Between Python 3.0 released in 2008 and Python 3.4 released in 2014, the Python filesystem encoding changed multiple times. It took 6 years to choose the best Python filesystem encoding on each platform.

I have been officially promoted as a core developer in January 2010 by Martin von Loewis. I spent the whole year of 2010 to fix dozens of encoding issues during the development of Python 3.2, following my Unicode work started in 2008.

This article is focused on the long discussions to choose the best Python filesystem encoding on each platform in 2010 for Python 3.2.

This article is the third in a series of articles telling the history and rationale of the Python 3 Unicode model for the operating system:


Python 3.0 loves UTF-8

When Python 3.0 was released, it was unclear which encodings should be used for:

  • File content: open().read()
  • Filenames: os.listdir(), open(), etc.
  • Command line arguments: sys.argv and subprocess.Popen arguments
  • Environment variables: os.environ
  • etc.

Python 3.0 was forked from Python 2.6 and functions were modified to use Unicode. Many Python 3 functions only used UTF-8 because the implementation were modified to use the default encoding which is UTF-8: it was not a deliberate choice.

While UTF-8 is a good choice in most cases, it is not the best choice in all cases. Almost everything worked well in Python 3.0 when all data used UTF-8, but Python 3.0 failed badly if the locale encoding was not UTF-8.

Python 3.1, 3.2 and 3.3 will get a lot of changes to adjust encodings in all corners of the standard library.

Python 3.1 got the surrogateescape error handler (PEP 383) which reduced Unicode errors: read my previous article Python 3.1 surrogateescape error handler (PEP 383).

Add sys.setfilesystemencoding()

September 2008, bpo-3187: To fix os.listdir(str) to support undecodable filenames, Martin v. Löwis proposed a new function to change the filesystem encoding:

Here is a patch that solves the issue in a different way: it introduces sys.setfilesystemencoding. If applications invoke sys.setfilesystemencoding("iso-8859-1"), all file names can be successfully converted into a character string.

The ISO-8859-1 encoding has a very interesting property for bytes: it maps exactly the 0x00 - 0xff byte range to the U+0000 - U+00ff Unicode range, the decoder cannot fail:

$ python3.6 -q
>>> all(ord((b'%c' % byte).decode('iso-8859-1')) == byte for byte in range(256))
>>> all(ord(('%c' % char).encode('iso-8859-1')) == char for char in range(256))

Guido van Rossum commented:

I will check in Victor's changes (with some edits).

Together this means that the various suggested higher-level solutions (like returning path-like objects, or some kind of roudtripping almost-but-not-quite-utf-8 encoding) can be implemented in pure Python.

October 2008, Martin v. Löwis pushed the commit 04dc25c5:

Issue #3187: Add sys.setfilesystemencoding.

Python 3.0 will be the first major release with this function.

In retrospective, I see this function as asking developers and users to be smart and choose the encoding themself.

While the ISO-8859-1 encoding trick is tempting, we will see later that setfilesystemencoding() is broken by design and so cannot be used in practice.

What if getting the locale encoding fails?

May 2010, I reported bpo-8610, "Python3/POSIX: errors if file system encoding is None":

On POSIX (but not on Mac OS X), Python3 calls get_codeset() to get the file system encoding. If this function fails, sys.getfilesystemencoding() returns None.

I pushed the commit b744ba1d:

Issue #8610: Load file system codec at startup, and display a fatal error on failure. Set the file system encoding to utf-8 (instead of None) if getting the locale encoding failed, or if nl_langinfo(CODESET) function is missing.

This change adds the function initfsencoding(): logic to initialize the filesystem encoding.

In practice, Python already used UTF-8 when the filesystem encoding was set to None, but this change makes the default more obvious. The change also makes the error case better defined: Python exits immediately with a fatal error.

Support locale encodings different than UTF-8

My biggest Unicode project in Python 3 was to fix the encoding in all corners of the standard library. This task kept me busy between Python 3.0 and Python 3.4, at least.

May 2010, I created bpo-8611:

Python3 is unable to start (bootstrap failure) on a POSIX system if the locale encoding is different than utf8 and the Python path (standard library path where the encoding module is stored) contains a non-ASCII character. (Windows and Mac OS X are not affected by this issue because the file system encoding is hardcoded.)

For example, bpo-8242 "Improve support of PEP 383 (surrogates) in Python3" is a meta issue tracking multiple issues:

  • bpo-7606: test_xmlrpc fails with non-ascii path
  • bpo-8092: utf8, backslashreplace and surrogates
  • bpo-8383: pickle is unable to encode unicode surrogates
  • bpo-8390: tarfile: use surrogates for undecode fields
  • bpo-8391: os.execvpe() doesn't support surrogates in env
  • bpo-8393: subprocess: support undecodable current working directory on POSIX OS
  • bpo-8394: ctypes.dlopen() doesn't support surrogates
  • bpo-8412: os.system() doesn't support surrogates nor bytes
  • bpo-8467: subprocess: surrogates of the error message (Python implementation on non-Windows)
  • bpo-8468: bz2: support surrogates in filename, and bytes/bytearray filename
  • bpo-8477: _ssl: support surrogates in filenames, and bytes/bytearray filenames
  • bpo-8485: Don't accept bytearray as filenames, or simplify the API

I fixed all these issues, and reported most of them.

October 2010, finally, five months later, I succeeded to close the issue!

Starting at r85691, the full test suite of Python 3.2 pass with ASCII, ISO-8859-1 and UTF-8 locale encodings in a non-ascii directory. The work on this issue is done.

At that time, I didn't know that it will take me a few more years to really fix all encoding issues. For example, it will take me 3 years to modify the core of the import machinery to pass filenames as Unicode on Windows: bpo-3080 Full unicode import system.

Add PYTHONFSENCODING environment variable

May 2010, while discussing how to fix bpo-8610 "Python3/POSIX: errors if file system encoding is None", I asked what is the best encoding if reading the locale encoding fails. As a follow-up, Marc-Andre Lemburg created bpo-8622:

As discussed on issue8610, we need a way to override the automatic detection of the file system encoding - for much the same reasons we also do for the I/O encoding: the detection mechanism isn't fail-safe.

We should add a new environment variable with the same functionality as PYTHONIOENCODING:

PYTHONFSENCODING: Encoding[:errors] used for file system.

I implemented the idea since I liked it. August 2010, I pushed the commit 94908bbc:

Issue #8622: Add PYTHONFSENCODING environment variable to override the filesystem encoding.

initfsencoding() displays also a better error message if get_codeset() failed.

Remove sys.setfilesystemencoding()

August 2010, just after adding PYTHONFSENCODING, I opened bpo-9632 to remove the sys.setfilesystemencoding() function:

The sys.setfilesystemencoding() function is dangerous because it introduces a lot of inconsistencies: this function is unable to reencode all filenames of all objects (eg. Python is unable to find filenames in user objects or 3rd party libraries). Eg. if you change the filesystem from utf8 to ascii, it will not be possible to use existing non-ascii (unicode) filenames: they will raise UnicodeEncodeError.

As sys.setdefaultencoding() in Python2, I think that sys.setfilesystemencoding() is the root of evil :-) PYTHONFSENCODING (issue #8622) is the right solution to set the filesysteme encoding.

Marc-Andre Lemburg complained that applications embedding Python may want to set the encoding used by Python. I proposed to use the PYTHONFSENCODING environment variable as a workaround, even if it was not the best option.

One month later, I pushed the commit 5b519e02:

Issue #9632: Remove sys.setfilesystemencoding() function: use PYTHONFSENCODING environment variable to set the filesystem encoding at Python startup. sys.setfilesystemencoding() created inconsistencies because it was unable to reencode all filenames of all objects.

Reencode filenames when setting the filesystem encoding

August 2010, I created bpo-9630: "Reencode filenames when setting the filesystem encoding".

Since the beginning of 2010, I identified a design flaw in the Python initialization. Python starts by decoding strings from the default encoding UTF-8. Later, Python reads the locale encoding and loads the Python codec of this encoding. Then Python decodes string from the locale encoding. Problem: if the locale encoding is not UTF-8, encoding strings decoded from UTF-8 to the locale encoding can fail in different ways.

I wrote a patch to "reencode" filenames of all module and code objects once the filesystem encoding is set, in initfsencoding(),

When I wrote the patch, I knew that it was an ugly hack and not the proper design. I proposed to try to avoid importing any Python module before the Python codec of the locale encoding is loaded, but there was a pratical issue. Python only has builtin implementation (written in C) of the most popular encodings like ASCII and UTF-8. Some encodings like ISO-8859-15 are only implemented in Python.

I also proposed to "unload all modules, clear all caches and delete all code objects" after setting the filesystem encoding. This option would be very inefficient and make Python startup slower, whereas Python 3 startup was already way slower than Python 2 startup.

September 2010, I pushed the commit c39211f5:

Issue #9630: Redecode filenames when setting the filesystem encoding

Redecode the filenames of:

  • all modules: __file__ and __path__ attributes
  • all code objects: co_filename attribute
  • sys.path
  • sys.meta_path
  • sys.executable
  • sys.path_importer_cache (keys)

Keep weak references to all code objects until initfsencoding() is called, to be able to redecode co_filename attribute of all code objects.

The list of weak references to code objects really looks like a hack and I disliked it, but I failed to find a better way to fix Python startup.


Even with my latest big and ugly "redecode filenames when setting the filesystem encoding" fix, there were issues when the filesystem encoding was different than the locale encoding. I identified 4 bugs:

  • bpo-9992, sys.argv: decoded from the locale encoding, but subprocess encodes process arguments to the filesystem encoding
  • bpo-10014, sys.path: decoded from the locale encoding, but import encodes paths to the filesystem encoding
  • bpo-10039, the script name: read on the command line (ex: python which is decoded from the locale encoding, whereas it is used to fill sys.path[0] and import encodes paths to the filesystem encoding.
  • bpo-9988, PYTHONWARNINGS environment variable: decoded from the locale encoding, but subprocess encodes environment variables to the filesystem encoding.

October 2010, I wrote an email to the python-dev list: Inconsistencies if locale and filesystem encodings are different. I proposed two solutions:

  • (a) use the same encoding to encode and decode values (it can be different for each issue).
  • (b) remove PYTHONFSENCODING variable and raise an error if locale and filesystem encodings are different (ensure that both encodings are the same).

Marc-Andre Lemburg replied:

You have to differentiate between the meaning of a file system encoding and the locale:

A file system encoding defines how the applications interact with the file system.

A locale defines how the user expects to interact with the application.

It is well possible that the two are different. Mac OS X is just one example. Another common example is having a Unix account using the C locale (=ASCII) while working on a UTF-8 file system.

This email is a good example of dilemma we had when having to choose one encoding. There is a big temptation to use multiple encodings, but at the end, data are not isolated. A filename can be found in command line arguments (python3 file.txt), in environment variables (LOG_FILE=log.txt), in file content (ex: Makefile or a configuration file), etc. Using multiple encodings does not work in practice.

Dead end


September 2010, I reported bpo-9992: Command-line arguments are not correctly decoded if locale and fileystem encodings are different.

I proposed a patch to use the locale encoding to decode and encode command line arguments, rather than using the filesystem encoding.

Martin v. Löwis proposed to use the locale encoding for the command line arguments, environment variables and all filenames. My summary:

You mean that we should use the following encoding:

  • Mac OS X: UTF-8
  • Windows: unicode for command line/env, mbcs to decode filenames
  • others OSes: locale encoding

To do that, we have to:

  • "others OSes": delete the PYTHONFSENCODING variable
  • Mac OS X: use UTF-8 to decode the command line arguments (we can use PyUnicode_DecodeUTF8() + PyUnicode_AsWideCharString() before Python is initialized)

October 2010, I pushed the commit 8f6b6b0c:

Issue #9992: Remove PYTHONFSENCODING environment variable.

Two days later, I pushed an important change to use the locale encoding and remove the ugly redecode_filenames() hack, commit f3170cce:

Use locale encoding if Py_FileSystemDefaultEncoding is not set

  • PyUnicode_EncodeFSDefault(), PyUnicode_DecodeFSDefaultAndSize() and PyUnicode_DecodeFSDefault() use the locale encoding instead of UTF-8 if Py_FileSystemDefaultEncoding is NULL
  • redecode_filenames() functions and _Py_code_object_list (issue #9630) are no more needed: remove them

This change has been made possible by enhancements of PyUnicode_EncodeFSDefault() and PyUnicode_DecodeFSDefaultAndSize(). Previously, these functions used UTF-8 before the filesystem was set. With my change, these functions now use the C implementation of the locale encoding: use mbstowcs() to decode and wcstombs() to encode. In practice, the code is more complex because Python uses the surrogateescape error handler.

Using the C implementation of the locale encoding fixed a lot of "bootstrap" issues of the Python initialization. It works because the Python codec of the locale encoding is 100% compatible with the C implementation of the locale codec.

Encodings used by Python 3.2

February 2011, Python 3.2 has been released. Summary of the used filesystem encodings:

  • ANSI code page on Windows;
  • UTF-8 on macOS;
  • locale encoding on other platforms.

Note: UTF-8 is used if the nl_langinfo(CODESET) function is not available.

Force ASCII encoding on FreeBSD and Solaris

November 2012, I created bpo-16455:

On FreeBSD and OpenIndiana, sys.getfilesystemencoding() returns 'ascii' when the locale is not set, whereas the locale encoding is ISO-8859-1 in practice.

This inconsistency causes different issues.

December 2012, I pushed the commit d45c7f8d:

Issue #16455: On FreeBSD and Solaris, if the locale is C, the ASCII/surrogateescape codec is now used, instead of the locale encoding, to decode the command line arguments. This change fixes inconsistencies with os.fsencode() and os.fsdecode() because these operating systems announces an ASCII locale encoding, whereas the ISO-8859-1 encoding is used in practice.

Extract of the main comment:

Workaround FreeBSD and OpenIndiana locale encoding issue with the C locale. On these operating systems, nl_langinfo(CODESET) announces an alias of the ASCII encoding, whereas mbstowcs() and wcstombs() functions use the ISO-8859-1 encoding. The problem is that os.fsencode() and os.fsdecode() use locale.getpreferredencoding() codec. For example, if command line arguments are decoded by mbstowcs() and encoded back by os.fsencode(), we get a UnicodeEncodeError instead of retrieving the original byte string.

The workaround is enabled if setlocale(LC_CTYPE, NULL) returns "C", nl_langinfo(CODESET) announces "ascii" (or an alias to ASCII), and at least one byte in range 0x80-0xff can be decoded from the locale encoding. The workaround is also enabled on error, for example if getting the locale failed.

Python 3.4 will be the first major release getting fix (March 2014), but I also backported the change to Python 3.2 and 3.3 branches.


It took 6 years to fix Python to use the best Python filesystem encoding.

Python 3.0 mostly uses UTF-8 everywhere, but it was not a deliberate choice and it caused many issues when the locale encoding was not UTF-8. Python 3.1 got the surrogateescape error handler (PEP 383) which reduced Unicode errors.

October 2008, Martin v. Löwis added sys.setfilesystemencoding() to Python 3.0.

August 2010, I added a new PYTHONFSENCODING environment variable, Marc-Andre Lemburg's idea.

September 2010, I removed the sys.setfilesystemencoding() function because it creates mojibake by design. I also pushed an ugly change to reencode filenames to fix many PYTHONFSENCODING bugs.

October 2010, I fixed all tests when Python lives in a non-ASCII directory: first milestone of supporting locale encodings different than UTF-8. I also removed the PYTHONFSENCODING environment variable after a long discussion. Moreover, I pushed the most important Python 3.2 change: Python now uses the locale encoding as the filesystem encoding. This change fixed many issues.

December 2012, I forced the filesystem encoding to ASCII on FreeBSD and Solaris when the announced locale encoding is wrong.