[Python-Dev] PEP 529: Change Windows filesystem encoding to UTF-8

Steve Dower steve.dower at python.org
Thu Sep 1 18:31:26 EDT 2016

I'm about to be offline for a few days, so I wanted to get my current 
draft PEPs out for people can read and review.

I don't believe there is a lot of change as a result of either PEP, but 
the impact of what change there is needs to be weighed against the benefits.

We've already had some thorough discussion on this one and failed to 
reach agreement on whether we can make this change in 3.6 or if it needs 
a deprecation cycle that is more visible than the one we started in 3.3. 
In the latter case, we need to determine how visible that should be 
(i.e. warnings visible by default, visible for non-Windows platforms, 
value-dependent warnings/errors, etc.). IMHO, the argument about having 
the change be on-by-default or off-by-default is irrelevant until we 
decide on the deprecation issue, at which point it is obvious what the 
default should be.

See https://bugs.python.org/issue27781 for the current proposed patch. I 
do need to update it in order to merge against default it seems (work 
for my upcoming flight).



PEP: 529
Title: Change Windows filesystem encoding to UTF-8
Version: $Revision$
Last-Modified: $Date$
Author: Steve Dower <steve.dower at python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 27-Aug-2016
Post-History: 01-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.

This PEP proposes changing the default filesystem encoding on Windows to 
and changing all filesystem functions to use the Unicode APIs for filesystem
paths. This will not affect code that uses strings to represent paths, 
those that use bytes for paths will now be able to correctly round-trip all
valid paths in Windows filesystems. Currently, the conversions between 
(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


File system paths are almost universally represented as text with an 
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 
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()`` or ``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 
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 
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 
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 
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 
the file system will still map the bytes back to the file. Making use of 
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'.'` 
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 
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 
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 
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' - 
we have
no ability to change this (though there is a user/machine configuration 
to change the encoding from CP_ACP to CP_OEM, so it won't necessarily always
match mbcs...)

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
Windows returns paths to Python as bytes, they will be decoded from 
utf-16-le to
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 decoded from utf-8 into utf-16-le and passed 
to the
*W APIs.

The use of utf-8 will not be configurable, with the possible exception of a
"legacy mode" environment variable or X-flag.

surrogateescape does not apply here, as the concern is not about retaining
non-sensical bytes. Any path returned from the operating system will be 
Unicode, while bytes paths created by the user may 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 without breaking the functionality of the ``os.path``
module, which assumes 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 
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 standard utf-8 codec with 
error mode, or if the legacy-mode switch is enabled the code page codec with
replace error 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 
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``. When this flag is set, the default 
encoding is set to mbcs rather than utf-8, and the error mode is set to
'replace' rather than 'strict'. The ``path_converter`` will continue to 
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.

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 
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 
some problems to be more obvious and lead to fixes, provided library 
are interested in supporting Windows and adding a separate code path to 
filesystem paths as strings.

Making the encoding mbcs without strict errors is equivalent to the 
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 
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.

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

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 
changes. For example::

     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
     filename = open('filename_in_mbcs.txt', 'r', encoding='mbcs').read()
     text = open(filename, 'r').read()

     # Fix 2: Decode path
     filename = open('filename_in_mbcs.txt', 'rb').read()
     text = open(filename.decode('mbcs'), 'r').read()

Explicitly using 'mbcs'

Code that explicitly encodes text using 'mbcs' before passing to file system
APIs. For example::

     filename = open('files.txt', 'r').readline()
     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()
     text = open(filename, 'r')

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


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