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diff --git a/ext/pybind11/docs/advanced/cast/strings.rst b/ext/pybind11/docs/advanced/cast/strings.rst new file mode 100644 index 000000000..c70fb0bec --- /dev/null +++ b/ext/pybind11/docs/advanced/cast/strings.rst @@ -0,0 +1,243 @@ +Strings, bytes and Unicode conversions +###################################### + +.. note:: + + 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``. + +Passing Python strings to C++ +============================= + +When 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. + +The 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/>`_. + +.. code-block:: c++ + + m.def("utf8_test", + [](const std::string &s) { + cout << "utf-8 is icing on the cake.\n"; + cout << s; + } + ); + m.def("utf8_charptr", + [](const char *s) { + cout << "My favorite food is\n"; + cout << s; + } + ); + +.. code-block:: python + + >>> utf8_test('🎂') + utf-8 is icing on the cake. + 🎂 + + >>> utf8_charptr('🍕') + My favorite food is + 🍕 + +.. note:: + + Some terminal emulators do not support UTF-8 or emoji fonts and may not display the example above correctly. + +The results are the same whether the C++ function accepts arguments by value or reference, and whether or not ``const`` is used. + +Passing bytes to C++ +-------------------- + +A Python ``bytes`` object will be passed to C++ functions that accept ``std::string`` or ``char*`` *without* conversion. + + +Returning C++ strings to Python +=============================== + +When 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``. + +.. code-block:: c++ + + m.def("std_string_return", + []() { + return std::string("This string needs to be UTF-8 encoded"); + } + ); + +.. code-block:: python + + >>> isinstance(example.std_string_return(), str) + True + + +Because 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. + +.. warning:: + + Implicit conversion assumes that a returned ``char *`` is null-terminated. If there is no null terminator a buffer overrun will occur. + +Explicit conversions +-------------------- + +If 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. + +.. code-block:: c++ + + // This uses the Python C API to convert Latin-1 to Unicode + m.def("str_output", + []() { + std::string s = "Send your r\xe9sum\xe9 to Alice in HR"; // Latin-1 + py::str py_s = PyUnicode_DecodeLatin1(s.data(), s.length()); + return py_s; + } + ); + +.. code-block:: python + + >>> str_output() + 'Send your résumé to Alice in HR' + +The `Python C API <https://docs.python.org/3/c-api/unicode.html#built-in-codecs>`_ provides several built-in codecs. + + +One could also use a third party encoding library such as libiconv to transcode to UTF-8. + +Return C++ strings without conversion +------------------------------------- + +If 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. + +.. code-block:: c++ + + m.def("return_bytes", + []() { + std::string s("\xba\xd0\xba\xd0"); // Not valid UTF-8 + return py::bytes(s); // Return the data without transcoding + } + ); + +.. code-block:: python + + >>> example.return_bytes() + b'\xba\xd0\xba\xd0' + + +Note the asymmetry: pybind11 will convert ``bytes`` to ``std::string`` without encoding, but cannot convert ``std::string`` back to ``bytes`` implicitly. + +.. code-block:: c++ + + m.def("asymmetry", + [](std::string s) { // Accepts str or bytes from Python + return s; // Looks harmless, but implicitly converts to str + } + ); + +.. code-block:: python + + >>> isinstance(example.asymmetry(b"have some bytes"), str) + True + + >>> example.asymmetry(b"\xba\xd0\xba\xd0") # invalid utf-8 as bytes + UnicodeDecodeError: 'utf-8' codec can't decode byte 0xba in position 0: invalid start byte + + +Wide character strings +====================== + +When 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``. + +.. code-block:: c++ + + #define UNICODE + #include <windows.h> + + m.def("set_window_text", + [](HWND hwnd, std::wstring s) { + // Call SetWindowText with null-terminated UTF-16 string + ::SetWindowText(hwnd, s.c_str()); + } + ); + m.def("get_window_text", + [](HWND hwnd) { + const int buffer_size = ::GetWindowTextLength(hwnd) + 1; + auto buffer = std::make_unique< wchar_t[] >(buffer_size); + + ::GetWindowText(hwnd, buffer.data(), buffer_size); + + std::wstring text(buffer.get()); + + // wstring will be converted to Python str + return text; + } + ); + +.. warning:: + + Wide character strings may not work as described on Python 2.7 or Python 3.3 compiled with ``--enable-unicode=ucs2``. + +Strings in multibyte encodings such as Shift-JIS must transcoded to a UTF-8/16/32 before being returned to Python. + + +Character literals +================== + +C++ 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. + +When 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. + +.. code-block:: c++ + + m.def("pass_char", [](char c) { return c; }); + m.def("pass_wchar", [](wchar_t w) { return w; }); + +.. code-block:: python + + >>> example.pass_char('A') + 'A' + +While 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. + +.. code-block:: python + + >>> example.pass_char(0x65) + TypeError + + >>> example.pass_char(chr(0x65)) + 'A' + +If the desire is to work with an 8-bit integer, use ``int8_t`` or ``uint8_t`` as the argument type. + +Grapheme clusters +----------------- + +A 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. + +.. code-block:: python + + >>> example.pass_wchar('é') + 'é' + + >>> combining_e_acute = 'e' + '\u0301' + + >>> combining_e_acute + 'é' + + >>> combining_e_acute == 'é' + False + + >>> example.pass_wchar(combining_e_acute) + 'e' + +Normalizing combining characters before passing the character literal to C++ may resolve *some* of these issues: + +.. code-block:: python + + >>> example.pass_wchar(unicodedata.normalize('NFC', combining_e_acute)) + 'é' + +In 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. + + +References +========== + +* `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/>`_ +* `C++ - Using STL Strings at Win32 API Boundaries <https://msdn.microsoft.com/en-ca/magazine/mt238407.aspx>`_
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