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PEP 529 -- Change Windows filesystem encoding to UTF-8

PEP: 529
Title: Change Windows filesystem encoding to UTF-8
Author: Steve Dower <steve.dower at>
Status: Final
Type: Standards Track
Created: 27-Aug-2016
Python-Version: 3.6
Post-History: 01-Sep-2016, 04-Sep-2016


Historically, Python uses the ANSI APIs for interacting with the Windows operating system, often via C Runtime functions. However, these have been long discouraged in favor of the UTF-16 APIs. Within the operating system, all text is represented as UTF-16, and the ANSI APIs perform encoding and decoding using the active code page. See Naming Files, Paths, and Namespaces [1] for more details.

This PEP proposes changing the default filesystem encoding on Windows to utf-8, and changing all filesystem functions to use the Unicode APIs for filesystem paths. This will not affect code that uses strings to represent paths, however those that use bytes for paths will now be able to correctly round-trip all valid paths in Windows filesystems. Currently, the conversions between Unicode (in the OS) and bytes (in Python) were lossy and would fail to round-trip characters outside of the user's active code page.

Notably, this does not impact the encoding of the contents of files. These will continue to default to locale.getpreferredencoding() (for text files) or plain bytes (for binary files). This only affects the encoding used when users pass a bytes object to Python where it is then passed to the operating system as a path name.


File system paths are almost universally represented as text with an encoding determined by the file system. In Python, we expose these paths via a number of interfaces, such as the os and io modules. Paths may be passed either direction across these interfaces, that is, from the filesystem to the application (for example, os.listdir() ), or from the application to the filesystem (for example, os.unlink() ).

When paths are passed between the filesystem and the application, they are either passed through as a bytes blob or converted to/from str using os.fsencode() and os.fsdecode() or explicit encoding using sys.getfilesystemencoding() . The result of encoding a string with sys.getfilesystemencoding() is a blob of bytes in the native format for the default file system.

On Windows, the native format for the filesystem is utf-16-le. The recommended platform APIs for accessing the filesystem all accept and return text encoded in this format. However, prior to Windows NT (and possibly further back), the native format was a configurable machine option and a separate set of APIs existed to accept this format. The option (the "active code page") and these APIs (the "*A functions") still exist in recent versions of Windows for backwards compatibility, though new functionality often only has a utf-16-le API (the "*W functions").

In Python, str is recommended because it can correctly round-trip all characters used in paths (on POSIX with surrogateescape handling; on Windows because str maps to the native representation). On Windows bytes cannot round-trip all characters used in paths, as Python internally uses the *A functions and hence the encoding is "whatever the active code page is". Since the active code page cannot represent all Unicode characters, the conversion of a path into bytes can lose information without warning or any available indication.

As a demonstration of this:

>>> open('test\uAB00.txt', 'wb').close()
>>> import glob
>>> glob.glob('test*')
>>> glob.glob(b'test*')

The Unicode character in the second call to glob has been replaced by a '?', which means passing the path back into the filesystem will result in a FileNotFoundError . The same results may be observed with os.listdir() or any function that matches the return type to the parameter type.

While one user-accessible fix is to use str everywhere, POSIX systems generally do not suffer from data loss when using bytes exclusively as the bytes are the canonical representation. Even if the encoding is "incorrect" by some standard, the file system will still map the bytes back to the file. Making use of this avoids the cost of decoding and reencoding, such that (theoretically, and only on POSIX), code such as this may be faster because of the use of b'.' compared to using '.' :

>>> for f in os.listdir(b'.'):
...     os.stat(f)

As a result, POSIX-focused library authors prefer to use bytes to represent paths. For some authors it is also a convenience, as their code may receive bytes already known to be encoded correctly, while others are attempting to simplify porting their code from Python 2. However, the correctness assumptions do not carry over to Windows where Unicode is the canonical representation, and errors may result. This potential data loss is why the use of bytes paths on Windows was deprecated in Python 3.3 - all of the above code snippets produce deprecation warnings on Windows.


Currently the default filesystem encoding is 'mbcs', which is a meta-encoder that uses the active code page. However, when bytes are passed to the filesystem they go through the *A APIs and the operating system handles encoding. In this case, paths are always encoded using the equivalent of 'mbcs:replace' with no opportunity for Python to override or change this.

This proposal would remove all use of the *A APIs and only ever call the *W APIs. When Windows returns paths to Python as str , they will be decoded from utf-16-le and returned as text (in whatever the minimal representation is). When Python code requests paths as bytes , the paths will be transcoded from utf-16-le into utf-8 using surrogatepass (Windows does not validate surrogate pairs, so it is possible to have invalid surrogates in filenames). Equally, when paths are provided as bytes , they are transcoded from utf-8 into utf-16-le and passed to the *W APIs.

The use of utf-8 will not be configurable, except for the provision of a "legacy mode" flag to revert to the previous behaviour.

The surrogateescape error mode does not apply here, as the concern is not about retaining non-sensical bytes. Any path returned from the operating system will be valid Unicode, while invalid paths created by the user should raise a decoding error (currently these would raise OSError or a subclass).

The choice of utf-8 bytes (as opposed to utf-16-le bytes) is to ensure the ability to round-trip path names and allow basic manipulation (for example, using the os.path module) when assuming an ASCII-compatible encoding. Using utf-16-le as the encoding is more pure, but will cause more issues than are resolved.

This change would also undeprecate the use of bytes paths on Windows. No change to the semantics of using bytes as a path is required - as before, they must be encoded with the encoding specified by sys.getfilesystemencoding() .

Specific Changes

Update sys.getfilesystemencoding

Remove the default value for Py_FileSystemDefaultEncoding and set it in initfsencoding() to utf-8, or if the legacy-mode switch is enabled to mbcs.

Update the implementations of PyUnicode_DecodeFSDefaultAndSize() and PyUnicode_EncodeFSDefault() to use the utf-8 codec, or if the legacy-mode switch is enabled the existing mbcs codec.

Add sys.getfilesystemencodeerrors

As the error mode may now change between surrogatepass and replace , Python code that manually performs encoding also needs access to the current error mode. This includes the implementation of os.fsencode() and os.fsdecode() , which currently assume an error mode based on the codec.

Add a public Py_FileSystemDefaultEncodeErrors , similar to the existing Py_FileSystemDefaultEncoding . The default value on Windows will be surrogatepass or in legacy mode, replace . The default value on all other platforms will be surrogateescape .

Add a public sys.getfilesystemencodeerrors() function that returns the current error mode.

Update the implementations of PyUnicode_DecodeFSDefaultAndSize() and PyUnicode_EncodeFSDefault() to use the variable for error mode rather than constant strings.

Update the implementations of os.fsencode() and os.fsdecode() to use sys.getfilesystemencodeerrors() instead of assuming the mode.

Update path_converter

Update the path converter to always decode bytes or buffer objects into text using PyUnicode_DecodeFSDefaultAndSize() .

Change the narrow field from a char* string into a flag that indicates whether the original object was bytes. This is required for functions that need to return paths using the same type as was originally provided.

Remove unused ANSI code

Remove all code paths using the narrow field, as these will no longer be reachable by any caller. These are only used within posixmodule.c . Other uses of paths should have use of bytes paths replaced with decoding and use of the *W APIs.

Add legacy mode

Add a legacy mode flag, enabled by the environment variable PYTHONLEGACYWINDOWSFSENCODING or by a function call to sys._enablelegacywindowsfsencoding() . The function call can only be used to enable the flag and should be used by programs as close to initialization as possible. Legacy mode cannot be disabled while Python is running.

When this flag is set, the default filesystem encoding is set to mbcs rather than utf-8, and the error mode is set to replace rather than surrogatepass . Paths will continue to decode to wide characters and only *W APIs will be called, however, the bytes passed in and received from Python will be encoded the same as prior to this change.

Undeprecate bytes paths on Windows

Using bytes as paths on Windows is currently deprecated. We would announce that this is no longer the case, and that paths when encoded as bytes should use whatever is returned from sys.getfilesystemencoding() rather than the user's active code page.

Beta experiment

To assist with determining the impact of this change, we propose applying it to 3.6.0b1 provisionally with the intent being to make a final decision before 3.6.0b4.

During the experiment period, decoding and encoding exception messages will be expanded to include a link to an active online discussion and encourage reporting of problems.

If it is decided to revert the functionality for 3.6.0b4, the implementation change would be to permanently enable the legacy mode flag, change the environment variable to PYTHONWINDOWSUTF8FSENCODING and function to sys._enablewindowsutf8fsencoding() to allow enabling the functionality on a case-by-case basis, as opposed to disabling it.

It is expected that if we cannot feasibly make the change for 3.6 due to compatibility concerns, it will not be possible to make the change at any later time in Python 3.x.

Affected Modules

This PEP implicitly includes all modules within the Python that either pass path names to the operating system, or otherwise use sys.getfilesystemencoding() .

As of 3.6.0a4, the following modules require modification:

  • os
  • _overlapped
  • _socket
  • subprocess
  • zipimport

The following modules use sys.getfilesystemencoding() but do not need modification:

  • gc (already assumes bytes are utf-8)
  • grp (not compiled for Windows)
  • http.server (correctly includes codec name with transmitted data)
  • idlelib.editor (should not be needed; has fallback handling)
  • nis (not compiled for Windows)
  • pwd (not compiled for Windows)
  • spwd (not compiled for Windows)
  • _ssl (only used for ASCII constants)
  • tarfile (code unused on Windows)
  • _tkinter (already assumes bytes are utf-8)
  • wsgiref (assumed as the default encoding for unknown environments)
  • zipapp (code unused on Windows)

The following native code uses one of the encoding or decoding functions, but do not require any modification:

  • Parser/parsetok.c (docs already specify sys.getfilesystemencoding() )
  • Python/ast.c (docs already specify sys.getfilesystemencoding() )
  • Python/compile.c (undocumented, but Python filesystem encoding implied)
  • Python/errors.c (docs already specify os.fsdecode() )
  • Python/fileutils.c (code unused on Windows)
  • Python/future.c (undocumented, but Python filesystem encoding implied)
  • Python/import.c (docs already specify utf-8)
  • Python/importdl.c (code unused on Windows)
  • Python/pythonrun.c (docs already specify sys.getfilesystemencoding() )
  • Python/symtable.c (undocumented, but Python filesystem encoding implied)
  • Python/thread.c (code unused on Windows)
  • Python/traceback.c (encodes correctly for comparing strings)
  • Python/_warnings.c (docs already specify os.fsdecode() )

Rejected Alternatives

Use strict mbcs decoding

This is essentially the same as the proposed change, but instead of changing sys.getfilesystemencoding() to utf-8 it is changed to mbcs (which dynamically maps to the active code page).

This approach allows the use of new functionality that is only available as *W APIs and also detection of encoding/decoding errors. For example, rather than silently replacing Unicode characters with '?', it would be possible to warn or fail the operation.

Compared to the proposed fix, this could enable some new functionality but does not fix any of the problems described initially. New runtime errors may cause some problems to be more obvious and lead to fixes, provided library maintainers are interested in supporting Windows and adding a separate code path to treat filesystem paths as strings.

Making the encoding mbcs without strict errors is equivalent to the legacy-mode switch being enabled by default. This is a possible course of action if there is significant breakage of actual code and a need to extend the deprecation period, but still a desire to have the simplifications to the CPython source.

Make bytes paths an error on Windows

By preventing the use of bytes paths on Windows completely we prevent users from hitting encoding issues.

However, the motivation for this PEP is to increase the likelihood that code written on POSIX will also work correctly on Windows. This alternative would move the other direction and make such code completely incompatible. As this does not benefit users in any way, we reject it.

Make bytes paths an error on all platforms

By deprecating and then disable the use of bytes paths on all platforms we prevent users from hitting encoding issues regardless of where the code was originally written. This would require a full deprecation cycle, as there are currently no warnings on platforms other than Windows.

This is likely to be seen as a hostile action against Python developers in general, and as such is rejected at this time.

Code that may break

The following code patterns may break or see different behaviour as a result of this change. Each of these examples would have been fragile in code intended for cross-platform use. The suggested fixes demonstrate the most compatible way to handle path encoding issues across all platforms and across multiple Python versions.

Note that all of these examples produce deprecation warnings on Python 3.3 and later.

Not managing encodings across boundaries

Code that does not manage encodings when crossing protocol boundaries may currently be working by chance, but could encounter issues when either encoding changes. Note that the source of filename may be any function that returns a bytes object, as illustrated in a second example below:

>>> filename = open('filename_in_mbcs.txt', 'rb').read()
>>> text = open(filename, 'r').read()

To correct this code, the encoding of the bytes in filename should be specified, either when reading from the file or before using the value:

>>> # Fix 1: Open file as text (default encoding)
>>> filename = open('filename_in_mbcs.txt', 'r').read()
>>> text = open(filename, 'r').read()

>>> # Fix 2: Open file as text (explicit encoding)
>>> filename = open('filename_in_mbcs.txt', 'r', encoding='mbcs').read()
>>> text = open(filename, 'r').read()

>>> # Fix 3: Explicitly decode the path
>>> filename = open('filename_in_mbcs.txt', 'rb').read()
>>> text = open(filename.decode('mbcs'), 'r').read()

Where the creator of filename is separated from the user of filename , the encoding is important information to include:

>>> some_object.filename = r'C:\Users\Steve\Documents\my_file.txt'.encode('mbcs')

>>> filename = some_object.filename
>>> type(filename)
<class 'bytes'>
>>> text = open(filename, 'r').read()

To fix this code for best compatibility across operating systems and Python versions, the filename should be exposed as str:

>>> # Fix 1: Expose as str
>>> some_object.filename = r'C:\Users\Steve\Documents\my_file.txt'

>>> filename = some_object.filename
>>> type(filename)
<class 'str'>
>>> text = open(filename, 'r').read()

Alternatively, the encoding used for the path needs to be made available to the user. Specifying os.fsencode() (or sys.getfilesystemencoding() ) is an acceptable choice, or a new attribute could be added with the exact encoding:

>>> # Fix 2: Use fsencode
>>> some_object.filename = os.fsencode(r'C:\Users\Steve\Documents\my_file.txt')

>>> filename = some_object.filename
>>> type(filename)
<class 'bytes'>
>>> text = open(filename, 'r').read()

>>> # Fix 3: Expose as explicit encoding
>>> some_object.filename = r'C:\Users\Steve\Documents\my_file.txt'.encode('cp437')
>>> some_object.filename_encoding = 'cp437'

>>> filename = some_object.filename
>>> type(filename)
<class 'bytes'>
>>> filename = filename.decode(some_object.filename_encoding)
>>> type(filename)
<class 'str'>
>>> text = open(filename, 'r').read()

Explicitly using 'mbcs'

Code that explicitly encodes text using 'mbcs' before passing to file system APIs is now passing incorrectly encoded bytes. Note that the source of filename in this example is not relevant, provided that it is a str:

>>> filename = open('files.txt', 'r').readline().rstrip()
>>> text = open(filename.encode('mbcs'), 'r')

To correct this code, the string should be passed without explicit encoding, or should use os.fsencode() :

>>> # Fix 1: Do not encode the string
>>> filename = open('files.txt', 'r').readline().rstrip()
>>> text = open(filename, 'r')

>>> # Fix 2: Use correct encoding
>>> filename = open('files.txt', 'r').readline().rstrip()
>>> text = open(os.fsencode(filename), 'r')