PEP 597: Rewrite (#1296)
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pep-0597.rst
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pep-0597.rst
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PEP: 597
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Title: Add PYTHONTEXTENCODING environment variable
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Title: Enable UTF-8 mode by default on Windows
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Author: Inada Naoki <songofacandy@gmail.com>
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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: 05-Jun-2019
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Python-Version: 3.9
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Python-Version: 3.10
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Abstract
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========
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Currently, ``TextIOWrapper`` uses ``locale.getpreferredencoding(False)``
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(hereinafter called "locale encoding") when ``encoding`` is not
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specified.
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This PEP proposes adding ``PYTHONTEXTENCODING`` environment
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variable to override the default text encoding since Python 3.9.
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This PEP proposes to make UTF-8 mode [#]_ enabled by default on
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Windows.
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The goal of this PEP is providing "UTF-8 by default" experience to
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Windows users, because macOS, Linux, Android, iOS users use UTF-8
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by default already.
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Windows users like Unix users.
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Motivation
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==========
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UTF-8 is the best encoding for saving unicode text
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--------------------------------------------------
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UTF-8 is the best encoding nowdays
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----------------------------------
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String in Python 3 is unicode. Encoding valid unicode strings with
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UTF-8 should not fail.
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Popular text editors like VS Code uses UTF-8 by default.
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Even Microsoft Notepad uses UTF-8 by default since the Windows 10
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May 2019 Update.
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Additionally, the default encoding of Python source files is UTF-8.
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On the other hand, most locale encoding used in Windows can not
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save all valid unicode string. It will cause UnicodeEncodeError
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or it may not round-trip. User may lost their data in such case.
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UTF-8 is the best encoding for saving text when user don't specify
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any encoding.
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Using UTF-8 by default is easier on new programmers
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---------------------------------------------------
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We can assume that most Python programmers use UTF-8 for most text
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files.
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Python is one of the most popular first programming languages.
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New programmers may not know about encoding. When they download text
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data written in UTF-8 from the Internet, they are forced to learn
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about encoding.
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Popular text editors like VS Code or Atom use UTF-8 by default.
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Even Microsoft Notepad uses UTF-8 by default since the Windows 10 May
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2019 Update. (Note that Python 3.9 will be released in 2021.)
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Additionally, the default encoding of Python source files is UTF-8.
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We can assume new Python programmers who don't know about encoding
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use editors which use UTF-8 by default.
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It would be nice if new programmers are not forced to learn about
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encoding until they need to handle text files encoded in encoding
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other than UTF-8.
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New programmers may not know about encoding. If the default encoding
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for text files is UTF-8, they can learn about encoding when they need
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to handle legacy encoding.
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People assume it is always UTF-8
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--------------------------------
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People assume the default encoding is UTF-8 already
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---------------------------------------------------
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Package authors using macOS or Linux may forget that the default
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encoding is not always UTF-8.
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Developers using macOS or Linux may forget that the default encoding
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is not always UTF-8.
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For example, ``long_description = open("README.md").read()`` in
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``setup.py`` is a common mistake. If there is at least one emoji or
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any other non-ASCII character in the ``README.md`` file, many Windows
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users cannot install the package due to a ``UnicodeDecodeError``.
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``setup.py`` is a common mistake. Many Windows users can not install
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the package if there is at least one emoji or any other non-ASCII
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character in the ``README.md`` file.
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Even Python experts assume that default encoding is UTF-8.
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It creates bugs that happen only on Windows. See [#]_ [#]_.
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Consistent with default encoding
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--------------------------------
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Python has ``sys.defaultencoding()`` which is always "UTF-8".
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``str.encode()`` uses "UTF-8" when encoding is omitted.
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Using "UTF-8" for text files are consistent with it. It makes Python
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more easy to learn language.
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Changing the default text encoding to UTF-8 will help many Windows
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users.
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Specification
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=============
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``PYTHONTEXTENCODING`` environment variable
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-------------------------------------------
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Enable UTF-8 mode on Windows unless it is disabled explicitly.
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``PYTHONTEXTENCODING`` environment variable can be used to specify the
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default text encoding.
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UTF-8 mode affects these areas:
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Unlike ``PYTHONIOENCODING``, it doesn't accept error handler.
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``PYTHONIOENCODING`` support it because changing error handler of
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stdio was difficult. But it is not true for regular files.
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* ``locale.getpreferredencoding`` returns "UTF-8".
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* ``open``, ``subprocess.Popen``, ``pathlib.Path.read_text``,
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``ZipFile.open``, and many other functions use UTF-8 when
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the ``encoding`` option is omitted.
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* The stdio uses "UTF-8" always.
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* Console I/O uses "UTF-8" already [#]_. So this affects
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only when the stdio are redirected.
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On the other hand, UTF-8 mode doesn't affect to "mbcs" encoding.
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Users can still use system encoding by chosing "mbcs" encoding
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explicitly.
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``sys.gettextencoding()``
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-------------------------
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Backwards Compatibility
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=======================
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When ``PYTHONTEXTENCODING`` is specified, this function return it.
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Some existing applications assuming the default text encoding is the
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system encoding (a.k.a. ANSI encoding) will be broken by this change.
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When it is not specified, this function returns
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``locale.getpreferredencoding(False)``.
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Users can disable the UTF-8 mode by environment variable
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(``PYTHONUTF8=0``) or command line option (``-Xutf8=0``) for backward
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compatibility.
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``encoding="locale"`` option
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----------------------------
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Rejected Ideas
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===============
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``TextIOWrapper`` now accepts ``encoding="locale"`` option.
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Change the default encoding of TextIOWrapper to "UTF-8"
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-------------------------------------------------------
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"locale" is not real encoding or alias.
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This is just a shortcut of
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``encoding=locale.getpreferredencoding(False)``.
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This idea changed the default encoding to UTF-8 always, regardless of
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platform, locale, and environment variables.
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While this idea looks ideal in terms of consistency, it will cause
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backward compatibility problems.
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Changes in stdlibs
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------------------
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``TextIOWrapper`` uses ``sys.gettextencoding()`` where
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``locale.getpreferredencoding(False)`` is used.
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But ``stdin``, ``stdout``, and ``stderr`` continue to respect
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locale encoding as well. ``PYTHONIOENCODING`` can be used to
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override their encoding.
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Pipes and TTY should use the "locale" encoding. UTF-8 mode [1]_
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can be used to override these encoding:
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* ``subprocess`` and ``os.popen`` use the "locale" encoding because
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the subprocess will use the locale encoding.
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* ``getpass.getpass`` uses the "locale" encoding when using TTY.
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All other code using the default encoding are not modified.
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They can be overridden by ``PYTHONTEXTENCODING``.
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This is an incomplete list:
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* ``lzma.open``, ``gzip.open``, ``bz2.open``, ``ZipFile.read_text``
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* ``socket.makefile``
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* ``tempfile.TemporaryFile``, ``tempfile.NamedTemporaryFile``
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* ``trace.CoverageResults.write_results_file``
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Rationale
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=========
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Why not just enable UTF-8 mode by default?
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------------------------------------------
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This PEP is not mutually exclusive to UTF-8 mode.
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If we enable UTF-8 mode by default, even people using Windows will
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forget the default encoding is not always UTF-8. More scripts will
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be written assuming the default encoding is UTF-8.
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So changing the default encoding of text files to UTF-8 would be
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better even if UTF-8 mode is enabled by default at some point.
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Why is "locale" not an alias codec?
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-----------------------------------
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For backward compatibility, ``io.TextIOWrapper`` calls
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``locale.getpreferredencoding(False)`` every time when
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``encoding="locale"`` is specified.
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It respects changing locale after Python startup.
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Why not change std(in|out|err) encoding too?
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--------------------------------------------
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Even when the locale encoding is not UTF-8, there can be many UTF-8
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text files. These files could be downloaded from the Internet or
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written by modern text editors.
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On the other hand, terminal encoding is assumed to be the same as
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locale encoding. And other tools are assumed to read and write the
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locale encoding as well.
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std(in|out|err) are likely to be connected to a terminal or other
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tools. So the locale encoding should be respected.
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Anyway, ``PYTHONIOENCODING`` can be used to change these encodings.
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Utilizing the UTF-8 mode seems better than adding one more backward
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compatibility option like ``PYTHONLEGACYWINDOWSSTDIO``.
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Reference Implementation
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@ -193,76 +112,13 @@ Reference Implementation
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To be written.
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Rejected Ideas
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==============
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Change the default text encoding
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--------------------------------
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Previous version of this PEP tried to change the default encoding
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to UTF-8.
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But we should have deprecation period long enough. Between the
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deprecation period, users can not change the default text encoding.
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And there are many difficulty there:
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* Omitting ``encoding`` option is very common.
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* If we raise ``DeprecationWarning`` always, it will be too noisy.
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* We can not assume how user use it. Complicated heuristics may be
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needed to raise ``DeprecationWarning`` only when it is really
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needed.
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* Users of legacy systems may dismiss warning.
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* They may not check the warning.
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* They may upgrade Python from 2.7 after 2020.
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Additionally, Microsoft is improving UTF-8 support of Windows 10
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recently.
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There are no public plan for future UTF-8 support yet. But Python may
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be able to change the default encoding without painful deprecation
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period in the future.
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Open Issues
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===========
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Easy way to set ``PYTHONTEXTENCODING``
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--------------------------------------
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UTF-8 is the best encoding for new users. But setting environment
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variables is not easy enough to new users.
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It would be helpfule if Python on Windows can provide easy way to set
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``PYTHONTEXTENCODING=UTF-8`` even after Python is installed.
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Commandline option
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------------------
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If there is reasonable use case for changing default text encoding
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per process, command line option should be considered.
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C-API
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-----
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The default text encoding should be able to configured from C.
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This will be considered when writing reference Implementation.
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Additionally, C-API like ``PySys_GetTextEncoding()`` should be
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considered too.
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References
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==========
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.. [1] PEP 540, Add a new UTF-8 Mode
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(https://www.python.org/dev/peps/pep-0540/)
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.. [#] `PEP 540 -- Add a new UTF-8 Mode <https://www.python.org/dev/peps/pep-0540/>`_
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.. [#] https://github.com/pypa/packaging.python.org/pull/682
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.. [#] https://bugs.python.org/issue33684
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.. [#] `PEP 528 -- Change Windows console encoding to UTF-8 <https://www.python.org/dev/peps/pep-0528/>`_
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Copyright
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