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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> 8 9<b> 10David M. Beazley <br> 11dave@dabeaz.com<br> 12</b> 13 14<p> 15<b>PLY Version: 2.3</b> 16<p> 17 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 --> 72 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 89<p> 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, 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 97this course. Most of PLY's interface and operation has been influenced by common 98usability problems encountered by students. 99 100<p> 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 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 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 111fully capable lex/yacc implementation written entirely in Python. 112 113<p> 114The rest of this document assumes that you are somewhat familar with 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. 122 123<H2><a name="ply_nn2"></a>2. PLY Overview</H2> 124 125 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 129collection of tokens specified by a collection of regular expression 130rules. <tt>yacc.py</tt> is used to recognize language syntax that has 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. 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 142simple one-pass compilers. 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 148resolution via precedence rules. In fact, everything that is possible in traditional yacc 149should be supported in PLY. 150 151<p> 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. 165 166<H2><a name="ply_nn3"></a>3. Lex</H2> 167 168 169<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: 171 172<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> 216# ------------------------------------------------------------ 217# calclex.py 218# 219# tokenizer for a simple expression evaluator for 220# numbers and +,-,*,/ 221# ------------------------------------------------------------ 222import ply.lex as lex 223 224# List of token names. This is always required 225tokens = ( 226 'NUMBER', 227 'PLUS', 228 'MINUS', 229 'TIMES', 230 'DIVIDE', 231 'LPAREN', 232 'RPAREN', 233) 234 235# Regular expression rules for simple tokens 236t_PLUS = r'\+' 237t_MINUS = r'-' 238t_TIMES = r'\*' 239t_DIVIDE = r'/' 240t_LPAREN = r'\(' 241t_RPAREN = r'\)' 242 243# A regular expression rule with some action code 244def t_NUMBER(t): 245 r'\d+' 246 try: 247 t.value = int(t.value) 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+' 256 t.lexer.lineno += len(t.value) 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] 264 t.lexer.skip(1) 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> 275 276# 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 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 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, 375a token rule can be specified as a function. For example, this rule matches numbers and 376converts the string into a Python integer. 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 391When a function is used, the regular expression rule is specified in the function documentation string. 392The function always takes a single argument which is an instance of 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. 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> 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. 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> 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> 553 554 555<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> 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 585rest of the input string that has not been tokenized. In the example, the error function 586was defined as follows: 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> 600 601 602<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 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: 616 617<blockquote> 618<pre> 619lexer = lex.lex() 620lexer.input(sometext) 621while 1: 622 tok = lexer.token() 623 if not tok: break 624 print tok 625</pre> 626</blockquote> 627 628<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. 631 632<p> 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. 635 636<H3><a name="ply_nn14"></a>3.11 The @TOKEN decorator</H3> 637 638 639In some applications, you may want to define build tokens from as a series of 640more complex regular expression rules. For example: 641 642<blockquote> 643<pre> 644digit = r'([0-9])' 645nondigit = r'([_A-Za-z])' 646identifier = r'(' + nondigit + r'(' + digit + r'|' + nondigit + r')*)' 647 648def t_ID(t): 649 # want docstring to be identifier above. ????? 650 ... 651</pre> 652</blockquote> 653 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: 657 658<blockquote> 659<pre> 660from ply.lex import TOKEN 661 662@TOKEN(identifier) 663def t_ID(t): 664 ... 665</pre> 666</blockquote> 667 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: 670 671<blockquote> 672<pre> 673def t_ID(t): 674 ... 675 676t_ID.__doc__ = identifier 677</pre> 678</blockquote> 679 680<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. 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. 707 708<p> 709To change the name of the lexer-generated file, use the <tt>lextab</tt> keyword argument. For example: 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. 751 752<p> 753For example, you might have a dedicated module that just contains 754the token rules: 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 933lexer = lex.lex() 934lexer.num_count = 0 # Set the initial count 935</pre> 936</blockquote> 937 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. 946 947<p> 948A third approach is to define the lexer as a class as shown in the previous example: 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> 983 984<p> 985If necessary, a lexer object can be quickly duplicated by invoking its <tt>clone()</tt> method. For example: 986 987<blockquote> 988<pre> 989lexer = lex.lex() 990... 991newlexer = lexer.clone() 992</pre> 993</blockquote> 994 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. 1002 1003<p> 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. 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 1013<blockquote> 1014<pre> 1015m = MyLexer() 1016a = lex.lex(object=m) # Create a lexer 1017 1018b = a.clone() # Clone the lexer 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 1065<p> 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> 1073 1074<p> 1075<tt>lexer.lineno</tt> 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> 1113<pre> 1114states = ( 1115 ('foo','exclusive'), 1116 ('bar','inclusive'), 1117) 1118</pre> 1119</blockquote> 1120 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. 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> 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 1319blazingly fast when used on very large input files. If 1320performance is concern, you might consider upgrading to the most 1321recent version of Python, creating a hand-written lexer, or offloading 1322the lexer into a C extension module. 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. 1332</ul> 1333 1334<H2><a name="ply_nn22"></a>4. Parsing basics</H2> 1335 1336 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 1339mentioned. First, <em>syntax</em> is usually specified in terms of a BNF grammar. 1340For example, if you wanted to parse 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 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 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> 1372<pre> 1373Grammar Action 1374-------------------------------- -------------------------------------------- 1375expression0 : expression1 + term expression0.val = expression1.val + term.val 1376 | expression1 - term expression0.val = expression1.val - term.val 1377 | term expression0.val = term.val 1378 1379term0 : term1 * factor term0.val = term1.val * factor.val 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 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 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 1402<tt>3 + 5 * (10 - 20)</tt> using the grammar defined above: 1403 1404<blockquote> 1405<pre> 1406Step Symbol Stack Input Tokens Action 1407---- --------------------- --------------------- ------------------------------- 14081 $ 3 + 5 * ( 10 - 20 )$ Shift 3 14092 $ 3 + 5 * ( 10 - 20 )$ Reduce factor : NUMBER 14103 $ factor + 5 * ( 10 - 20 )$ Reduce term : factor 14114 $ term + 5 * ( 10 - 20 )$ Reduce expr : term 14125 $ expr + 5 * ( 10 - 20 )$ Shift + 14136 $ expr + 5 * ( 10 - 20 )$ Shift 5 14147 $ expr + 5 * ( 10 - 20 )$ Reduce factor : NUMBER 14158 $ expr + factor * ( 10 - 20 )$ Reduce term : factor 14169 $ expr + term * ( 10 - 20 )$ Shift * 141710 $ expr + term * ( 10 - 20 )$ Shift ( 141811 $ expr + term * ( 10 - 20 )$ Shift 10 141912 $ expr + term * ( 10 - 20 )$ Reduce factor : NUMBER 142013 $ expr + term * ( factor - 20 )$ Reduce term : factor 142114 $ expr + term * ( term - 20 )$ Reduce expr : term 142215 $ expr + term * ( expr - 20 )$ Shift - 142316 $ expr + term * ( expr - 20 )$ Shift 20 142417 $ expr + term * ( expr - 20 )$ Reduce factor : NUMBER 142518 $ expr + term * ( expr - factor )$ Reduce term : factor 142619 $ expr + term * ( expr - term )$ Reduce expr : expr - term 142720 $ expr + term * ( expr )$ Shift ) 142821 $ expr + term * ( expr ) $ Reduce factor : (expr) 142922 $ expr + term * factor $ Reduce term : term * factor 143023 $ expr + term $ Reduce expr : expr + term 143124 $ expr $ Reduce expr 143225 $ $ Success! 1433</pre> 1434</blockquote> 1435 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> 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. 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 1449<H2><a name="ply_nn23"></a>5. Yacc reference</H2> 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 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 1464import ply.yacc as yacc 1465 1466# Get the token map from the lexer. This is required. 1467from calclex import tokens 1468 1469def p_expression_plus(p): 1470 'expression : expression PLUS term' 1471 p[0] = p[1] + p[3] 1472 1473def p_expression_minus(p): 1474 'expression : expression MINUS term' 1475 p[0] = p[1] - p[3] 1476 1477def p_expression_term(p): 1478 'expression : term' 1479 p[0] = p[1] 1480 1481def p_term_times(p): 1482 'term : term TIMES factor' 1483 p[0] = p[1] * p[3] 1484 1485def p_term_div(p): 1486 'term : term DIVIDE factor' 1487 p[0] = p[1] / p[3] 1488 1489def p_term_factor(p): 1490 'term : factor' 1491 p[0] = p[1] 1492 1493def p_factor_num(p): 1494 'factor : NUMBER' 1495 p[0] = p[1] 1496 1497def p_factor_expr(p): 1498 'factor : LPAREN expression RPAREN' 1499 p[0] = p[2] 1500 1501# Error rule for syntax errors 1502def p_error(p): 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 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 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: 1526 1527<blockquote> 1528<pre> 1529def p_expression_plus(p): 1530 'expression : expression PLUS term' 1531 # ^ ^ ^ ^ 1532 # p[0] p[1] p[2] p[3] 1533 1534 p[0] = p[1] + p[3] 1535</pre> 1536</blockquote> 1537 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 1540in the lexer module. For non-terminals, the value is determined by 1541whatever is placed in <tt>p[0]</tt> when rules are reduced. This 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 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 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>. 1560 1561<p>The <tt>p_error(p)</tt> rule is defined to catch syntax errors. See the error handling section 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 1573yacc: Generating LALR parsing table... 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 1585regenerated). Note: The names of parser output files can be changed if necessary. See the notes that follow later. 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 1602<H3><a name="ply_nn25"></a>5.2 Combining Grammar Rule Functions</H3> 1603 1604 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> 1610def p_expression_plus(p): 1611 'expression : expression PLUS term' 1612 p[0] = p[1] + p[3] 1613 1614def p_expression_minus(t): 1615 'expression : expression MINUS term' 1616 p[0] = p[1] - p[3] 1617</pre> 1618</blockquote> 1619 1620Instead of writing two functions, you might write a single function like this: 1621 1622<blockquote> 1623<pre> 1624def p_expression(p): 1625 '''expression : expression PLUS term 1626 | expression MINUS term''' 1627 if p[2] == '+': 1628 p[0] = p[1] + p[3] 1629 elif p[2] == '-': 1630 p[0] = p[1] - p[3] 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> 1639def p_binary_operators(p): 1640 '''expression : expression PLUS term 1641 | expression MINUS term 1642 term : term TIMES factor 1643 | term DIVIDE factor''' 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] 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 1657complicated than necessary. However, it is possible to handle simple cases using len(). For example: 1658 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> 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 1710<tt>yacc.py</tt> can handle empty productions by defining a rule like this: 1711 1712<blockquote> 1713<pre> 1714def p_empty(p): 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> 1724def p_optitem(p): 1725 'optitem : item' 1726 ' | empty' 1727 ... 1728</pre> 1729</blockquote> 1730 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. 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 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. 1783For example, does the expression mean "(3 * 4) + 5" or is it "3 * (4+5)"? 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> 1792<pre> 1793Step Symbol Stack Input Tokens Action 1794---- --------------------- --------------------- ------------------------------- 17951 $ 3 * 4 + 5$ Shift 3 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 1804In this case, when the parser reaches step 6, it has two options. One is to reduce the 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 1814reduced <tt>expr * expr</tt> since multiplication has higher mathematical precedence than addition. 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 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 1834precedence than <tt>PLUS</tt>/<tt>MINUS</tt> (since they appear later in the 1835precedence specification). 1836 1837<p> 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: 1840 1841<blockquote> 1842<pre> 1843PLUS : level = 1, assoc = 'left' 1844MINUS : level = 1, assoc = 'left' 1845TIMES : level = 2, assoc = 'left' 1846DIVIDE : level = 2, assoc = 'left' 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 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 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. 1891However, in our precedence specifier, MINUS has a lower precedence than TIMES. To deal with this, 1892precedence rules can be given for fictitious tokens like this: 1893 1894<blockquote> 1895<pre> 1896precedence = ( 1897 ('left', 'PLUS', 'MINUS'), 1898 ('left', 'TIMES', 'DIVIDE'), 1899 ('right', 'UMINUS'), # Unary minus operator 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> 1908def p_expr_uminus(p): 1909 'expression : MINUS expression %prec UMINUS' 1910 p[0] = -p[2] 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> 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 1926you wanted to support comparison operators like <tt><</tt> and <tt>></tt> but you didn't want to allow 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> 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> 1953<pre> 1954assignment : ID EQUALS NUMBER 1955 | ID EQUALS expression 1956 1957expression : expression PLUS expression 1958 | expression MINUS expression 1959 | expression TIMES expression 1960 | expression DIVIDE expression 1961 | LPAREN expression RPAREN 1962 | NUMBER 1963</pre> 1964</blockquote> 1965 1966In this case, a reduce/reduce conflict exists between these two rules: 1967 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 1976is supposed to be reduced as <tt>assignment : ID EQUALS NUMBER</tt> or 1977whether it's supposed to reduce the 5 as an expression and then reduce 1978the rule <tt>assignment : ID EQUALS expression</tt>. 1979 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. 1984 1985<H3><a name="ply_nn28"></a>5.7 The parser.out file</H3> 1986 1987 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 1996 1997Grammar 1998 1999Rule 1 expression -> expression PLUS expression 2000Rule 2 expression -> expression MINUS expression 2001Rule 3 expression -> expression TIMES expression 2002Rule 4 expression -> expression DIVIDE expression 2003Rule 5 expression -> NUMBER 2004Rule 6 expression -> LPAREN expression RPAREN 2005 2006Terminals, with rules where they appear 2007 2008TIMES : 3 2009error : 2010MINUS : 2 2011RPAREN : 6 2012LPAREN : 6 2013DIVIDE : 4 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 2022Parsing method: LALR 2023 2024 2025state 0 2026 2027 S' -> . expression 2028 expression -> . expression PLUS expression 2029 expression -> . expression MINUS expression 2030 expression -> . expression TIMES expression 2031 expression -> . expression DIVIDE expression 2032 expression -> . NUMBER 2033 expression -> . LPAREN expression RPAREN 2034 2035 NUMBER shift and go to state 3 2036 LPAREN shift and go to state 2 2037 2038 2039state 1 2040 2041 S' -> expression . 2042 expression -> expression . PLUS expression 2043 expression -> expression . MINUS expression 2044 expression -> expression . TIMES expression 2045 expression -> expression . DIVIDE expression 2046 2047 PLUS shift and go to state 6 2048 MINUS shift and go to state 5 2049 TIMES shift and go to state 4 2050 DIVIDE shift and go to state 7 2051 2052 2053state 2 2054 2055 expression -> LPAREN . expression RPAREN 2056 expression -> . expression PLUS expression 2057 expression -> . expression MINUS expression 2058 expression -> . expression TIMES expression 2059 expression -> . expression DIVIDE expression 2060 expression -> . NUMBER 2061 expression -> . LPAREN expression RPAREN 2062 2063 NUMBER shift and go to state 3 2064 LPAREN shift and go to state 2 2065 2066 2067state 3 2068 2069 expression -> NUMBER . 2070 2071 $ reduce using rule 5 2072 PLUS reduce using rule 5 2073 MINUS reduce using rule 5 2074 TIMES reduce using rule 5 2075 DIVIDE reduce using rule 5 2076 RPAREN reduce using rule 5 2077 2078 2079state 4 2080 2081 expression -> expression TIMES . expression 2082 expression -> . expression PLUS expression 2083 expression -> . expression MINUS expression 2084 expression -> . expression TIMES expression 2085 expression -> . expression DIVIDE expression 2086 expression -> . NUMBER 2087 expression -> . LPAREN expression RPAREN 2088 2089 NUMBER shift and go to state 3 2090 LPAREN shift and go to state 2 2091 2092 2093state 5 2094 2095 expression -> expression MINUS . expression 2096 expression -> . expression PLUS expression 2097 expression -> . expression MINUS expression 2098 expression -> . expression TIMES expression 2099 expression -> . expression DIVIDE expression 2100 expression -> . NUMBER 2101 expression -> . LPAREN expression RPAREN 2102 2103 NUMBER shift and go to state 3 2104 LPAREN shift and go to state 2 2105 2106 2107state 6 2108 2109 expression -> expression PLUS . expression 2110 expression -> . expression PLUS expression 2111 expression -> . expression MINUS expression 2112 expression -> . expression TIMES expression 2113 expression -> . expression DIVIDE expression 2114 expression -> . NUMBER 2115 expression -> . LPAREN expression RPAREN 2116 2117 NUMBER shift and go to state 3 2118 LPAREN shift and go to state 2 2119 2120 2121state 7 2122 2123 expression -> expression DIVIDE . expression 2124 expression -> . expression PLUS expression 2125 expression -> . expression MINUS expression 2126 expression -> . expression TIMES expression 2127 expression -> . expression DIVIDE expression 2128 expression -> . NUMBER 2129 expression -> . LPAREN expression RPAREN 2130 2131 NUMBER shift and go to state 3 2132 LPAREN shift and go to state 2 2133 2134 2135state 8 2136 2137 expression -> LPAREN expression . RPAREN 2138 expression -> expression . PLUS expression 2139 expression -> expression . MINUS expression 2140 expression -> expression . TIMES expression 2141 expression -> expression . DIVIDE expression 2142 2143 RPAREN shift and go to state 13 2144 PLUS shift and go to state 6 2145 MINUS shift and go to state 5 2146 TIMES shift and go to state 4 2147 DIVIDE shift and go to state 7 2148 2149 2150state 9 2151 2152 expression -> expression TIMES expression . 2153 expression -> expression . PLUS expression 2154 expression -> expression . MINUS expression 2155 expression -> expression . TIMES expression 2156 expression -> expression . DIVIDE expression 2157 2158 $ reduce using rule 3 2159 PLUS reduce using rule 3 2160 MINUS reduce using rule 3 2161 TIMES reduce using rule 3 2162 DIVIDE reduce using rule 3 2163 RPAREN reduce using rule 3 2164 2165 ! PLUS [ shift and go to state 6 ] 2166 ! MINUS [ shift and go to state 5 ] 2167 ! TIMES [ shift and go to state 4 ] 2168 ! DIVIDE [ shift and go to state 7 ] 2169 2170state 10 2171 2172 expression -> expression MINUS expression . 2173 expression -> expression . PLUS expression 2174 expression -> expression . MINUS expression 2175 expression -> expression . TIMES expression 2176 expression -> expression . DIVIDE expression 2177 2178 $ reduce using rule 2 2179 PLUS reduce using rule 2 2180 MINUS reduce using rule 2 2181 RPAREN reduce using rule 2 2182 TIMES shift and go to state 4 2183 DIVIDE shift and go to state 7 2184 2185 ! TIMES [ reduce using rule 2 ] 2186 ! DIVIDE [ reduce using rule 2 ] 2187 ! PLUS [ shift and go to state 6 ] 2188 ! MINUS [ shift and go to state 5 ] 2189 2190state 11 2191 2192 expression -> expression PLUS expression . 2193 expression -> expression . PLUS expression 2194 expression -> expression . MINUS expression 2195 expression -> expression . TIMES expression 2196 expression -> expression . DIVIDE expression 2197 2198 $ reduce using rule 1 2199 PLUS reduce using rule 1 2200 MINUS reduce using rule 1 2201 RPAREN reduce using rule 1 2202 TIMES shift and go to state 4 2203 DIVIDE shift and go to state 7 2204 2205 ! TIMES [ reduce using rule 1 ] 2206 ! DIVIDE [ reduce using rule 1 ] 2207 ! PLUS [ shift and go to state 6 ] 2208 ! MINUS [ shift and go to state 5 ] 2209 2210state 12 2211 2212 expression -> expression DIVIDE expression . 2213 expression -> expression . PLUS expression 2214 expression -> expression . MINUS expression 2215 expression -> expression . TIMES expression 2216 expression -> expression . DIVIDE expression 2217 2218 $ reduce using rule 4 2219 PLUS reduce using rule 4 2220 MINUS reduce using rule 4 2221 TIMES reduce using rule 4 2222 DIVIDE reduce using rule 4 2223 RPAREN reduce using rule 4 2224 2225 ! PLUS [ shift and go to state 6 ] 2226 ! MINUS [ shift and go to state 5 ] 2227 ! TIMES [ shift and go to state 4 ] 2228 ! DIVIDE [ shift and go to state 7 ] 2229 2230state 13 2231 2232 expression -> LPAREN expression RPAREN . 2233 2234 $ reduce using rule 6 2235 PLUS reduce using rule 6 2236 MINUS reduce using rule 6 2237 TIMES reduce using rule 6 2238 DIVIDE reduce using rule 6 2239 RPAREN reduce using rule 6 2240</pre> 2241</blockquote> 2242 2243In the file, each state of the grammar is described. Within each state the "." indicates the current 2244location of the parse within any applicable grammar rules. In addition, the actions for each valid 2245input token are listed. When a shift/reduce or reduce/reduce conflict arises, rules <em>not</em> selected 2246are prefixed with an !. For example: 2247 2248<blockquote> 2249<pre> 2250 ! TIMES [ reduce using rule 2 ] 2251 ! DIVIDE [ reduce using rule 2 ] 2252 ! PLUS [ shift and go to state 6 ] 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 2261<H3><a name="ply_nn29"></a>5.8 Syntax Error Handling</H3> 2262 2263 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> 2272When a syntax error occurs, <tt>yacc.py</tt> performs the following steps: 2273 2274<ol> 2275<li>On the first occurrence of an error, the user-defined <tt>p_error()</tt> function 2276is called with the offending token as an argument. Afterwards, the parser enters 2277an "error-recovery" mode in which it will not make future calls to <tt>p_error()</tt> until it 2278has successfully shifted at least 3 tokens onto the parsing stack. 2279 2280<p> 2281<li>If no recovery action is taken in <tt>p_error()</tt>, the offending lookahead token is replaced 2282with a special <tt>error</tt> token. 2283 2284<p> 2285<li>If the offending lookahead token is already set to <tt>error</tt>, the top item of the parsing stack is 2286deleted. 2287 2288<p> 2289<li>If the entire parsing stack is unwound, the parser enters a restart state and attempts to start 2290parsing from its initial state. 2291 2292<p> 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 2301<H4><a name="ply_nn30"></a>5.8.1 Recovery and resynchronization with error rules</H4> 2302 2303 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> 2309def p_statement_print(p): 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> 2319def p_statement_print_error(p): 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 2328encountered. Once the semicolon is reached, the rule will be 2329invoked and the <tt>error</tt> token will go away. 2330 2331<p> 2332This type of recovery is sometimes known as parser resynchronization. 2333The <tt>error</tt> token acts as a wildcard for any bad input text and 2334the token immediately following <tt>error</tt> acts as a 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> 2343def p_statement_print_error(p): 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 2353<H4><a name="ply_nn31"></a>5.8.2 Panic mode recovery</H4> 2354 2355 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> 2366def p_error(p): 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> 2381def p_error(p): 2382 print "Syntax error at token", p.type 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: 2391<p> 2392<ul> 2393<li><tt>yacc.errok()</tt>. This resets the parser state so it doesn't think it's in error-recovery 2394mode. This will prevent an <tt>error</tt> token from being generated and will reset the internal 2395error counters so that the next syntax error will call <tt>p_error()</tt> again. 2396 2397<p> 2398<li><tt>yacc.token()</tt>. This returns the next token on the input stream. 2399 2400<p> 2401<li><tt>yacc.restart()</tt>. This discards the entire parsing stack and resets the parser 2402to its initial state. 2403</ul> 2404 2405Note: these functions are only available when invoking <tt>p_error()</tt> and are not available 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> 2414def p_error(p): 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 2426<H4><a name="ply_nn32"></a>5.8.3 General comments on error handling</H4> 2427 2428 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 2434<H3><a name="ply_nn33"></a>5.9 Line Number and Position Tracking</H3> 2435 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: 2438 2439<ul> 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> 2442</ul> 2443 2444For example: 2445 2446<blockquote> 2447<pre> 2448def p_expression(p): 2449 'expression : expression PLUS 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 2483 ... 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 2486 2487</pre> 2488</blockquote> 2489 2490Note: The <tt>lexspan()</tt> function only returns the range of values up to the start of the last grammar symbol. 2491 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: 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 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 2529and assign nodes to <tt>p[0]</tt> in each rule. 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 2549def p_expression_binop(p): 2550 '''expression : expression PLUS expression 2551 | expression MINUS expression 2552 | expression TIMES expression 2553 | expression DIVIDE expression''' 2554 2555 p[0] = BinOp(p[1],p[2],p[3]) 2556 2557def p_expression_group(p): 2558 'expression : LPAREN expression RPAREN' 2559 p[0] = p[2] 2560 2561def p_expression_number(p): 2562 'expression : NUMBER' 2563 p[0] = Number(p[1]) 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 2581def p_expression_binop(p): 2582 '''expression : expression PLUS expression 2583 | expression MINUS expression 2584 | expression TIMES expression 2585 | expression DIVIDE expression''' 2586 2587 p[0] = Node("binop", [p[1],p[3]], p[2]) 2588</pre> 2589</blockquote> 2590 2591<H3><a name="ply_nn35"></a>5.11 Embedded Actions</H3> 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 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 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> 2727in this case, <tt>x</tt> must be a Lexer object that minimally has a <tt>x.token()</tt> method for retrieving the next 2728token. If an input string is given to <tt>yacc.parse()</tt>, the lexer must also have an <tt>x.input()</tt> method. 2729 2730<p> 2731<li>By default, the yacc generates tables in debugging mode (which produces the parser.out file and other output). 2732To disable this, use 2733 2734<blockquote> 2735<pre> 2736yacc.yacc(debug=0) 2737</pre> 2738</blockquote> 2739 2740<p> 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 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> 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> 2801<li>Since the generation of the LALR tables is relatively expensive, previously generated tables are 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> 2811<li>Since LR parsing is driven by tables, the performance of the parser is largely independent of the 2812size of the grammar. The biggest bottlenecks will be the lexer and the complexity of the code in your grammar rules. 2813</ul> 2814 2815<H2><a name="ply_nn37"></a>6. Parser and Lexer State Management</H2> 2816 2817 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 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> 2859def p_expr_plus(p): 2860 'expr : expr PLUS expr' 2861 ... 2862 print p.parser # Show parser object 2863 print p.lexer # Show lexer object 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 2871<H2><a name="ply_nn38"></a>7. Using Python's Optimized Mode</H2> 2872 2873 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) 2882yacc.yacc(optimize=1) 2883</pre> 2884</blockquote> 2885 2886then PLY can later be used when Python runs in optimized mode. To make this work, 2887make sure you first run Python in normal mode. Once the lexing and parsing tables 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 2896<H2><a name="ply_nn39"></a>8. Where to go from here?</H2> 2897 2898 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 2911