PEP 597: Add PYTHONTEXTENCODING envvar (#1102)
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pep-0597.rst
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pep-0597.rst
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@ -1,5 +1,5 @@
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PEP: 597
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Title: Use UTF-8 for default text file encoding
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Title: Add PYTHONTEXTENCODING environment variable
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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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@ -12,46 +12,32 @@ 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 specified.
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(hereinafter called "locale encoding") when ``encoding`` is not
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specified.
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This PEP proposes changing the default text encoding to "UTF-8"
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regardless of platform or locale.
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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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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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Motivation
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==========
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People assume it is always UTF-8
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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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Package authors using macOS or Linux may forget that the default encoding
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is not always UTF-8.
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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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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 any
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other non-ASCII character in the ``README.md`` file, many Windows users
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cannot install the package due to a ``UnicodeDecodeError``.
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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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Active code page is not stable
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------------------------------
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Some tools on Windows change the active code page to 65001 (UTF-8), and
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Microsoft is using UTF-8 and cp65001 more widely in recent versions of
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Windows 10.
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For example, "Command Prompt" uses the legacy code page by default.
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But the Windows Subsystem for Linux (WSL) changes the active code page to
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65001, and ``python.exe`` can be executed from the WSL. So ``python.exe``
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executed from the legacy console and from the WSL cannot read text files
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written by each other.
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But many Windows users don't understand which code page is active.
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So changing the default text file encoding based on the active code page
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causes confusion.
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Consistent default text encoding will make Python behavior more expectable
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and easier to learn.
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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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@ -59,77 +45,104 @@ Using UTF-8 by default is easier on new programmers
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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 data
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written in UTF-8 from the Internet, they are forced to learn about encoding.
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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 2019
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Update. (Note that Python 3.9 will be released in 2021.)
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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 encoding
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until they need to handle text files encoded in encoding other than UTF-8.
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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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People assume it is always UTF-8
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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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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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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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Specification
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=============
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From Python 3.9, the default encoding of ``TextIOWrapper`` and ``open()`` is
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changed from ``locale.getpreferredencoding(False)`` to "UTF-8".
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``PYTHONTEXTENCODING`` environment variable
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-------------------------------------------
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When there is device encoding (``os.device_encoding(buffer.fileno())``),
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it still supersedes the default encoding.
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``PYTHONTEXTENCODING`` environment variable can be used to specify the
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default text encoding.
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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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Unaffected areas
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----------------
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``sys.gettextencoding()``
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-------------------------
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Unlike UTF-8 mode, ``locale.getpreferredencoding(False)`` still respects
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locale encoding.
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When ``PYTHONTEXTENCODING`` is specified, this function return it.
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``stdin``, ``stdout``, and ``stderr`` continue to respect locale encoding
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as well. For example, these commands do not cause mojibake regardless of the
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active code page::
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> python -c "print('こんにちは')" | more
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こんにちは
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> python -c "print('こんにちは')" > temp.txt
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> type temp.txt
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こんにちは
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Pipes and TTY should use the locale encoding:
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* ``subprocess`` and ``os.popen`` use the locale encoding because the
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subprocess will use the locale encoding.
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* ``getpass.getpass`` uses the locale encoding when using TTY.
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When it is not specified, this function returns
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``locale.getpreferredencoding(False)``.
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Affected APIs
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-------------
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``encoding="locale"`` option
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----------------------------
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All other code using the default encoding of ``TextIOWrapper`` or ``open`` are
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affected. This is an incomplete list of APIs affected by this PEP:
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``TextIOWrapper`` now accepts ``encoding="locale"`` option.
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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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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 thier 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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These APIs will always use "UTF-8" when opening text files.
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Deprecation Warning
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-------------------
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From 3.8 onwards, ``DeprecationWarning`` is shown when encoding is omitted and
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the locale encoding is not UTF-8. This helps not only when writing
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forward-compatible code, but also when investigating an unexpected
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``UnicodeDecodeError`` caused by assuming the default text encoding is UTF-8.
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(See `People assume it is always UTF-8`_ above.)
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Rationale
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=========
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@ -139,12 +152,22 @@ Why not just enable UTF-8 mode by default?
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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 forget
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the default encoding is not always UTF-8. More scripts will be written
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assuming the default encoding is UTF-8.
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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 better
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even if UTF-8 mode is enabled by default at some point.
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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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@ -158,55 +181,10 @@ 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 tools.
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So the locale encoding should be respected.
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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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Why not always warn when encoding is omitted?
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---------------------------------------------
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Omitting encoding is a common mistake when writing portable code.
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But when portability does not matter, assuming UTF-8 is not so bad because
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Python already implements locale coercion (:pep:`538`) and UTF-8 mode
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(:pep:`540`).
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And these scripts will become portable when the default encoding is changed
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to UTF-8.
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Backward compatibility
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======================
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There may be scripts relying on the locale encoding or active code page not
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being UTF-8. They must be rewritten to specify ``encoding`` explicitly.
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* If the script assumes ``latin1`` or ``cp932``, ``encoding="latin1"``
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or ``encoding="cp932"`` should be used.
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* If the script is designed to respect locale encoding,
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``locale.getpreferredencoding(False)`` should be used.
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There are non-portable short forms of
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``locale.getpreferredencoding(False)``.
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* On Windows, ``"mbcs"`` can be used instead.
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* On Unix, ``os.fsencoding()`` can be used instead.
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Note that such scripts will be broken even without upgrading Python, such as
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when:
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* Upgrading Windows
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* Changing the language setting
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* Changing terminal from legacy console to a modern one
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* Using tools which do ``chcp 65001``
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How to Teach This
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=================
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When opening text files, "UTF-8" is used by default. It is consistent with
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the default encoding used for ``text.encode()``.
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Anyway, ``PYTHONIOENCODING`` can be used to change these encodings.
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Reference Implementation
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@ -218,35 +196,74 @@ To be written.
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Rejected Ideas
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==============
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To be discussed.
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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 difficulity 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 heuritics 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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Alias for locale encoding
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-------------------------
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Easy way to set ``PYTHONTEXTENCODING``
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--------------------------------------
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``encoding=locale.getpreferredencoding(False)`` is too long, and
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``"mbcs"`` and ``os.fsencoding()`` are not portable.
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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 may be possible to add a new "locale" encoding alias as an easy and
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portable version of ``locale.getpreferredencoding(False)``.
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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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The difficulty of this is uncertain because ``encodings`` is currently
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imported prior to ``_bootlocale``.
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Another option is for ``TextIOWrapper`` to treat `"locale"` as a special
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case::
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Commandline option
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------------------
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if encoding == "locale":
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encoding = locale.getpreferredencoding(False)
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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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Copyright
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=========
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@ -261,4 +278,3 @@ This document has been placed in the public domain.
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fill-column: 70
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coding: utf-8
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End:
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