PEP: 529
Title: Change Windows filesystem encoding to UTF-8
Author: Steve Dower <steve.dower@python.org>
Status: Final
Type: Standards Track
Created: 27-Aug-2016
Python-Version: 3.6
Post-History: 01-Sep-2016, 04-Sep-2016
Resolution: https://mail.python.org/pipermail/python-dev/2016-September/146277.html

Abstract
========

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`_ 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.

Background
==========

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*')
    ['test\uab00.txt']
    >>> glob.glob(b'test*')
    [b'test?.txt']

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.

Proposal
========

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 nonsensical 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')


References
==========

.. _Naming Files, Paths, and Namespaces:
   https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx

Copyright
=========

This document has been placed in the public domain.
