PEP 686: Make UTF-8 mode default (#2443)

pep-0686.rst

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+PEP: 686
+Title: Make UTF-8 mode default
+Author: Inada Naoki <[email protected]>
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 18-Mar-2022
+Python-Version: 3.12
+
+
+Abstract
+========
+
+This PEP proposes making UTF-8 mode [1]_ on by default.
+
+With this change, Python uses UTF-8 for default encoding of files, stdio, and
+pipes consistently.
+
+
+Motivation
+==========
+
+UTF-8 becomes de-facto standard text encoding.
+
+* Default encoding of Python source files is UTF-8.
+* JSON, TOML, YAML uses UTF-8.
+* Most text editors including VS Code and Windows notepad use UTF-8 by
+  default.
+* Most websites and text data on the internet uses UTF-8.
+* And many other popular programming languages including node.js, Go, Rust,
+  Ruby, and Java uses UTF-8 by default.
+
+Changing the default encoding to UTF-8 makes Python easier to interoperate
+with them.
+
+Additionally, many Python developers using Unix forget that the default
+encoding is platform dependant. 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 caused many bugs.
+
+
+Specification
+=============
+
+Changes to UTF-8 mode
+---------------------
+
+Currently, UTF-8 mode affects to ``locale.getpreferredencoding()``.
+
+This PEP proposes to remove this override. UTF-8 mode will not affect to
+``locale`` module.
+
+After this change, UTF-8 mode affects to:
+
+* stdin, stdout, stderr
+
+  * User can override it with ``PYTHONIOENCODING``.
+
+* filesystem encoding
+
+* ``TextIOWrapper`` and APIs using it including ``open()``,
+  ``Path.read_text()``, ``subprocess.Popen(cmd, text=True)``, etc...
+
+This change will be introduced in Python 3.11 if possible.
+
+
+Enable UTF-8 mode by default
+----------------------------
+
+Python enables UTF-8 mode by default.
+
+User can still disable UTF-8 mode by setting ``PYTHONUTF8=0`` or ``-X utf8=0``.
+
+
+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 very 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 to keep using locale encoding. [2]_
+
+
+Preceding examples
+==================
+
+* Ruby changed the default ``external_encoding`` to UTF-8 on Windows in Ruby
+  3.0 (2020). [3]_
+* Java changed the default text encoding to UTF-8 in JDK 18. (2022). [4]_
+
+Both Ruby and Java have an option for backward compatibility.
+They don't provide any warning like ``EncodingWarning`` [2]_ in Python for use
+of the default encoding.
+
+
+Rejected Alternative
+====================
+
+Deprecate implicit encoding
+---------------------------
+
+Deprecating use of the default encoding is considered.
+
+But there are many cases user uses the default encoding when just they need
+ASCII. And some users use Python only on Unix with UTF-8 locale.
+
+So forcing users to specify the ``encoding`` option everywhere is too painful.
+
+Java also rejected this idea [4]_.
+
+
+How to teach this
+=================
+
+For new users, this change reduces things that need to teach.
+
+Users can delay learning about text encoding until they need to handle
+non-UTF-8 text files.
+
+For existing users, see `Backward compatibility`_ section.
+
+
+Resources
+=========
+
+.. [1] `PEP 540 – Add a new UTF-8 Mode`__
+
+  __ https://peps.python.org/pep-0540/
+
+.. [2] `PEP 597 – Add optional EncodingWarning`__
+
+  __ https://peps.python.org/pep-0597/
+
+.. [3] `Set default for Encoding.default_external to UTF-8 on Windows`__
+
+   __ https://bugs.ruby-lang.org/issues/16604
+
+.. [4] `JEP 400: UTF-8 by Default`__
+
+   __ 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.