PEP: 686
Title: Make UTF-8 mode default
Author: Inada Naoki <songofacandy@gmail.com>
Discussions-To: https://discuss.python.org/t/14435
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 18-Mar-2022
Python-Version: 3.13
Post-History: `18-Mar-2022 <https://discuss.python.org/t/14435>`__


Abstract
========

This PEP proposes enabling :pep:`UTF-8 mode <540>` by default.

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


Motivation
==========

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.


Specification
=============

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``.


``locale.get_encoding()``
-------------------------

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 <Feature #16604_>`__ the default ``external_encoding``
  to UTF-8 on Windows in Ruby 3.0 (2020).
* Java `changed <JEP 400_>`__ 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.


References
==========

.. _Feature #16604: https://bugs.ruby-lang.org/issues/16604

.. _JEP 400: https://openjdk.java.net/jeps/400


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.