prec = 2, left 842 | expression DIVIDE expression # prec = 2, left 843 | LPAREN expression RPAREN # prec = unknown 844 | NUMBER # prec = unknown | 1843PLUS : level = 1, assoc = 'left' 1844MINUS : level = 1, assoc = 'left' 1845TIMES : level = 2, assoc = 'left' 1846DIVIDE : level = 2, assoc = 'left' |
| 845</pre> 846</blockquote> 847 | 1847</pre> 1848</blockquote> 1849 |
| 1850These values are then used to attach a numerical precedence value and associativity direction 1851to each grammar rule. <em>This is always determined by looking at the precedence of the right-most terminal symbol.</em> 1852For example: 1853 1854<blockquote> 1855<pre> 1856expression : expression PLUS expression # level = 1, left 1857 | expression MINUS expression # level = 1, left 1858 | expression TIMES expression # level = 2, left 1859 | expression DIVIDE expression # level = 2, left 1860 | LPAREN expression RPAREN # level = None (not specified) 1861 | NUMBER # level = None (not specified) 1862</pre> 1863</blockquote> 1864 |
|
| 848When shift/reduce conflicts are encountered, the parser generator resolves the conflict by 849looking at the precedence rules and associativity specifiers. 850 851<p> 852<ol> 853<li>If the current token has higher precedence, it is shifted. 854<li>If the grammar rule on the stack has higher precedence, the rule is reduced. 855<li>If the current token and the grammar rule have the same precedence, the 856rule is reduced for left associativity, whereas the token is shifted for right associativity. 857<li>If nothing is known about the precedence, shift/reduce conflicts are resolved in 858favor of shifting (the default). 859</ol> 860 | 1865When shift/reduce conflicts are encountered, the parser generator resolves the conflict by 1866looking at the precedence rules and associativity specifiers. 1867 1868<p> 1869<ol> 1870<li>If the current token has higher precedence, it is shifted. 1871<li>If the grammar rule on the stack has higher precedence, the rule is reduced. 1872<li>If the current token and the grammar rule have the same precedence, the 1873rule is reduced for left associativity, whereas the token is shifted for right associativity. 1874<li>If nothing is known about the precedence, shift/reduce conflicts are resolved in 1875favor of shifting (the default). 1876</ol> 1877 |
| 1878For example, if "expression PLUS expression" has been parsed and the next token 1879is "TIMES", the action is going to be a shift because "TIMES" has a higher precedence level than "PLUS". On the other 1880hand, if "expression TIMES expression" has been parsed and the next token is "PLUS", the action 1881is going to be reduce because "PLUS" has a lower precedence than "TIMES." 1882 |
|
| 861<p> 862When shift/reduce conflicts are resolved using the first three techniques (with the help of 863precedence rules), <tt>yacc.py</tt> will report no errors or conflicts in the grammar. 864 865<p> 866One problem with the precedence specifier technique is that it is sometimes necessary to 867change the precedence of an operator in certain contents. For example, consider a unary-minus operator 868in "3 + 4 * -5". Normally, unary minus has a very high precedence--being evaluated before the multiply. --- 9 unchanged lines hidden (view full) --- 878) 879</pre> 880</blockquote> 881 882Now, in the grammar file, we can write our unary minus rule like this: 883 884<blockquote> 885<pre> | 1883<p> 1884When shift/reduce conflicts are resolved using the first three techniques (with the help of 1885precedence rules), <tt>yacc.py</tt> will report no errors or conflicts in the grammar. 1886 1887<p> 1888One problem with the precedence specifier technique is that it is sometimes necessary to 1889change the precedence of an operator in certain contents. For example, consider a unary-minus operator 1890in "3 + 4 * -5". Normally, unary minus has a very high precedence--being evaluated before the multiply. --- 9 unchanged lines hidden (view full) --- 1900) 1901</pre> 1902</blockquote> 1903 1904Now, in the grammar file, we can write our unary minus rule like this: 1905 1906<blockquote> 1907<pre> |
| 886def p_expr_uminus(t): | 1908def p_expr_uminus(p): |
| 887 'expression : MINUS expression %prec UMINUS' | 1909 'expression : MINUS expression %prec UMINUS' |
| 888 t[0] = -t[2] | 1910 p[0] = -p[2] |
| 889</pre> 890</blockquote> 891 892In this case, <tt>%prec UMINUS</tt> overrides the default rule precedence--setting it to that 893of UMINUS in the precedence specifier. 894 895<p> | 1911</pre> 1912</blockquote> 1913 1914In this case, <tt>%prec UMINUS</tt> overrides the default rule precedence--setting it to that 1915of UMINUS in the precedence specifier. 1916 1917<p> |
| 1918At first, the use of UMINUS in this example may appear very confusing. 1919UMINUS is not an input token or a grammer rule. Instead, you should 1920think of it as the name of a special marker in the precedence table. When you use the <tt>%prec</tt> qualifier, you're simply 1921telling yacc that you want the precedence of the expression to be the same as for this special marker instead of the usual precedence. 1922 1923<p> |
|
| 896It is also possible to specify non-associativity in the <tt>precedence</tt> table. This would 897be used when you <em>don't</em> want operations to chain together. For example, suppose | 1924It is also possible to specify non-associativity in the <tt>precedence</tt> table. This would 1925be used when you <em>don't</em> want operations to chain together. For example, suppose |
| 898you wanted to support a comparison operators like <tt><</tt> and <tt>></tt> but you didn't want to allow | 1926you wanted to support comparison operators like < and > but you didn't want to allow |
| 899combinations like <tt>a < b < c</tt>. To do this, simply specify a rule like this: 900 901<blockquote> 902<pre> 903precedence = ( 904 ('nonassoc', 'LESSTHAN', 'GREATERTHAN'), # Nonassociative operators 905 ('left', 'PLUS', 'MINUS'), 906 ('left', 'TIMES', 'DIVIDE'), 907 ('right', 'UMINUS'), # Unary minus operator 908) 909</pre> 910</blockquote> 911 912<p> | 1927combinations like <tt>a < b < c</tt>. To do this, simply specify a rule like this: 1928 1929<blockquote> 1930<pre> 1931precedence = ( 1932 ('nonassoc', 'LESSTHAN', 'GREATERTHAN'), # Nonassociative operators 1933 ('left', 'PLUS', 'MINUS'), 1934 ('left', 'TIMES', 'DIVIDE'), 1935 ('right', 'UMINUS'), # Unary minus operator 1936) 1937</pre> 1938</blockquote> 1939 1940<p> |
| 1941If you do this, the occurrence of input text such as <tt> a < b < c</tt> will result in a syntax error. However, simple 1942expressions such as <tt>a < b</tt> will still be fine. 1943 1944<p> |
|
| 913Reduce/reduce conflicts are caused when there are multiple grammar 914rules that can be applied to a given set of symbols. This kind of 915conflict is almost always bad and is always resolved by picking the 916rule that appears first in the grammar file. Reduce/reduce conflicts 917are almost always caused when different sets of grammar rules somehow 918generate the same set of symbols. For example: 919 920<blockquote> --- 15 unchanged lines hidden (view full) --- 936<blockquote> 937<pre> 938assignment : ID EQUALS NUMBER 939expression : NUMBER 940</pre> 941</blockquote> 942 943For example, if you wrote "a = 5", the parser can't figure out if this | 1945Reduce/reduce conflicts are caused when there are multiple grammar 1946rules that can be applied to a given set of symbols. This kind of 1947conflict is almost always bad and is always resolved by picking the 1948rule that appears first in the grammar file. Reduce/reduce conflicts 1949are almost always caused when different sets of grammar rules somehow 1950generate the same set of symbols. For example: 1951 1952<blockquote> --- 15 unchanged lines hidden (view full) --- 1968<blockquote> 1969<pre> 1970assignment : ID EQUALS NUMBER 1971expression : NUMBER 1972</pre> 1973</blockquote> 1974 1975For example, if you wrote "a = 5", the parser can't figure out if this |
| 944is supposed to reduced as assignment : ID EQUALS NUMBER or | 1976is supposed to be reduced as <tt>assignment : ID EQUALS NUMBER</tt> or |
| 945whether it's supposed to reduce the 5 as an expression and then reduce 946the rule <tt>assignment : ID EQUALS expression</tt>. 947 | 1977whether it's supposed to reduce the 5 as an expression and then reduce 1978the rule <tt>assignment : ID EQUALS expression</tt>. 1979 |
| 948<h2>The parser.out file</h2> | 1980<p> 1981It should be noted that reduce/reduce conflicts are notoriously difficult to spot 1982simply looking at the input grammer. To locate these, it is usually easier to look at the 1983<tt>parser.out</tt> debugging file with an appropriately high level of caffeination. |
| 949 | 1984 |
| 1985<H3><a name="ply_nn28"></a>5.7 The parser.out file</H3> 1986 1987 |
|
| 950Tracking down shift/reduce and reduce/reduce conflicts is one of the finer pleasures of using an LR 951parsing algorithm. To assist in debugging, <tt>yacc.py</tt> creates a debugging file called 952'parser.out' when it generates the parsing table. The contents of this file look like the following: 953 954<blockquote> 955<pre> 956Unused terminals: 957 --- 18 unchanged lines hidden (view full) --- 976PLUS : 1 977NUMBER : 5 978 979Nonterminals, with rules where they appear 980 981expression : 1 1 2 2 3 3 4 4 6 0 982 983 | 1988Tracking down shift/reduce and reduce/reduce conflicts is one of the finer pleasures of using an LR 1989parsing algorithm. To assist in debugging, <tt>yacc.py</tt> creates a debugging file called 1990'parser.out' when it generates the parsing table. The contents of this file look like the following: 1991 1992<blockquote> 1993<pre> 1994Unused terminals: 1995 --- 18 unchanged lines hidden (view full) --- 2014PLUS : 1 2015NUMBER : 5 2016 2017Nonterminals, with rules where they appear 2018 2019expression : 1 1 2 2 3 3 4 4 6 0 2020 2021 |
| 984Parsing method: SLR | 2022Parsing method: LALR |
| 985 986 987state 0 988 989 S' -> . expression 990 expression -> . expression PLUS expression 991 expression -> . expression MINUS expression 992 expression -> . expression TIMES expression --- 222 unchanged lines hidden (view full) --- 1215 ! MINUS [ shift and go to state 5 ] 1216</pre> 1217</blockquote> 1218 1219By looking at these rules (and with a little practice), you can usually track down the source 1220of most parsing conflicts. It should also be stressed that not all shift-reduce conflicts are 1221bad. However, the only way to be sure that they are resolved correctly is to look at <tt>parser.out</tt>. 1222 | 2023 2024 2025state 0 2026 2027 S' -> . expression 2028 expression -> . expression PLUS expression 2029 expression -> . expression MINUS expression 2030 expression -> . expression TIMES expression --- 222 unchanged lines hidden (view full) --- 2253 ! MINUS [ shift and go to state 5 ] 2254</pre> 2255</blockquote> 2256 2257By looking at these rules (and with a little practice), you can usually track down the source 2258of most parsing conflicts. It should also be stressed that not all shift-reduce conflicts are 2259bad. However, the only way to be sure that they are resolved correctly is to look at <tt>parser.out</tt>. 2260 |
| 1223<h2>Syntax Error Handling</h2> | 2261<H3><a name="ply_nn29"></a>5.8 Syntax Error Handling</H3> |
| 1224 | 2262 |
| 2263 |
|
| 1225When a syntax error occurs during parsing, the error is immediately 1226detected (i.e., the parser does not read any more tokens beyond the 1227source of the error). Error recovery in LR parsers is a delicate 1228topic that involves ancient rituals and black-magic. The recovery mechanism 1229provided by <tt>yacc.py</tt> is comparable to Unix yacc so you may want 1230consult a book like O'Reilly's "Lex and Yacc" for some of the finer details. 1231 1232<p> --- 21 unchanged lines hidden (view full) --- 1254<li>If a grammar rule accepts <tt>error</tt> as a token, it will be 1255shifted onto the parsing stack. 1256 1257<p> 1258<li>If the top item of the parsing stack is <tt>error</tt>, lookahead tokens will be discarded until the 1259parser can successfully shift a new symbol or reduce a rule involving <tt>error</tt>. 1260</ol> 1261 | 2264When a syntax error occurs during parsing, the error is immediately 2265detected (i.e., the parser does not read any more tokens beyond the 2266source of the error). Error recovery in LR parsers is a delicate 2267topic that involves ancient rituals and black-magic. The recovery mechanism 2268provided by <tt>yacc.py</tt> is comparable to Unix yacc so you may want 2269consult a book like O'Reilly's "Lex and Yacc" for some of the finer details. 2270 2271<p> --- 21 unchanged lines hidden (view full) --- 2293<li>If a grammar rule accepts <tt>error</tt> as a token, it will be 2294shifted onto the parsing stack. 2295 2296<p> 2297<li>If the top item of the parsing stack is <tt>error</tt>, lookahead tokens will be discarded until the 2298parser can successfully shift a new symbol or reduce a rule involving <tt>error</tt>. 2299</ol> 2300 |
| 1262<h4>Recovery and resynchronization with error rules</h4> | 2301<H4><a name="ply_nn30"></a>5.8.1 Recovery and resynchronization with error rules</H4> |
| 1263 | 2302 |
| 2303 |
|
| 1264The most well-behaved approach for handling syntax errors is to write grammar rules that include the <tt>error</tt> 1265token. For example, suppose your language had a grammar rule for a print statement like this: 1266 1267<blockquote> 1268<pre> | 2304The most well-behaved approach for handling syntax errors is to write grammar rules that include the <tt>error</tt> 2305token. For example, suppose your language had a grammar rule for a print statement like this: 2306 2307<blockquote> 2308<pre> |
| 1269def p_statement_print(t): | 2309def p_statement_print(p): |
| 1270 'statement : PRINT expr SEMI' 1271 ... 1272</pre> 1273</blockquote> 1274 1275To account for the possibility of a bad expression, you might write an additional grammar rule like this: 1276 1277<blockquote> 1278<pre> | 2310 'statement : PRINT expr SEMI' 2311 ... 2312</pre> 2313</blockquote> 2314 2315To account for the possibility of a bad expression, you might write an additional grammar rule like this: 2316 2317<blockquote> 2318<pre> |
| 1279def p_statement_print_error(t): | 2319def p_statement_print_error(p): |
| 1280 'statement : PRINT error SEMI' 1281 print "Syntax error in print statement. Bad expression" 1282 1283</pre> 1284</blockquote> 1285 1286In this case, the <tt>error</tt> token will match any sequence of 1287tokens that might appear up to the first semicolon that is --- 7 unchanged lines hidden (view full) --- 1295synchronization token. 1296 1297<p> 1298It is important to note that the <tt>error</tt> token usually does not appear as the last token 1299on the right in an error rule. For example: 1300 1301<blockquote> 1302<pre> | 2320 'statement : PRINT error SEMI' 2321 print "Syntax error in print statement. Bad expression" 2322 2323</pre> 2324</blockquote> 2325 2326In this case, the <tt>error</tt> token will match any sequence of 2327tokens that might appear up to the first semicolon that is --- 7 unchanged lines hidden (view full) --- 2335synchronization token. 2336 2337<p> 2338It is important to note that the <tt>error</tt> token usually does not appear as the last token 2339on the right in an error rule. For example: 2340 2341<blockquote> 2342<pre> |
| 1303def p_statement_print_error(t): | 2343def p_statement_print_error(p): |
| 1304 'statement : PRINT error' 1305 print "Syntax error in print statement. Bad expression" 1306</pre> 1307</blockquote> 1308 1309This is because the first bad token encountered will cause the rule to 1310be reduced--which may make it difficult to recover if more bad tokens 1311immediately follow. 1312 | 2344 'statement : PRINT error' 2345 print "Syntax error in print statement. Bad expression" 2346</pre> 2347</blockquote> 2348 2349This is because the first bad token encountered will cause the rule to 2350be reduced--which may make it difficult to recover if more bad tokens 2351immediately follow. 2352 |
| 1313<h4>Panic mode recovery</h4> | 2353<H4><a name="ply_nn31"></a>5.8.2 Panic mode recovery</H4> |
| 1314 | 2354 |
| 2355 |
|
| 1315An alternative error recovery scheme is to enter a panic mode recovery in which tokens are 1316discarded to a point where the parser might be able to recover in some sensible manner. 1317 1318<p> 1319Panic mode recovery is implemented entirely in the <tt>p_error()</tt> function. For example, this 1320function starts discarding tokens until it reaches a closing '}'. Then, it restarts the 1321parser in its initial state. 1322 1323<blockquote> 1324<pre> | 2356An alternative error recovery scheme is to enter a panic mode recovery in which tokens are 2357discarded to a point where the parser might be able to recover in some sensible manner. 2358 2359<p> 2360Panic mode recovery is implemented entirely in the <tt>p_error()</tt> function. For example, this 2361function starts discarding tokens until it reaches a closing '}'. Then, it restarts the 2362parser in its initial state. 2363 2364<blockquote> 2365<pre> |
| 1325def p_error(t): | 2366def p_error(p): |
| 1326 print "Whoa. You are seriously hosed." 1327 # Read ahead looking for a closing '}' 1328 while 1: 1329 tok = yacc.token() # Get the next token 1330 if not tok or tok.type == 'RBRACE': break 1331 yacc.restart() 1332</pre> 1333</blockquote> 1334 1335<p> 1336This function simply discards the bad token and tells the parser that the error was ok. 1337 1338<blockquote> 1339<pre> | 2367 print "Whoa. You are seriously hosed." 2368 # Read ahead looking for a closing '}' 2369 while 1: 2370 tok = yacc.token() # Get the next token 2371 if not tok or tok.type == 'RBRACE': break 2372 yacc.restart() 2373</pre> 2374</blockquote> 2375 2376<p> 2377This function simply discards the bad token and tells the parser that the error was ok. 2378 2379<blockquote> 2380<pre> |
| 1340def p_error(t): 1341 print "Syntax error at token", t.type | 2381def p_error(p): 2382 print "Syntax error at token", p.type |
| 1342 # Just discard the token and tell the parser it's okay. 1343 yacc.errok() 1344</pre> 1345</blockquote> 1346 1347<P> 1348Within the <tt>p_error()</tt> function, three functions are available to control the behavior 1349of the parser: --- 15 unchanged lines hidden (view full) --- 1365at any other time. 1366 1367<p> 1368To supply the next lookahead token to the parser, <tt>p_error()</tt> can return a token. This might be 1369useful if trying to synchronize on special characters. For example: 1370 1371<blockquote> 1372<pre> | 2383 # Just discard the token and tell the parser it's okay. 2384 yacc.errok() 2385</pre> 2386</blockquote> 2387 2388<P> 2389Within the <tt>p_error()</tt> function, three functions are available to control the behavior 2390of the parser: --- 15 unchanged lines hidden (view full) --- 2406at any other time. 2407 2408<p> 2409To supply the next lookahead token to the parser, <tt>p_error()</tt> can return a token. This might be 2410useful if trying to synchronize on special characters. For example: 2411 2412<blockquote> 2413<pre> |
| 1373def p_error(t): | 2414def p_error(p): |
| 1374 # Read ahead looking for a terminating ";" 1375 while 1: 1376 tok = yacc.token() # Get the next token 1377 if not tok or tok.type == 'SEMI': break 1378 yacc.errok() 1379 1380 # Return SEMI to the parser as the next lookahead token 1381 return tok 1382</pre> 1383</blockquote> 1384 | 2415 # Read ahead looking for a terminating ";" 2416 while 1: 2417 tok = yacc.token() # Get the next token 2418 if not tok or tok.type == 'SEMI': break 2419 yacc.errok() 2420 2421 # Return SEMI to the parser as the next lookahead token 2422 return tok 2423</pre> 2424</blockquote> 2425 |
| 1385<h4>General comments on error handling</h4> | 2426<H4><a name="ply_nn32"></a>5.8.3 General comments on error handling</H4> |
| 1386 | 2427 |
| 2428 |
|
| 1387For normal types of languages, error recovery with error rules and resynchronization characters is probably the most reliable 1388technique. This is because you can instrument the grammar to catch errors at selected places where it is relatively easy 1389to recover and continue parsing. Panic mode recovery is really only useful in certain specialized applications where you might want 1390to discard huge portions of the input text to find a valid restart point. 1391 | 2429For normal types of languages, error recovery with error rules and resynchronization characters is probably the most reliable 2430technique. This is because you can instrument the grammar to catch errors at selected places where it is relatively easy 2431to recover and continue parsing. Panic mode recovery is really only useful in certain specialized applications where you might want 2432to discard huge portions of the input text to find a valid restart point. 2433 |
| 1392<h2>Line Number Tracking</h2> | 2434<H3><a name="ply_nn33"></a>5.9 Line Number and Position Tracking</H3> |
| 1393 | 2435 |
| 1394<tt>yacc.py</tt> automatically tracks line numbers for all of the grammar symbols and tokens it processes. To retrieve the line 1395numbers, two functions are used in grammar rules: | 2436Position tracking is often a tricky problem when writing compilers. By default, PLY tracks the line number and position of 2437all tokens. This information is available using the following functions: |
| 1396 1397<ul> | 2438 2439<ul> |
| 1398<li><tt>t.lineno(num)</tt>. Return the starting line number for symbol <em>num</em> 1399<li><tt>t.linespan(num)</tt>. Return a tuple (startline,endline) with the starting and ending line number for symbol <em>num</em>. | 2440<li><tt>p.lineno(num)</tt>. Return the line number for symbol <em>num</em> 2441<li><tt>p.lexpos(num)</tt>. Return the lexing position for symbol <em>num</em> |
| 1400</ul> 1401 1402For example: 1403 1404<blockquote> 1405<pre> | 2442</ul> 2443 2444For example: 2445 2446<blockquote> 2447<pre> |
| 1406def t_expression(t): | 2448def p_expression(p): |
| 1407 'expression : expression PLUS expression' | 2449 'expression : expression PLUS expression' |
| 1408 t.lineno(1) # Line number of the left expression 1409 t.lineno(2) # line number of the PLUS operator 1410 t.lineno(3) # line number of the right expression | 2450 line = p.lineno(2) # line number of the PLUS token 2451 index = p.lexpos(2) # Position of the PLUS token 2452</pre> 2453</blockquote> 2454 2455As an optional feature, <tt>yacc.py</tt> can automatically track line numbers and positions for all of the grammar symbols 2456as well. However, this 2457extra tracking requires extra processing and can significantly slow down parsing. Therefore, it must be enabled by passing the 2458<tt>tracking=True</tt> option to <tt>yacc.parse()</tt>. For example: 2459 2460<blockquote> 2461<pre> 2462yacc.parse(data,tracking=True) 2463</pre> 2464</blockquote> 2465 2466Once enabled, the <tt>lineno()</tt> and <tt>lexpos()</tt> methods work for all grammar symbols. In addition, two 2467additional methods can be used: 2468 2469<ul> 2470<li><tt>p.linespan(num)</tt>. Return a tuple (startline,endline) with the starting and ending line number for symbol <em>num</em>. 2471<li><tt>p.lexspan(num)</tt>. Return a tuple (start,end) with the starting and ending positions for symbol <em>num</em>. 2472</ul> 2473 2474For example: 2475 2476<blockquote> 2477<pre> 2478def p_expression(p): 2479 'expression : expression PLUS expression' 2480 p.lineno(1) # Line number of the left expression 2481 p.lineno(2) # line number of the PLUS operator 2482 p.lineno(3) # line number of the right expression |
| 1411 ... | 2483 ... |
| 1412 start,end = t.linespan(3) # Start,end lines of the right expression | 2484 start,end = p.linespan(3) # Start,end lines of the right expression 2485 starti,endi = p.lexspan(3) # Start,end positions of right expression |
| 1413 1414</pre> 1415</blockquote> 1416 | 2486 2487</pre> 2488</blockquote> 2489 |
| 1417Since line numbers are managed internally by the parser, there is usually no need to modify the line 1418numbers. However, if you want to save the line numbers in a parse-tree node, you will need to make your own 1419private copy. | 2490Note: The <tt>lexspan()</tt> function only returns the range of values up to the start of the last grammar symbol. |
| 1420 | 2491 |
| 1421<h2>AST Construction</h2> | 2492<p> 2493Although it may be convenient for PLY to track position information on 2494all grammar symbols, this is often unnecessary. For example, if you 2495are merely using line number information in an error message, you can 2496often just key off of a specific token in the grammar rule. For 2497example: |
| 1422 | 2498 |
| 2499<blockquote> 2500<pre> 2501def p_bad_func(p): 2502 'funccall : fname LPAREN error RPAREN' 2503 # Line number reported from LPAREN token 2504 print "Bad function call at line", p.lineno(2) 2505</pre> 2506</blockquote> 2507 2508<p> 2509Similarly, you may get better parsing performance if you only propagate line number 2510information where it's needed. For example: 2511 2512<blockquote> 2513<pre> 2514def p_fname(p): 2515 'fname : ID' 2516 p[0] = (p[1],p.lineno(1)) 2517</pre> 2518</blockquote> 2519 2520Finally, it should be noted that PLY does not store position information after a rule has been 2521processed. If it is important for you to retain this information in an abstract syntax tree, you 2522must make your own copy. 2523 2524<H3><a name="ply_nn34"></a>5.10 AST Construction</H3> 2525 2526 |
|
| 1423<tt>yacc.py</tt> provides no special functions for constructing an abstract syntax tree. However, such 1424construction is easy enough to do on your own. Simply create a data structure for abstract syntax tree nodes | 2527<tt>yacc.py</tt> provides no special functions for constructing an abstract syntax tree. However, such 2528construction is easy enough to do on your own. Simply create a data structure for abstract syntax tree nodes |
| 1425and assign nodes to <tt>t[0]</tt> in each rule. | 2529and assign nodes to <tt>p[0]</tt> in each rule. |
| 1426 1427For example: 1428 1429<blockquote> 1430<pre> 1431class Expr: pass 1432 1433class BinOp(Expr): 1434 def __init__(self,left,op,right): 1435 self.type = "binop" 1436 self.left = left 1437 self.right = right 1438 self.op = op 1439 1440class Number(Expr): 1441 def __init__(self,value): 1442 self.type = "number" 1443 self.value = value 1444 | 2530 2531For example: 2532 2533<blockquote> 2534<pre> 2535class Expr: pass 2536 2537class BinOp(Expr): 2538 def __init__(self,left,op,right): 2539 self.type = "binop" 2540 self.left = left 2541 self.right = right 2542 self.op = op 2543 2544class Number(Expr): 2545 def __init__(self,value): 2546 self.type = "number" 2547 self.value = value 2548 |
| 1445def p_expression_binop(t): | 2549def p_expression_binop(p): |
| 1446 '''expression : expression PLUS expression 1447 | expression MINUS expression 1448 | expression TIMES expression 1449 | expression DIVIDE expression''' 1450 | 2550 '''expression : expression PLUS expression 2551 | expression MINUS expression 2552 | expression TIMES expression 2553 | expression DIVIDE expression''' 2554 |
| 1451 t[0] = BinOp(t[1],t[2],t[3]) | 2555 p[0] = BinOp(p[1],p[2],p[3]) |
| 1452 | 2556 |
| 1453def p_expression_group(t): | 2557def p_expression_group(p): |
| 1454 'expression : LPAREN expression RPAREN' | 2558 'expression : LPAREN expression RPAREN' |
| 1455 t[0] = t[2] | 2559 p[0] = p[2] |
| 1456 | 2560 |
| 1457def p_expression_number(t): | 2561def p_expression_number(p): |
| 1458 'expression : NUMBER' | 2562 'expression : NUMBER' |
| 1459 t[0] = Number(t[1]) | 2563 p[0] = Number(p[1]) |
| 1460</pre> 1461</blockquote> 1462 1463To simplify tree traversal, it may make sense to pick a very generic tree structure for your parse tree nodes. 1464For example: 1465 1466<blockquote> 1467<pre> 1468class Node: 1469 def __init__(self,type,children=None,leaf=None): 1470 self.type = type 1471 if children: 1472 self.children = children 1473 else: 1474 self.children = [ ] 1475 self.leaf = leaf 1476 | 2564</pre> 2565</blockquote> 2566 2567To simplify tree traversal, it may make sense to pick a very generic tree structure for your parse tree nodes. 2568For example: 2569 2570<blockquote> 2571<pre> 2572class Node: 2573 def __init__(self,type,children=None,leaf=None): 2574 self.type = type 2575 if children: 2576 self.children = children 2577 else: 2578 self.children = [ ] 2579 self.leaf = leaf 2580 |
| 1477def p_expression_binop(t): | 2581def p_expression_binop(p): |
| 1478 '''expression : expression PLUS expression 1479 | expression MINUS expression 1480 | expression TIMES expression 1481 | expression DIVIDE expression''' 1482 | 2582 '''expression : expression PLUS expression 2583 | expression MINUS expression 2584 | expression TIMES expression 2585 | expression DIVIDE expression''' 2586 |
| 1483 t[0] = Node("binop", [t[1],t[3]], t[2]) | 2587 p[0] = Node("binop", [p[1],p[3]], p[2]) |
| 1484</pre> 1485</blockquote> 1486 | 2588</pre> 2589</blockquote> 2590 |
| 1487<h2>Yacc implementation notes</h2> | 2591<H3><a name="ply_nn35"></a>5.11 Embedded Actions</H3> |
| 1488 | 2592 |
| 2593 2594The parsing technique used by yacc only allows actions to be executed at the end of a rule. For example, 2595suppose you have a rule like this: 2596 2597<blockquote> 2598<pre> 2599def p_foo(p): 2600 "foo : A B C D" 2601 print "Parsed a foo", p[1],p[2],p[3],p[4] 2602</pre> 2603</blockquote> 2604 2605<p> 2606In this case, the supplied action code only executes after all of the 2607symbols <tt>A</tt>, <tt>B</tt>, <tt>C</tt>, and <tt>D</tt> have been 2608parsed. Sometimes, however, it is useful to execute small code 2609fragments during intermediate stages of parsing. For example, suppose 2610you wanted to perform some action immediately after <tt>A</tt> has 2611been parsed. To do this, you can write a empty rule like this: 2612 2613<blockquote> 2614<pre> 2615def p_foo(p): 2616 "foo : A seen_A B C D" 2617 print "Parsed a foo", p[1],p[3],p[4],p[5] 2618 print "seen_A returned", p[2] 2619 2620def p_seen_A(p): 2621 "seen_A :" 2622 print "Saw an A = ", p[-1] # Access grammar symbol to left 2623 p[0] = some_value # Assign value to seen_A 2624 2625</pre> 2626</blockquote> 2627 2628<p> 2629In this example, the empty <tt>seen_A</tt> rule executes immediately 2630after <tt>A</tt> is shifted onto the parsing stack. Within this 2631rule, <tt>p[-1]</tt> refers to the symbol on the stack that appears 2632immediately to the left of the <tt>seen_A</tt> symbol. In this case, 2633it would be the value of <tt>A</tt> in the <tt>foo</tt> rule 2634immediately above. Like other rules, a value can be returned from an 2635embedded action by simply assigning it to <tt>p[0]</tt> 2636 2637<p> 2638The use of embedded actions can sometimes introduce extra shift/reduce conflicts. For example, 2639this grammar has no conflicts: 2640 2641<blockquote> 2642<pre> 2643def p_foo(p): 2644 """foo : abcd 2645 | abcx""" 2646 2647def p_abcd(p): 2648 "abcd : A B C D" 2649 2650def p_abcx(p): 2651 "abcx : A B C X" 2652</pre> 2653</blockquote> 2654 2655However, if you insert an embedded action into one of the rules like this, 2656 2657<blockquote> 2658<pre> 2659def p_foo(p): 2660 """foo : abcd 2661 | abcx""" 2662 2663def p_abcd(p): 2664 "abcd : A B C D" 2665 2666def p_abcx(p): 2667 "abcx : A B seen_AB C X" 2668 2669def p_seen_AB(p): 2670 "seen_AB :" 2671</pre> 2672</blockquote> 2673 2674an extra shift-reduce conflict will be introduced. This conflict is caused by the fact that the same symbol <tt>C</tt> appears next in 2675both the <tt>abcd</tt> and <tt>abcx</tt> rules. The parser can either shift the symbol (<tt>abcd</tt> rule) or reduce the empty rule <tt>seen_AB</tt> (<tt>abcx</tt> rule). 2676 2677<p> 2678A common use of embedded rules is to control other aspects of parsing 2679such as scoping of local variables. For example, if you were parsing C code, you might 2680write code like this: 2681 2682<blockquote> 2683<pre> 2684def p_statements_block(p): 2685 "statements: LBRACE new_scope statements RBRACE""" 2686 # Action code 2687 ... 2688 pop_scope() # Return to previous scope 2689 2690def p_new_scope(p): 2691 "new_scope :" 2692 # Create a new scope for local variables 2693 s = new_scope() 2694 push_scope(s) 2695 ... 2696</pre> 2697</blockquote> 2698 2699In this case, the embedded action <tt>new_scope</tt> executes immediately after a <tt>LBRACE</tt> (<tt>{</tt>) symbol is parsed. This might 2700adjust internal symbol tables and other aspects of the parser. Upon completion of the rule <tt>statements_block</tt>, code might undo the operations performed in the embedded action (e.g., <tt>pop_scope()</tt>). 2701 2702<H3><a name="ply_nn36"></a>5.12 Yacc implementation notes</H3> 2703 2704 |
|
| 1489<ul> | 2705<ul> |
| 2706<li>The default parsing method is LALR. To use SLR instead, run yacc() as follows: 2707 2708<blockquote> 2709<pre> 2710yacc.yacc(method="SLR") 2711</pre> 2712</blockquote> 2713Note: LALR table generation takes approximately twice as long as SLR table generation. There is no 2714difference in actual parsing performance---the same code is used in both cases. LALR is preferred when working 2715with more complicated grammars since it is more powerful. 2716 2717<p> 2718 |
|
| 1490<li>By default, <tt>yacc.py</tt> relies on <tt>lex.py</tt> for tokenizing. However, an alternative tokenizer 1491can be supplied as follows: 1492 1493<blockquote> 1494<pre> 1495yacc.parse(lexer=x) 1496</pre> 1497</blockquote> --- 14 unchanged lines hidden (view full) --- 1512<li>To change the name of the <tt>parsetab.py</tt> file, use: 1513 1514<blockquote> 1515<pre> 1516yacc.yacc(tabmodule="foo") 1517</pre> 1518</blockquote> 1519 | 2719<li>By default, <tt>yacc.py</tt> relies on <tt>lex.py</tt> for tokenizing. However, an alternative tokenizer 2720can be supplied as follows: 2721 2722<blockquote> 2723<pre> 2724yacc.parse(lexer=x) 2725</pre> 2726</blockquote> --- 14 unchanged lines hidden (view full) --- 2741<li>To change the name of the <tt>parsetab.py</tt> file, use: 2742 2743<blockquote> 2744<pre> 2745yacc.yacc(tabmodule="foo") 2746</pre> 2747</blockquote> 2748 |
| 2749<p> 2750<li>To change the directory in which the <tt>parsetab.py</tt> file (and other output files) are written, use: 2751<blockquote> 2752<pre> 2753yacc.yacc(tabmodule="foo",outputdir="somedirectory") 2754</pre> 2755</blockquote> 2756 2757<p> 2758<li>To prevent yacc from generating any kind of parser table file, use: 2759<blockquote> 2760<pre> 2761yacc.yacc(write_tables=0) 2762</pre> 2763</blockquote> 2764 2765Note: If you disable table generation, yacc() will regenerate the parsing tables 2766each time it runs (which may take awhile depending on how large your grammar is). 2767 |
|
| 1520<P> 1521<li>To print copious amounts of debugging during parsing, use: 1522 1523<blockquote> 1524<pre> 1525yacc.parse(debug=1) 1526</pre> 1527</blockquote> 1528 1529<p> | 2768<P> 2769<li>To print copious amounts of debugging during parsing, use: 2770 2771<blockquote> 2772<pre> 2773yacc.parse(debug=1) 2774</pre> 2775</blockquote> 2776 2777<p> |
| 2778<li>To redirect the debugging output to a filename of your choosing, use: 2779 2780<blockquote> 2781<pre> 2782yacc.parse(debug=1, debugfile="debugging.out") 2783</pre> 2784</blockquote> 2785 2786<p> |
|
| 1530<li>The <tt>yacc.yacc()</tt> function really returns a parser object. If you want to support multiple 1531parsers in the same application, do this: 1532 1533<blockquote> 1534<pre> 1535p = yacc.yacc() 1536... 1537p.parse() 1538</pre> 1539</blockquote> 1540 1541Note: The function <tt>yacc.parse()</tt> is bound to the last parser that was generated. 1542 1543<p> | 2787<li>The <tt>yacc.yacc()</tt> function really returns a parser object. If you want to support multiple 2788parsers in the same application, do this: 2789 2790<blockquote> 2791<pre> 2792p = yacc.yacc() 2793... 2794p.parse() 2795</pre> 2796</blockquote> 2797 2798Note: The function <tt>yacc.parse()</tt> is bound to the last parser that was generated. 2799 2800<p> |
| 1544<li>Since the generation of the SLR tables is relatively expensive, previously generated tables are | 2801<li>Since the generation of the LALR tables is relatively expensive, previously generated tables are |
| 1545cached and reused if possible. The decision to regenerate the tables is determined by taking an MD5 1546checksum of all grammar rules and precedence rules. Only in the event of a mismatch are the tables regenerated. 1547 1548<p> 1549It should be noted that table generation is reasonably efficient, even for grammars that involve around a 100 rules 1550and several hundred states. For more complex languages such as C, table generation may take 30-60 seconds on a slow 1551machine. Please be patient. 1552 1553<p> | 2802cached and reused if possible. The decision to regenerate the tables is determined by taking an MD5 2803checksum of all grammar rules and precedence rules. Only in the event of a mismatch are the tables regenerated. 2804 2805<p> 2806It should be noted that table generation is reasonably efficient, even for grammars that involve around a 100 rules 2807and several hundred states. For more complex languages such as C, table generation may take 30-60 seconds on a slow 2808machine. Please be patient. 2809 2810<p> |
| 1554<li>Since LR parsing is mostly driven by tables, the performance of the parser is largely independent of the 1555size of the grammar. The biggest bottlenecks will be the lexer and the complexity of your grammar rules. | 2811 2812size of the grammar. The biggest bottlenecks will be the lexer and the complexity of the code in your grammar rules. |
| 1556</ul> 1557 | 2813</ul> 2814 |
| 1558<h2>Parser and Lexer State Management</h2> | 2815<H2><a name="ply_nn37"></a>6. Parser and Lexer State Management</H2> |
| 1559 | 2816 |
| 2817 |
|
| 1560In advanced parsing applications, you may want to have multiple 1561parsers and lexers. Furthermore, the parser may want to control the 1562behavior of the lexer in some way. 1563 1564<p> 1565To do this, it is important to note that both the lexer and parser are 1566actually implemented as objects. These objects are returned by the 1567<tt>lex()</tt> and <tt>yacc()</tt> functions respectively. For example: 1568 1569<blockquote> 1570<pre> 1571lexer = lex.lex() # Return lexer object 1572parser = yacc.yacc() # Return parser object 1573</pre> 1574</blockquote> 1575 | 2818In advanced parsing applications, you may want to have multiple 2819parsers and lexers. Furthermore, the parser may want to control the 2820behavior of the lexer in some way. 2821 2822<p> 2823To do this, it is important to note that both the lexer and parser are 2824actually implemented as objects. These objects are returned by the 2825<tt>lex()</tt> and <tt>yacc()</tt> functions respectively. For example: 2826 2827<blockquote> 2828<pre> 2829lexer = lex.lex() # Return lexer object 2830parser = yacc.yacc() # Return parser object 2831</pre> 2832</blockquote> 2833 |
| 2834To attach the lexer and parser together, make sure you use the <tt>lexer</tt> argumemnt to parse. For example: 2835 2836<blockquote> 2837<pre> 2838parser.parse(text,lexer=lexer) 2839</pre> 2840</blockquote> 2841 |
|
| 1576Within lexer and parser rules, these objects are also available. In the lexer, 1577the "lexer" attribute of a token refers to the lexer object in use. For example: 1578 1579<blockquote> 1580<pre> 1581def t_NUMBER(t): 1582 r'\d+' 1583 ... 1584 print t.lexer # Show lexer object 1585</pre> 1586</blockquote> 1587 1588In the parser, the "lexer" and "parser" attributes refer to the lexer 1589and parser objects respectively. 1590 1591<blockquote> 1592<pre> | 2842Within lexer and parser rules, these objects are also available. In the lexer, 2843the "lexer" attribute of a token refers to the lexer object in use. For example: 2844 2845<blockquote> 2846<pre> 2847def t_NUMBER(t): 2848 r'\d+' 2849 ... 2850 print t.lexer # Show lexer object 2851</pre> 2852</blockquote> 2853 2854In the parser, the "lexer" and "parser" attributes refer to the lexer 2855and parser objects respectively. 2856 2857<blockquote> 2858<pre> |
| 1593def p_expr_plus(t): | 2859def p_expr_plus(p): |
| 1594 'expr : expr PLUS expr' 1595 ... | 2860 'expr : expr PLUS expr' 2861 ... |
| 1596 print t.parser # Show parser object 1597 print t.lexer # Show lexer object | 2862 print p.parser # Show parser object 2863 print p.lexer # Show lexer object |
| 1598</pre> 1599</blockquote> 1600 1601If necessary, arbitrary attributes can be attached to the lexer or parser object. 1602For example, if you wanted to have different parsing modes, you could attach a mode 1603attribute to the parser object and look at it later. 1604 | 2864</pre> 2865</blockquote> 2866 2867If necessary, arbitrary attributes can be attached to the lexer or parser object. 2868For example, if you wanted to have different parsing modes, you could attach a mode 2869attribute to the parser object and look at it later. 2870 |
| 1605<h2>Using Python's Optimized Mode</h2> | 2871<H2><a name="ply_nn38"></a>7. Using Python's Optimized Mode</H2> |
| 1606 | 2872 |
| 2873 |
|
| 1607Because PLY uses information from doc-strings, parsing and lexing 1608information must be gathered while running the Python interpreter in 1609normal mode (i.e., not with the -O or -OO options). However, if you 1610specify optimized mode like this: 1611 1612<blockquote> 1613<pre> 1614lex.lex(optimize=1) --- 6 unchanged lines hidden (view full) --- 1621have been generated the first time, run Python in optimized mode. PLY will use 1622the tables without the need for doc strings. 1623 1624<p> 1625Beware: running PLY in optimized mode disables a lot of error 1626checking. You should only do this when your project has stabilized 1627and you don't need to do any debugging. 1628 | 2874Because PLY uses information from doc-strings, parsing and lexing 2875information must be gathered while running the Python interpreter in 2876normal mode (i.e., not with the -O or -OO options). However, if you 2877specify optimized mode like this: 2878 2879<blockquote> 2880<pre> 2881lex.lex(optimize=1) --- 6 unchanged lines hidden (view full) --- 2888have been generated the first time, run Python in optimized mode. PLY will use 2889the tables without the need for doc strings. 2890 2891<p> 2892Beware: running PLY in optimized mode disables a lot of error 2893checking. You should only do this when your project has stabilized 2894and you don't need to do any debugging. 2895 |
| 1629<h2>Where to go from here?</h2> | 2896<H2><a name="ply_nn39"></a>8. Where to go from here?</H2> |
| 1630 | 2897 |
| 2898 |
|
| 1631The <tt>examples</tt> directory of the PLY distribution contains several simple examples. Please consult a 1632compilers textbook for the theory and underlying implementation details or LR parsing. 1633 1634</body> 1635</html> 1636 1637 1638 1639 1640 1641 1642 | 2899The <tt>examples</tt> directory of the PLY distribution contains several simple examples. Please consult a 2900compilers textbook for the theory and underlying implementation details or LR parsing. 2901 2902</body> 2903</html> 2904 2905 2906 2907 2908 2909 2910 |