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Python Enhancement Proposals

PEP 686 – Make UTF-8 mode default

Inada Naoki <songofacandy at>
Discourse thread
Standards Track
18-Mar-2022, 31-Mar-2022
Discourse message

Table of Contents


This PEP proposes enabling UTF-8 mode by default.

With this change, Python consistently uses UTF-8 for default encoding of files, stdio, and pipes.


UTF-8 becomes de facto standard text encoding.

  • The default encoding of Python source files is UTF-8.
  • JSON, TOML, YAML use UTF-8.
  • Most text editors, including Visual Studio Code and Windows Notepad use UTF-8 by default.
  • Most websites and text data on the internet use UTF-8.
  • And many other popular programming languages, including Node.js, Go, Rust, and Java uses UTF-8 by default.

Changing the default encoding to UTF-8 makes it easier for Python to interoperate with them.

Additionally, many Python developers using Unix forget that the default encoding is platform dependent. They omit to specify encoding="utf-8" when they read text files encoded in UTF-8 (e.g. JSON, TOML, Markdown, and Python source files). Inconsistent default encoding causes many bugs.


Enable UTF-8 mode by default

Python will enable UTF-8 mode by default from Python 3.15.

Users can still disable UTF-8 mode by setting PYTHONUTF8=0 or -X utf8=0.


Since UTF-8 mode affects locale.getpreferredencoding(False), we need an API to get locale encoding regardless of UTF-8 mode.

locale.getencoding() will be added for this purpose. It returns locale encoding too, but ignores UTF-8 mode.

When warn_default_encoding option is specified, locale.getpreferredencoding() will emit EncodingWarning like open() (see also PEP 597).

This API was added in Python 3.11.

Fixing encoding="locale" option

PEP 597 added the encoding="locale" option to the TextIOWrapper. This option is used to specify the locale encoding explicitly. TextIOWrapper should use locale encoding when the option is specified, regardless of default text encoding.

But TextIOWrapper uses "UTF-8" in UTF-8 mode even if encoding="locale" is specified for now. This behavior is inconsistent with the PEP 597 motivation. It is because we didn’t expect making UTF-8 mode default when Python changes its default text encoding.

This inconsistency should be fixed before making UTF-8 mode default. TextIOWrapper should use locale encoding when encoding="locale" is passed even in UTF-8 mode.

This issue was fixed in Python 3.11.

Backward Compatibility

Most Unix systems use UTF-8 locale and Python enables UTF-8 mode when its locale is C or POSIX. So this change mostly affects Windows users.

When a Python program depends on the default encoding, this change may cause UnicodeError, mojibake, or even silent data corruption. So this change should be announced loudly.

This is the guideline to fix this backward compatibility issue:

  1. Disable UTF-8 mode.
  2. Use EncodingWarning (PEP 597) to find every places UTF-8 mode affects.
    • If encoding option is omitted, consider using encoding="utf-8" or encoding="locale".
    • If locale.getpreferredencoding() is used, consider using "utf-8" or locale.getencoding().
  3. Test the application with UTF-8 mode.

Preceding examples

  • Ruby changed the default external_encoding to UTF-8 on Windows in Ruby 3.0 (2020).
  • Java changed the default text encoding to UTF-8 in JDK 18. (2022).

Both Ruby and Java have an option for backward compatibility. They don’t provide any warning like PEP 597’s EncodingWarning in Python for use of the default encoding.

Rejected Alternative

Deprecate implicit encoding

Deprecating the use of the default encoding is considered.

But there are many cases that the default encoding is used for reading/writing only ASCII text. Additionally, such warnings are not useful for non-cross platform applications run on Unix.

So forcing users to specify the encoding everywhere is too painful. Emitting a lot of DeprecationWarning will lead users ignore warnings.

PEP 387 requires adding a warning for backward incompatible changes. But it doesn’t require using DeprecationWarning. So using optional EncodingWarning doesn’t violate the PEP 387.

Java also rejected this idea in JEP 400.


To ease backward compatibility issue, using PYTHONIOENCODING as the default encoding of PIPEs in the subprocess module is considered.

With this idea, users can use legacy encoding for subprocess.Popen(text=True) even in UTF-8 mode.

But this idea makes “default encoding” complicated. And this idea is also backward incompatible.

So this idea is rejected. Users can disable UTF-8 mode until they replace text=True with encoding="utf-8" or encoding="locale".

How to teach this

For new users, this change reduces things that need to teach. Users don’t need to learn about text encoding in their first year. They should learn it when they need to use non-UTF-8 text files.

For existing users, see the Backward compatibility section.


Last modified: 2023-09-09 17:39:29 GMT