195 lines
5.8 KiB
ReStructuredText
195 lines
5.8 KiB
ReStructuredText
PEP: 686
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Title: Make UTF-8 mode default
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Author: Inada Naoki <songofacandy@gmail.com>
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Discussions-To: https://discuss.python.org/t/14737
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 18-Mar-2022
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Python-Version: 3.13
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Post-History: `18-Mar-2022 <https://discuss.python.org/t/14435>`__,
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`31-Mar-2022 <https://discuss.python.org/t/14737>`__
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Abstract
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========
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This PEP proposes enabling :pep:`UTF-8 mode <540>` by default.
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With this change, Python consistently uses UTF-8 for default encoding of
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files, stdio, and pipes.
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Motivation
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==========
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UTF-8 becomes de facto standard text encoding.
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* The default encoding of Python source files is UTF-8.
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* JSON, TOML, YAML use UTF-8.
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* Most text editors, including Visual Studio Code and Windows Notepad use
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UTF-8 by default.
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* Most websites and text data on the internet use UTF-8.
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* And many other popular programming languages, including Node.js, Go, Rust,
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and Java uses UTF-8 by default.
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Changing the default encoding to UTF-8 makes it easier for Python to
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interoperate with them.
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Additionally, many Python developers using Unix forget that the default
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encoding is platform dependent.
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They omit to specify ``encoding="utf-8"`` when they read text files encoded
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in UTF-8 (e.g. JSON, TOML, Markdown, and Python source files).
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Inconsistent default encoding causes many bugs.
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Specification
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=============
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Enable UTF-8 mode by default
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----------------------------
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Python will enable UTF-8 mode by default from Python 3.13.
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Users can still disable UTF-8 mode by setting ``PYTHONUTF8=0`` or
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``-X utf8=0``.
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``locale.getencoding()``
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------------------------
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Since UTF-8 mode affects ``locale.getpreferredencoding(False)``,
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we need an API to get locale encoding regardless of UTF-8 mode.
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``locale.getencoding()`` will be added for this purpose.
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It returns locale encoding too, but ignores UTF-8 mode.
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When ``warn_default_encoding`` option is specified,
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``locale.getpreferredencoding()`` will emit ``EncodingWarning`` like
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``open()`` (see also :pep:`597`).
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This API was added in Python 3.11.
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Fixing ``encoding="locale"`` option
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-----------------------------------
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:pep:`597` added the ``encoding="locale"`` option to the ``TextIOWrapper``.
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This option is used to specify the locale encoding explicitly.
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``TextIOWrapper`` should use locale encoding when the option is specified,
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regardless of default text encoding.
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But ``TextIOWrapper`` uses ``"UTF-8"`` in UTF-8 mode even if
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``encoding="locale"`` is specified for now.
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This behavior is inconsistent with the :pep:`597` motivation.
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It is because we didn't expect making UTF-8 mode default when Python
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changes its default text encoding.
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This inconsistency should be fixed before making UTF-8 mode default.
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``TextIOWrapper`` should use locale encoding when ``encoding="locale"`` is
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passed even in UTF-8 mode.
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This issue was fixed in Python 3.11.
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Backward Compatibility
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======================
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Most Unix systems use UTF-8 locale and Python enables UTF-8 mode when its
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locale is C or POSIX.
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So this change mostly affects Windows users.
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When a Python program depends on the default encoding, this change may cause
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``UnicodeError``, mojibake, or even silent data corruption.
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So this change should be announced loudly.
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This is the guideline to fix this backward compatibility issue:
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1. Disable UTF-8 mode.
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2. Use ``EncodingWarning`` (:pep:`597`) to find every places UTF-8 mode
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affects.
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* If ``encoding`` option is omitted, consider using ``encoding="utf-8"``
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or ``encoding="locale"``.
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* If ``locale.getpreferredencoding()`` is used, consider using
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``"utf-8"`` or ``locale.getencoding()``.
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3. Test the application with UTF-8 mode.
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Preceding examples
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==================
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* Ruby `changed <Feature #16604_>`__ the default ``external_encoding``
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to UTF-8 on Windows in Ruby 3.0 (2020).
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* Java `changed <JEP 400_>`__ the default text encoding
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to UTF-8 in JDK 18. (2022).
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Both Ruby and Java have an option for backward compatibility.
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They don't provide any warning like :pep:`597`'s ``EncodingWarning``
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in Python for use of the default encoding.
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Rejected Alternative
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====================
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Deprecate implicit encoding
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---------------------------
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Deprecating the use of the default encoding is considered.
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But there are many cases that the default encoding is used for reading/writing
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only ASCII text.
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Additionally, such warnings are not useful for non-cross platform applications
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run on Unix.
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So forcing users to specify the ``encoding`` everywhere is too painful.
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Emitting a lot of ``DeprecationWarning`` will lead users ignore warnings.
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:pep:`387` requires adding a warning for backward incompatible changes.
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But it doesn't require using ``DeprecationWarning``.
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So using optional ``EncodingWarning`` doesn't violate the :pep:`387`.
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Java also rejected this idea in `JEP 400`_.
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Use ``PYTHONIOENCODING`` for PIPEs
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----------------------------------
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To ease backward compatibility issue, using ``PYTHONIOENCODING`` as the
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default encoding of PIPEs in the ``subprocess`` module is considered.
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With this idea, users can use legacy encoding for
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``subprocess.Popen(text=True)`` even in UTF-8 mode.
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But this idea makes "default encoding" complicated.
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And this idea is also backward incompatible.
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So this idea is rejected. Users can disable UTF-8 mode until they replace
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``text=True`` with ``encoding="utf-8"`` or ``encoding="locale"``.
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How to teach this
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=================
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For new users, this change reduces things that need to teach.
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Users don't need to learn about text encoding in their first year.
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They should learn it when they need to use non-UTF-8 text files.
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For existing users, see the `Backward compatibility`_ section.
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References
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==========
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.. _Feature #16604: https://bugs.ruby-lang.org/issues/16604
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.. _JEP 400: https://openjdk.java.net/jeps/400
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Copyright
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=========
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This document is placed in the public domain or under the
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CC0-1.0-Universal license, whichever is more permissive.
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