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Python Enhancement Proposals

PEP 277 – Unicode file name support for Windows NT

Neil Hodgson <neilh at>
Standards Track

Table of Contents


This PEP discusses supporting access to all files possible on Windows NT by passing Unicode file names directly to the system’s wide-character functions.


Python 2.2 on Win32 platforms converts Unicode file names passed to open and to functions in the os module into the ‘mbcs’ encoding before passing the result to the operating system. This is often successful in the common case where the script is operating with the locale set to the same value as when the file was created. Most machines are set up as one locale and rarely if ever changed from this locale. For some users, locale is changed more often and on servers there are often files saved by users using different locales.

On Windows NT and descendent operating systems, including Windows 2000 and Windows XP, wide-character APIs are available that provide direct access to all file names, including those that are not representable using the current locale. The purpose of this proposal is to provide access to these wide-character APIs through the standard Python file object and posix module and so provide access to all files on Windows NT.


On Windows platforms which provide wide-character file APIs, when Unicode arguments are provided to file APIs, wide-character calls are made instead of the standard C library and posix calls.

The Python file object is extended to use a Unicode file name argument directly rather than converting it. This affects the file object constructor file(filename[, mode[, bufsize]]) and also the open function which is an alias of this constructor. When a Unicode filename argument is used here then the name attribute of the file object will be Unicode. The representation of a file object, repr(f) will display Unicode file names as an escaped string in a similar manner to the representation of Unicode strings.

The posix module contains functions that take file or directory names: chdir, listdir, mkdir, open, remove, rename, rmdir, stat, and _getfullpathname. These will use Unicode arguments directly rather than converting them. For the rename function, this behaviour is triggered when either of the arguments is Unicode and the other argument converted to Unicode using the default encoding.

The listdir function currently returns a list of strings. Under this proposal, it will return a list of Unicode strings when its path argument is Unicode.


On the consumer Windows operating systems, Windows 95, Windows 98, and Windows ME, there are no wide-character file APIs so behaviour is unchanged under this proposal. It may be possible in the future to extend this proposal to cover these operating systems as the VFAT-32 file system used by them does support Unicode file names but access is difficult and so implementing this would require much work. The “Microsoft Layer for Unicode” could be a starting point for implementing this.

Python can be compiled with the size of Unicode characters set to 4 bytes rather than 2 by defining PY_UNICODE_TYPE to be a 4 byte type and Py_UNICODE_SIZE to be 4. As the Windows API does not accept 4 byte characters, the features described in this proposal will not work in this mode so the implementation falls back to the current ‘mbcs’ encoding technique. This restriction could be lifted in the future by performing extra conversions using PyUnicode_AsWideChar but for now that would add too much complexity for a very rarely used feature.

Reference Implementation

The implementation is available at [2].


[1] Microsoft Windows APIs


Last modified: 2023-09-09 17:39:29 GMT