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PEP 540 -- Add a new UTF-8 Mode

Title:Add a new UTF-8 Mode
Author:Victor Stinner <victor.stinner at>
BDFL-Delegate:INADA Naoki
Type:Standards Track


Add a new "UTF-8 Mode" to enhance Python's use of UTF-8. When UTF-8 Mode is active, Python will:

  • use the utf-8 locale, irregardless of the locale currently set by the current platform, and
  • change the stdin and stdout error handlers to surrogateescape.

This mode is off by default, but is automatically activated when using the "POSIX" locale.

Add the -X utf8 command line option and PYTHONUTF8 environment variable to control UTF-8 Mode.


Locale encoding and UTF-8

Python 3.6 uses the locale encoding for filenames, environment variables, standard streams, etc. The locale encoding is inherited from the locale; the encoding and the locale are tightly coupled.

Many users inherit the ASCII encoding from the POSIX locale, aka the "C" locale, but are unable change the locale for various reasons. This encoding is very limited in term of Unicode support: any non-ASCII character is likely to cause trouble.

It isn't always easy to get an accurate locale. Locales don't get the exact same name on different Linux distributions, FreeBSD, macOS, etc. And some locales, like the recent C.UTF-8 locale, are only supported by a few platforms. The current locale can even vary on the same platform depending on context; for example, a SSH connection can use a different encoding than the filesystem or local terminal encoding on the same machine.

On the flip side, Python 3.6 is already using UTF-8 by default on macOS, Android and Windows (PEP 529) for most functions -- although open() is a notable exception here. UTF-8 is also the default encoding of Python scripts, XML and JSON file formats. The Go programming language uses UTF-8 for all strings.

UTF-8 support is nearly ubiquitous for data read and written by modern platforms. It also has excellent support in Python. The problem is simply that the locale is frequently misconfigured. An obvious solution suggests itself: ignore the locale encoding and use UTF-8.

Passthough for undecodable bytes: surrogateescape

When decoding bytes from UTF-8 using the default strict error handler, Python 3 raises a UnicodeDecodeError on the first undecodable byte.

Unix command line tools like cat or grep and most Python 2 applications simply do not have this class of bugs: they don't decode data, but process data as a raw bytes sequence.

Python 3 already has a solution to behave like Unix tools and Python 2: the surrogateescape error handler (PEP 383). It allows processing data as if it were bytes, but uses Unicode in practice; undecodable bytes are stored as surrogate characters.

UTF-8 Mode sets the surrogateescape error handler for stdin and stdout, since these streams as commonly associated to Unix command line tools.

However, users have a different expectation on files. Files are expected to be properly encoded, and Python is expected to fail early when open() is called with the wrong options, like opening a JPEG picture in text mode. The open() default error handler remains strict for these reasons.

No change by default for best backward compatibility

While UTF-8 is perfect in most cases, sometimes the locale encoding is actually the best encoding.

This PEP changes the behaviour for the POSIX locale since this locale is usually equivalent to the ASCII encoding, whereas UTF-8 is a much better choice. It does not change the behaviour for other locales to prevent any risk or regression.

As users are responsible to enable explicitly the new UTF-8 Mode for these other locales, they are responsible for any potential mojibake issues caused by UTF-8 Mode.


Add a new UTF-8 Mode to use the UTF-8 encoding, ignore the locale encoding, and change stdin and stdout error handlers to surrogateescape.

Add the new -X utf8 command line option and PYTHONUTF8 environment variable. Users can explicitly activate UTF-8 Mode with the command-line option -X utf8 or by setting the environment variable PYTHONUTF8=1.

This mode is disabled by default and enabled by the POSIX locale. Users can explicitly disable UTF-8 Mode with the command-line option -X utf8=0 or by setting the environment variable PYTHONUTF8=0.

For standard streams, the PYTHONIOENCODING environment variable has priority over UTF-8 Mode.

On Windows, the PYTHONLEGACYWINDOWSFSENCODING environment variable (PEP 529) has the priority over UTF-8 Mode.

Effects of UTF-8 Mode:

  • sys.getfilesystemencoding() returns 'UTF-8'.
  • locale.getpreferredencoding() returns UTF-8; its do_setlocale argument, and the locale encoding, are ignored.
  • sys.stdin and sys.stdout error handler is set to surrogateescape.

Side effects:

  • open() uses the UTF-8 encoding by default. However, it still uses the strict error handler by default.
  • os.fsdecode() and os.fsencode() use the UTF-8 encoding.
  • Command line arguments, environment variables and filenames use the UTF-8 encoding.

Relationship with the locale coercion (PEP 538)

The POSIX locale enables the locale coercion (PEP 538) and the UTF-8 mode (PEP 540). When the locale coercion is enabled, enabling the UTF-8 mode has no additional effect.

The UTF-8 Mode has the same effect as locale coercion:

  • sys.getfilesystemencoding() returns 'UTF-8',
  • locale.getpreferredencoding() returns UTF-8, and
  • the sys.stdin and sys.stdout error handlers are set to surrogateescape.

These changes only affect Python code. But the locale coercion has addiditonal effects: the LC_CTYPE environment variable and the LC_CTYPE locale are set to a UTF-8 locale like C.UTF-8. One side effect is that non-Python code is also impacted by the locale coercion. The two PEPs are complementary.

On platforms like Centos 7 where locale coercion is not supported, the POSIX locale only enables UTF-8 Mode. In this case, Python code uses the UTF-8 encoding and ignores the locale encoding, whereas non-Python code uses the locale encoding, which is usually ASCII for the POSIX locale.

While the UTF-8 Mode is supported on all platforms and can be enabled with any locale, the locale coercion is not supported by all platforms and is restricted to the POSIX locale.

The UTF-8 Mode has only an impact on Python child processes when the PYTHONUTF8 environment variable is set to 1, whereas the locale coercion sets the LC_CTYPE environment variables which impacts all child processes.

The benefit of the locale coercion approach is that it helps ensure that encoding handling in binary extension modules and child processes is consistent with Python's encoding handling. The upside of the UTF-8 Mode approach is that it allows an embedding application to change the interpreter's behaviour without having to change the process global locale settings.

Backward Compatibility

The only backward incompatible change is that the POSIX locale now enables the UTF-8 Mode by default: it will now use the UTF-8 encoding, ignore the locale encoding, and change stdin and stdout error handlers to surrogateescape.

Annex: Encodings And Error Handlers

UTF-8 Mode changes the default encoding and error handler used by open(), os.fsdecode(), os.fsencode(), sys.stdin, sys.stdout and sys.stderr.

Encoding and error handler

Function Default UTF-8 Mode or POSIX locale
open() locale/strict UTF-8/strict
os.fsdecode(), os.fsencode() locale/surrogateescape UTF-8/surrogateescape
sys.stdin, sys.stdout locale/strict UTF-8/surrogateescape
sys.stderr locale/backslashreplace UTF-8/backslashreplace

By comparison, Python 3.6 uses:

Function Default POSIX locale
open() locale/strict locale/strict
os.fsdecode(), os.fsencode() locale/surrogateescape locale/surrogateescape
sys.stdin, sys.stdout locale/strict locale/surrogateescape
sys.stderr locale/backslashreplace locale/backslashreplace

Encoding and error handler on Windows

On Windows, the encodings and error handlers are different:

Function Default Legacy Windows FS encoding UTF-8 Mode
open() mbcs/strict mbcs/strict UTF-8/strict
os.fsdecode(), os.fsencode() UTF-8/surrogatepass mbcs/replace UTF-8/surrogatepass
sys.stdin, sys.stdout UTF-8/surrogateescape UTF-8/surrogateescape UTF-8/surrogateescape
sys.stderr UTF-8/backslashreplace UTF-8/backslashreplace UTF-8/backslashreplace

By comparison, Python 3.6 uses:

Function Default Legacy Windows FS encoding
open() mbcs/strict mbcs/strict
os.fsdecode(), os.fsencode() UTF-8/surrogatepass mbcs/replace
sys.stdin, sys.stdout UTF-8/surrogateescape UTF-8/surrogateescape
sys.stderr UTF-8/backslashreplace UTF-8/backslashreplace

The "Legacy Windows FS encoding" is enabled by the PYTHONLEGACYWINDOWSFSENCODING environment variable.

If stdin and/or stdout is redirected to a pipe, sys.stdin and/or sys.output uses mbcs encoding by default rather than UTF-8. But in UTF-8 Mode, sys.stdin and sys.stdout always use the UTF-8 encoding.

Post History

Version History

  • Version 4: locale.getpreferredencoding() now returns 'UTF-8' in the UTF-8 Mode.
  • Version 3: The UTF-8 Mode does not change the open() default error handler (strict) anymore, and the Strict UTF-8 Mode has been removed.
  • Version 2: Rewrite the PEP from scratch to make it much shorter and easier to understand.
  • Version 1: First version posted to python-dev.