Python Enhancement Proposals

PEP 686 – Make UTF-8 mode default

Make UTF-8 mode default
Inada Naoki <songofacandy at>
Standards Track



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 enables UTF-8 mode by default.

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


Currently, TextIOWrapper uses locale.getpreferredencoding(False) when encoding="locale" option is specified. It is "UTF-8" in UTF-8 mode.

This behavior is inconsistent with the PEP 597 motivation. TextIOWrapper should use locale encoding when encoding="locale" is passed before/after the default encoding is changed to UTF-8.

To fix this inconsistency, we will add locale.get_encoding(). It is the same as locale.getpreferredencoding(False) but it ignores the UTF-8 mode.

This change will be released in Python 3.11 so that users can use UTF-8 mode that is the same as Python 3.13.

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.

To resolve this backward incompatibility, users can do:

  • Disable UTF-8 mode.
  • Use EncodingWarning to find where the default encoding is used and use encoding="locale" option if locale encoding should be used (as defined in PEP 597).
  • Find every occurrence of locale.getpreferredencoding(False) in the application, and replace it with locale.get_locale_encoding() if locale encoding should be used.
  • 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.

Java also rejected this idea in JEP 400.

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: 2022-03-25 04:11:38 GMT