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RFC - Add documentation for strings and Unicode issues (#636)
* Add documentation for strings and Unicode issues * More Unicode documentation on character literals and wide characters
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jbarlow83
Wenzel Jakob
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0558a9a739
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40db2c757a
@ -33,6 +33,7 @@ the last case of the above list.
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:maxdepth: 1
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overview
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strings
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stl
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functional
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chrono
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243
docs/advanced/cast/strings.rst
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243
docs/advanced/cast/strings.rst
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@ -0,0 +1,243 @@
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Strings, bytes and Unicode conversions
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######################################
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.. note::
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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``.
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Passing Python strings to C++
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=============================
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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.
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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/>`_.
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.. code-block:: c++
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m.def("utf8_test",
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[](const std::string &s) {
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cout << "utf-8 is icing on the cake.\n";
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cout << s;
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}
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);
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m.def("utf8_charptr",
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[](const char *s) {
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cout << "My favorite food is\n";
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cout << s;
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}
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);
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.. code-block:: python
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>>> utf8_test('🎂')
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utf-8 is icing on the cake.
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🎂
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>>> utf8_charptr('🍕')
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My favorite food is
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🍕
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.. note::
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Some terminal emulators do not support UTF-8 or emoji fonts and may not display the example above correctly.
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The results are the same whether the C++ function accepts arguments by value or reference, and whether or not ``const`` is used.
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Passing bytes to C++
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--------------------
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A Python ``bytes`` object will be passed to C++ functions that accept ``std::string`` or ``char*`` *without* conversion.
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Returning C++ strings to Python
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===============================
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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``.
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.. code-block:: c++
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m.def("std_string_return",
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[]() {
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return std::string("This string needs to be UTF-8 encoded");
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}
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);
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.. code-block:: python
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>>> isinstance(example.std_string_return(), str)
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True
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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.
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.. warning::
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Implicit conversion assumes that a returned ``char *`` is null-terminated. If there is no null terminator a buffer overrun will occur.
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Explicit conversions
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--------------------
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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.
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.. code-block:: c++
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// This uses the Python C API to convert Latin-1 to Unicode
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m.def("str_output",
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[]() {
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std::string s = "Send your r\xe9sum\xe9 to Alice in HR"; // Latin-1
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py::str py_s = PyUnicode_DecodeLatin1(s.data(), s.length());
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return py_s;
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}
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);
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.. code-block:: python
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>>> str_output()
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'Send your résumé to Alice in HR'
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The `Python C API <https://docs.python.org/3/c-api/unicode.html#built-in-codecs>`_ provides several built-in codecs.
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One could also use a third party encoding library such as libiconv to transcode to UTF-8.
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Return C++ strings without conversion
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-------------------------------------
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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.
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.. code-block:: c++
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m.def("return_bytes",
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[]() {
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std::string s("\xba\xd0\xba\xd0"); // Not valid UTF-8
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return py::bytes(s); // Return the data without transcoding
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}
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);
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.. code-block:: python
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>>> example.return_bytes()
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b'\xba\xd0\xba\xd0'
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Note the asymmetry: pybind11 will convert ``bytes`` to ``std::string`` without encoding, but cannot convert ``std::string`` back to ``bytes`` implicitly.
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.. code-block:: c++
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m.def("asymmetry",
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[](std::string s) { // Accepts str or bytes from Python
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return s; // Looks harmless, but implicitly converts to str
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}
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);
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.. code-block:: python
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>>> isinstance(example.asymmetry(b"have some bytes"), str)
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True
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>>> example.asymmetry(b"\xba\xd0\xba\xd0") # invalid utf-8 as bytes
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UnicodeDecodeError: 'utf-8' codec can't decode byte 0xba in position 0: invalid start byte
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Wide character strings
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======================
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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``.
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.. code-block:: c++
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#define UNICODE
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#include <windows.h>
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m.def("set_window_text",
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[](HWND hwnd, std::wstring s) {
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// Call SetWindowText with null-terminated UTF-16 string
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::SetWindowText(hwnd, s.c_str());
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}
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);
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m.def("get_window_text",
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[](HWND hwnd) {
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const int buffer_size = ::GetWindowTextLength(hwnd) + 1;
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auto buffer = std::make_unique< wchar_t[] >(buffer_size);
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::GetWindowText(hwnd, buffer.data(), buffer_size);
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std::wstring text(buffer.get());
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// wstring will be converted to Python str
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return text;
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}
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);
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.. warning::
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Wide character strings may not work as described on Python 2.7 or Python 3.3 compiled with ``--enable-unicode=ucs2``.
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Strings in multibyte encodings such as Shift-JIS must transcoded to a UTF-8/16/32 before being returned to Python.
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Character literals
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==================
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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.
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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.
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.. code-block:: c++
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m.def("pass_char", [](char c) { return c; });
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m.def("pass_wchar", [](wchar_t w) { return w; });
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.. code-block:: python
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>>> example.pass_char('A')
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'A'
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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.
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.. code-block:: python
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>>> example.pass_char(0x65)
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TypeError
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>>> example.pass_char(chr(0x65))
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'A'
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If the desire is to work with an 8-bit integer, use ``int8_t`` or ``uint8_t`` as the argument type.
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Grapheme clusters
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-----------------
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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.
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.. code-block:: python
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>>> example.pass_wchar('é')
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'é'
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>>> combining_e_acute = 'e' + '\u0301'
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>>> combining_e_acute
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'é'
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>>> combining_e_acute == 'é'
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False
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>>> example.pass_wchar(combining_e_acute)
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'e'
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Normalizing combining characters before passing the character literal to C++ may resolve *some* of these issues:
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.. code-block:: python
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>>> example.pass_wchar(unicodedata.normalize('NFC', combining_e_acute))
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'é'
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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.
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References
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==========
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* `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/>`_
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* `C++ - Using STL Strings at Win32 API Boundaries <https://msdn.microsoft.com/en-ca/magazine/mt238407.aspx>`_
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