strings.rst revision 12037
112037Sandreas.sandberg@arm.comStrings, bytes and Unicode conversions 212037Sandreas.sandberg@arm.com###################################### 312037Sandreas.sandberg@arm.com 412037Sandreas.sandberg@arm.com.. note:: 512037Sandreas.sandberg@arm.com 612037Sandreas.sandberg@arm.com This section discusses string handling in terms of Python 3 strings. For Python 2.7, replace all occurrences of ``str`` with ``unicode`` and ``bytes`` with ``str``. Python 2.7 users may find it best to use ``from __future__ import unicode_literals`` to avoid unintentionally using ``str`` instead of ``unicode``. 712037Sandreas.sandberg@arm.com 812037Sandreas.sandberg@arm.comPassing Python strings to C++ 912037Sandreas.sandberg@arm.com============================= 1012037Sandreas.sandberg@arm.com 1112037Sandreas.sandberg@arm.comWhen a Python ``str`` is passed from Python to a C++ function that accepts ``std::string`` or ``char *`` as arguments, pybind11 will encode the Python string to UTF-8. All Python ``str`` can be encoded in UTF-8, so this operation does not fail. 1212037Sandreas.sandberg@arm.com 1312037Sandreas.sandberg@arm.comThe C++ language is encoding agnostic. It is the responsibility of the programmer to track encodings. It's often easiest to simply `use UTF-8 everywhere <http://utf8everywhere.org/>`_. 1412037Sandreas.sandberg@arm.com 1512037Sandreas.sandberg@arm.com.. code-block:: c++ 1612037Sandreas.sandberg@arm.com 1712037Sandreas.sandberg@arm.com m.def("utf8_test", 1812037Sandreas.sandberg@arm.com [](const std::string &s) { 1912037Sandreas.sandberg@arm.com cout << "utf-8 is icing on the cake.\n"; 2012037Sandreas.sandberg@arm.com cout << s; 2112037Sandreas.sandberg@arm.com } 2212037Sandreas.sandberg@arm.com ); 2312037Sandreas.sandberg@arm.com m.def("utf8_charptr", 2412037Sandreas.sandberg@arm.com [](const char *s) { 2512037Sandreas.sandberg@arm.com cout << "My favorite food is\n"; 2612037Sandreas.sandberg@arm.com cout << s; 2712037Sandreas.sandberg@arm.com } 2812037Sandreas.sandberg@arm.com ); 2912037Sandreas.sandberg@arm.com 3012037Sandreas.sandberg@arm.com.. code-block:: python 3112037Sandreas.sandberg@arm.com 3212037Sandreas.sandberg@arm.com >>> utf8_test('') 3312037Sandreas.sandberg@arm.com utf-8 is icing on the cake. 3412037Sandreas.sandberg@arm.com 3512037Sandreas.sandberg@arm.com 3612037Sandreas.sandberg@arm.com >>> utf8_charptr('') 3712037Sandreas.sandberg@arm.com My favorite food is 3812037Sandreas.sandberg@arm.com 3912037Sandreas.sandberg@arm.com 4012037Sandreas.sandberg@arm.com.. note:: 4112037Sandreas.sandberg@arm.com 4212037Sandreas.sandberg@arm.com Some terminal emulators do not support UTF-8 or emoji fonts and may not display the example above correctly. 4312037Sandreas.sandberg@arm.com 4412037Sandreas.sandberg@arm.comThe results are the same whether the C++ function accepts arguments by value or reference, and whether or not ``const`` is used. 4512037Sandreas.sandberg@arm.com 4612037Sandreas.sandberg@arm.comPassing bytes to C++ 4712037Sandreas.sandberg@arm.com-------------------- 4812037Sandreas.sandberg@arm.com 4912037Sandreas.sandberg@arm.comA Python ``bytes`` object will be passed to C++ functions that accept ``std::string`` or ``char*`` *without* conversion. 5012037Sandreas.sandberg@arm.com 5112037Sandreas.sandberg@arm.com 5212037Sandreas.sandberg@arm.comReturning C++ strings to Python 5312037Sandreas.sandberg@arm.com=============================== 5412037Sandreas.sandberg@arm.com 5512037Sandreas.sandberg@arm.comWhen a C++ function returns a ``std::string`` or ``char*`` to a Python caller, **pybind11 will assume that the string is valid UTF-8** and will decode it to a native Python ``str``, using the same API as Python uses to perform ``bytes.decode('utf-8')``. If this implicit conversion fails, pybind11 will raise a ``UnicodeDecodeError``. 5612037Sandreas.sandberg@arm.com 5712037Sandreas.sandberg@arm.com.. code-block:: c++ 5812037Sandreas.sandberg@arm.com 5912037Sandreas.sandberg@arm.com m.def("std_string_return", 6012037Sandreas.sandberg@arm.com []() { 6112037Sandreas.sandberg@arm.com return std::string("This string needs to be UTF-8 encoded"); 6212037Sandreas.sandberg@arm.com } 6312037Sandreas.sandberg@arm.com ); 6412037Sandreas.sandberg@arm.com 6512037Sandreas.sandberg@arm.com.. code-block:: python 6612037Sandreas.sandberg@arm.com 6712037Sandreas.sandberg@arm.com >>> isinstance(example.std_string_return(), str) 6812037Sandreas.sandberg@arm.com True 6912037Sandreas.sandberg@arm.com 7012037Sandreas.sandberg@arm.com 7112037Sandreas.sandberg@arm.comBecause UTF-8 is inclusive of pure ASCII, there is never any issue with returning a pure ASCII string to Python. If there is any possibility that the string is not pure ASCII, it is necessary to ensure the encoding is valid UTF-8. 7212037Sandreas.sandberg@arm.com 7312037Sandreas.sandberg@arm.com.. warning:: 7412037Sandreas.sandberg@arm.com 7512037Sandreas.sandberg@arm.com Implicit conversion assumes that a returned ``char *`` is null-terminated. If there is no null terminator a buffer overrun will occur. 7612037Sandreas.sandberg@arm.com 7712037Sandreas.sandberg@arm.comExplicit conversions 7812037Sandreas.sandberg@arm.com-------------------- 7912037Sandreas.sandberg@arm.com 8012037Sandreas.sandberg@arm.comIf some C++ code constructs a ``std::string`` that is not a UTF-8 string, one can perform a explicit conversion and return a ``py::str`` object. Explicit conversion has the same overhead as implicit conversion. 8112037Sandreas.sandberg@arm.com 8212037Sandreas.sandberg@arm.com.. code-block:: c++ 8312037Sandreas.sandberg@arm.com 8412037Sandreas.sandberg@arm.com // This uses the Python C API to convert Latin-1 to Unicode 8512037Sandreas.sandberg@arm.com m.def("str_output", 8612037Sandreas.sandberg@arm.com []() { 8712037Sandreas.sandberg@arm.com std::string s = "Send your r\xe9sum\xe9 to Alice in HR"; // Latin-1 8812037Sandreas.sandberg@arm.com py::str py_s = PyUnicode_DecodeLatin1(s.data(), s.length()); 8912037Sandreas.sandberg@arm.com return py_s; 9012037Sandreas.sandberg@arm.com } 9112037Sandreas.sandberg@arm.com ); 9212037Sandreas.sandberg@arm.com 9312037Sandreas.sandberg@arm.com.. code-block:: python 9412037Sandreas.sandberg@arm.com 9512037Sandreas.sandberg@arm.com >>> str_output() 9612037Sandreas.sandberg@arm.com 'Send your résumé to Alice in HR' 9712037Sandreas.sandberg@arm.com 9812037Sandreas.sandberg@arm.comThe `Python C API <https://docs.python.org/3/c-api/unicode.html#built-in-codecs>`_ provides several built-in codecs. 9912037Sandreas.sandberg@arm.com 10012037Sandreas.sandberg@arm.com 10112037Sandreas.sandberg@arm.comOne could also use a third party encoding library such as libiconv to transcode to UTF-8. 10212037Sandreas.sandberg@arm.com 10312037Sandreas.sandberg@arm.comReturn C++ strings without conversion 10412037Sandreas.sandberg@arm.com------------------------------------- 10512037Sandreas.sandberg@arm.com 10612037Sandreas.sandberg@arm.comIf the data in a C++ ``std::string`` does not represent text and should be returned to Python as ``bytes``, then one can return the data as a ``py::bytes`` object. 10712037Sandreas.sandberg@arm.com 10812037Sandreas.sandberg@arm.com.. code-block:: c++ 10912037Sandreas.sandberg@arm.com 11012037Sandreas.sandberg@arm.com m.def("return_bytes", 11112037Sandreas.sandberg@arm.com []() { 11212037Sandreas.sandberg@arm.com std::string s("\xba\xd0\xba\xd0"); // Not valid UTF-8 11312037Sandreas.sandberg@arm.com return py::bytes(s); // Return the data without transcoding 11412037Sandreas.sandberg@arm.com } 11512037Sandreas.sandberg@arm.com ); 11612037Sandreas.sandberg@arm.com 11712037Sandreas.sandberg@arm.com.. code-block:: python 11812037Sandreas.sandberg@arm.com 11912037Sandreas.sandberg@arm.com >>> example.return_bytes() 12012037Sandreas.sandberg@arm.com b'\xba\xd0\xba\xd0' 12112037Sandreas.sandberg@arm.com 12212037Sandreas.sandberg@arm.com 12312037Sandreas.sandberg@arm.comNote the asymmetry: pybind11 will convert ``bytes`` to ``std::string`` without encoding, but cannot convert ``std::string`` back to ``bytes`` implicitly. 12412037Sandreas.sandberg@arm.com 12512037Sandreas.sandberg@arm.com.. code-block:: c++ 12612037Sandreas.sandberg@arm.com 12712037Sandreas.sandberg@arm.com m.def("asymmetry", 12812037Sandreas.sandberg@arm.com [](std::string s) { // Accepts str or bytes from Python 12912037Sandreas.sandberg@arm.com return s; // Looks harmless, but implicitly converts to str 13012037Sandreas.sandberg@arm.com } 13112037Sandreas.sandberg@arm.com ); 13212037Sandreas.sandberg@arm.com 13312037Sandreas.sandberg@arm.com.. code-block:: python 13412037Sandreas.sandberg@arm.com 13512037Sandreas.sandberg@arm.com >>> isinstance(example.asymmetry(b"have some bytes"), str) 13612037Sandreas.sandberg@arm.com True 13712037Sandreas.sandberg@arm.com 13812037Sandreas.sandberg@arm.com >>> example.asymmetry(b"\xba\xd0\xba\xd0") # invalid utf-8 as bytes 13912037Sandreas.sandberg@arm.com UnicodeDecodeError: 'utf-8' codec can't decode byte 0xba in position 0: invalid start byte 14012037Sandreas.sandberg@arm.com 14112037Sandreas.sandberg@arm.com 14212037Sandreas.sandberg@arm.comWide character strings 14312037Sandreas.sandberg@arm.com====================== 14412037Sandreas.sandberg@arm.com 14512037Sandreas.sandberg@arm.comWhen a Python ``str`` is passed to a C++ function expecting ``std::wstring``, ``wchar_t*``, ``std::u16string`` or ``std::u32string``, the ``str`` will be encoded to UTF-16 or UTF-32 depending on how the C++ compiler implements each type, in the platform's endian. When strings of these types are returned, they are assumed to contain valid UTF-16 or UTF-32, and will be decoded to Python ``str``. 14612037Sandreas.sandberg@arm.com 14712037Sandreas.sandberg@arm.com.. code-block:: c++ 14812037Sandreas.sandberg@arm.com 14912037Sandreas.sandberg@arm.com #define UNICODE 15012037Sandreas.sandberg@arm.com #include <windows.h> 15112037Sandreas.sandberg@arm.com 15212037Sandreas.sandberg@arm.com m.def("set_window_text", 15312037Sandreas.sandberg@arm.com [](HWND hwnd, std::wstring s) { 15412037Sandreas.sandberg@arm.com // Call SetWindowText with null-terminated UTF-16 string 15512037Sandreas.sandberg@arm.com ::SetWindowText(hwnd, s.c_str()); 15612037Sandreas.sandberg@arm.com } 15712037Sandreas.sandberg@arm.com ); 15812037Sandreas.sandberg@arm.com m.def("get_window_text", 15912037Sandreas.sandberg@arm.com [](HWND hwnd) { 16012037Sandreas.sandberg@arm.com const int buffer_size = ::GetWindowTextLength(hwnd) + 1; 16112037Sandreas.sandberg@arm.com auto buffer = std::make_unique< wchar_t[] >(buffer_size); 16212037Sandreas.sandberg@arm.com 16312037Sandreas.sandberg@arm.com ::GetWindowText(hwnd, buffer.data(), buffer_size); 16412037Sandreas.sandberg@arm.com 16512037Sandreas.sandberg@arm.com std::wstring text(buffer.get()); 16612037Sandreas.sandberg@arm.com 16712037Sandreas.sandberg@arm.com // wstring will be converted to Python str 16812037Sandreas.sandberg@arm.com return text; 16912037Sandreas.sandberg@arm.com } 17012037Sandreas.sandberg@arm.com ); 17112037Sandreas.sandberg@arm.com 17212037Sandreas.sandberg@arm.com.. warning:: 17312037Sandreas.sandberg@arm.com 17412037Sandreas.sandberg@arm.com Wide character strings may not work as described on Python 2.7 or Python 3.3 compiled with ``--enable-unicode=ucs2``. 17512037Sandreas.sandberg@arm.com 17612037Sandreas.sandberg@arm.comStrings in multibyte encodings such as Shift-JIS must transcoded to a UTF-8/16/32 before being returned to Python. 17712037Sandreas.sandberg@arm.com 17812037Sandreas.sandberg@arm.com 17912037Sandreas.sandberg@arm.comCharacter literals 18012037Sandreas.sandberg@arm.com================== 18112037Sandreas.sandberg@arm.com 18212037Sandreas.sandberg@arm.comC++ functions that accept character literals as input will receive the first character of a Python ``str`` as their input. If the string is longer than one Unicode character, trailing characters will be ignored. 18312037Sandreas.sandberg@arm.com 18412037Sandreas.sandberg@arm.comWhen a character literal is returned from C++ (such as a ``char`` or a ``wchar_t``), it will be converted to a ``str`` that represents the single character. 18512037Sandreas.sandberg@arm.com 18612037Sandreas.sandberg@arm.com.. code-block:: c++ 18712037Sandreas.sandberg@arm.com 18812037Sandreas.sandberg@arm.com m.def("pass_char", [](char c) { return c; }); 18912037Sandreas.sandberg@arm.com m.def("pass_wchar", [](wchar_t w) { return w; }); 19012037Sandreas.sandberg@arm.com 19112037Sandreas.sandberg@arm.com.. code-block:: python 19212037Sandreas.sandberg@arm.com 19312037Sandreas.sandberg@arm.com >>> example.pass_char('A') 19412037Sandreas.sandberg@arm.com 'A' 19512037Sandreas.sandberg@arm.com 19612037Sandreas.sandberg@arm.comWhile C++ will cast integers to character types (``char c = 0x65;``), pybind11 does not convert Python integers to characters implicitly. The Python function ``chr()`` can be used to convert integers to characters. 19712037Sandreas.sandberg@arm.com 19812037Sandreas.sandberg@arm.com.. code-block:: python 19912037Sandreas.sandberg@arm.com 20012037Sandreas.sandberg@arm.com >>> example.pass_char(0x65) 20112037Sandreas.sandberg@arm.com TypeError 20212037Sandreas.sandberg@arm.com 20312037Sandreas.sandberg@arm.com >>> example.pass_char(chr(0x65)) 20412037Sandreas.sandberg@arm.com 'A' 20512037Sandreas.sandberg@arm.com 20612037Sandreas.sandberg@arm.comIf the desire is to work with an 8-bit integer, use ``int8_t`` or ``uint8_t`` as the argument type. 20712037Sandreas.sandberg@arm.com 20812037Sandreas.sandberg@arm.comGrapheme clusters 20912037Sandreas.sandberg@arm.com----------------- 21012037Sandreas.sandberg@arm.com 21112037Sandreas.sandberg@arm.comA single grapheme may be represented by two or more Unicode characters. For example 'é' is usually represented as U+00E9 but can also be expressed as the combining character sequence U+0065 U+0301 (that is, the letter 'e' followed by a combining acute accent). The combining character will be lost if the two-character sequence is passed as an argument, even though it renders as a single grapheme. 21212037Sandreas.sandberg@arm.com 21312037Sandreas.sandberg@arm.com.. code-block:: python 21412037Sandreas.sandberg@arm.com 21512037Sandreas.sandberg@arm.com >>> example.pass_wchar('é') 21612037Sandreas.sandberg@arm.com 'é' 21712037Sandreas.sandberg@arm.com 21812037Sandreas.sandberg@arm.com >>> combining_e_acute = 'e' + '\u0301' 21912037Sandreas.sandberg@arm.com 22012037Sandreas.sandberg@arm.com >>> combining_e_acute 22112037Sandreas.sandberg@arm.com 'é' 22212037Sandreas.sandberg@arm.com 22312037Sandreas.sandberg@arm.com >>> combining_e_acute == 'é' 22412037Sandreas.sandberg@arm.com False 22512037Sandreas.sandberg@arm.com 22612037Sandreas.sandberg@arm.com >>> example.pass_wchar(combining_e_acute) 22712037Sandreas.sandberg@arm.com 'e' 22812037Sandreas.sandberg@arm.com 22912037Sandreas.sandberg@arm.comNormalizing combining characters before passing the character literal to C++ may resolve *some* of these issues: 23012037Sandreas.sandberg@arm.com 23112037Sandreas.sandberg@arm.com.. code-block:: python 23212037Sandreas.sandberg@arm.com 23312037Sandreas.sandberg@arm.com >>> example.pass_wchar(unicodedata.normalize('NFC', combining_e_acute)) 23412037Sandreas.sandberg@arm.com 'é' 23512037Sandreas.sandberg@arm.com 23612037Sandreas.sandberg@arm.comIn some languages (Thai for example), there are `graphemes that cannot be expressed as a single Unicode code point <http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries>`_, so there is no way to capture them in a C++ character type. 23712037Sandreas.sandberg@arm.com 23812037Sandreas.sandberg@arm.com 23912037Sandreas.sandberg@arm.comReferences 24012037Sandreas.sandberg@arm.com========== 24112037Sandreas.sandberg@arm.com 24212037Sandreas.sandberg@arm.com* `The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) <https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/>`_ 24312037Sandreas.sandberg@arm.com* `C++ - Using STL Strings at Win32 API Boundaries <https://msdn.microsoft.com/en-ca/magazine/mt238407.aspx>`_