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PyUnicode_AsWideChar() doesn't merge surrogate pairs on a system with 32 bits wchar_t and Python compiled in narrow mode (sizeof(wchar_t) == 4 and sizeof(Py_UNICODE) == 2) => see issue #8670.
It is not easy to fix this problem because the callers of PyUnicode_AsWideChar() suppose that the output (wide character) string has the same length (in character) than the input (PyUnicode) string (suppose that sizeof(wchar_t) == sizeof(Py_UNICODE)). And PyUnicode_AsWideChar() doesn't write nul character at the end if the output string is truncated.
To prepare this change, a new PyUnicode_AsWideCharString() function would help because it does compute the size of the output buffer (whereas PyUnicode_AsWideChar() requires the output buffer in an argument).
Attached patch implements it:
-------
/* Convert the Unicode object to a wide character string. The output string
 always ends with a nul character. If size is not NULL, write the number of
 wide characters (including the final nul character) into *size.
 Returns a buffer allocated by PyMem_Alloc() (use PyMem_Free() to free it) on
 success. On error, returns NULL and *size is undefined. */
PyAPI_FUNC(wchar_t*) PyUnicode_AsWideCharString(
 PyUnicodeObject *unicode, /* Unicode object */
 Py_ssize_t *size /* number of characters of the result */
 );
-------

New version of the patch:
 - fix PyUnicode_AsWideCharString() :-)
 - replace PyUnicode_AsWideChar() by PyUnicode_AsWideCharString() in most functions using PyUnicode_AsWideChar()
 - indicate that PyUnicode_AsWideCharString() raises a MemoryError on error
Keep the call to PyUnicode_AsWideChar() in:
 - Modules/getpath.c because getpath.c uses a global limitation of MAXPATHLEN+1 characters
 - WCharArray_set_value() and U_set() of ctypes because the output buffer size is fixed

STINNER Victor wrote:
> 
> New submission from STINNER Victor <victor.stinner@haypocalc.com>:
> 
> PyUnicode_AsWideChar() doesn't merge surrogate pairs on a system with 32 bits wchar_t and Python compiled in narrow mode (sizeof(wchar_t) == 4 and sizeof(Py_UNICODE) == 2) => see issue #8670.
> 
> It is not easy to fix this problem because the callers of PyUnicode_AsWideChar() suppose that the output (wide character) string has the same length (in character) than the input (PyUnicode) string (suppose that sizeof(wchar_t) == sizeof(Py_UNICODE)). And PyUnicode_AsWideChar() doesn't write nul character at the end if the output string is truncated.
> 
> To prepare this change, a new PyUnicode_AsWideCharString() function would help because it does compute the size of the output buffer (whereas PyUnicode_AsWideChar() requires the output buffer in an argument).
Great idea !

+1 from me as well.
But shouldn't PyUnicode_AsWideCharString() merge surrogate pairs when it can? The implementation doesn't do this.

> But shouldn't PyUnicode_AsWideCharString() merge surrogate pairs when it
> can? The implementation doesn't do this.
I don't want to do two different things at the same time. My plan is:
 - create PyUnicode_AsWideCharString()
 - use PyUnicode_AsWideCharString() everywhere
 - patch unicode_aswidechar() (used by PyUnicode_AsWideChar() and 
PyUnicode_AsWideCharString()) to convert surrogates when needed
So, you agree with the API (and the documentation)?

I fixed in this issue in multiple commits:
 - r85093: create PyUnicode_AsWideCharString()
 - r85094: use it in import.c
 - r85095: use it for _locale.strcoll()
 - r85096: use it for time.strftime()
 - r85097: use it in _ctypes module
> So, you agree with the API (and the documentation)?
Well, you can now directly patch the documentation. I think that the API is simple and fine :-)