python-peps/pep-0597.rst

PEP: 597
Title: Add optional EncodingWarning
Last-Modified: 21-Mar-2021
Author: Inada Naoki <songofacandy@gmail.com>
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 05-Jun-2019
Python-Version: 3.10


Abstract
========

Add a new warning category ``EncodingWarning``. It is emitted when
``encoding`` option is omitted and the default encoding is a locale
encoding.

The warning is disabled by default. New ``-X warn_default_encoding``
command-line option and ``PYTHONWARNDEFAULTENCODING`` environment
variable are used to enable the warnings.

``encoding="locale"`` option is added too. It is used to specify
locale encoding explicitly.


Motivation
==========

Using the default encoding is a common mistake
----------------------------------------------

Developers using macOS or Linux may forget that the default encoding
is not always UTF-8.

For example, ``long_description = open("README.md").read()`` in
``setup.py`` is a common mistake. Many Windows users can not install
the package if there is at least one non-ASCII character (e.g. emoji)
in the ``README.md`` file which is encoded in UTF-8.

For example, 489 packages of the 4000 most downloaded packages from
PyPI used non-ASCII characters in README. And 82 packages of them
can not be installed from source package when locale encoding is
ASCII. [1]_ They used the default encoding to read README or TOML
file.

Another example is ``logging.basicConfig(filename="log.txt")``.
Some users expect UTF-8 is used by default, but locale encoding is
used actually. [2]_

Even Python experts assume that default encoding is UTF-8.
It creates bugs that happen only on Windows. See [3]_, [4]_, [5]_,
and [6]_ for example.

Emitting a warning when the ``encoding`` option is omitted will help
to find such mistakes.


Explicit way to use locale-specific encoding
--------------------------------------------

``open(filename)`` isn't explicit about which encoding is expected:

* Expects ASCII (not a bug, but inefficient on Windows)
* Expects UTF-8 (bug or platform-specific script)
* Expects the locale encoding.

In this point of view, ``open(filename)`` is not readable.

``encoding=locale.getpreferredencoding(False)`` can be used to
specify the locale encoding explicitly. But it is too long and easy
to misuse. (e.g. forget to pass ``False`` to its parameter)

This PEP provides an explicit way to specify the locale encoding.


Prepare to change the default encoding to UTF-8
-----------------------------------------------

Since UTF-8 becomes de-facto standard text encoding, we might change
the default text encoding to UTF-8 in the future.

But this change will affect many applications and libraries. If we
start emitting ``DeprecationWarning`` everywhere ``encoding`` option
is omitted, it will be too noisy and painful.

Although this PEP doesn't propose to change the default encoding,
this PEP will help the change:

* Reduce the number of omitted ``encoding`` options in many libraries
  before we start emitting the ``DeprecationWarning`` by default.

* Users will be able to use ``encoding="locale"`` option to suppress
  the warning without dropping Python 3.10 support.


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

``EncodingWarning``
--------------------

Add a new ``EncodingWarning`` warning class which is a subclass of
``Warning``. It is used to warn when the ``encoding`` option is
omitted and the default encoding is locale-specific.


Options to enable the warning
------------------------------

``-X warn_default_encoding`` option and the
``PYTHONWARNDEFAULTENCODING`` environment variable are added. They
are used to enable ``EncodingWarning``.

``sys.flags.encoding_warning`` is also added. The flag represents
``EncodingWarning`` is enabled.

When the option is enabled, ``io.TextIOWrapper()``, ``open()``, and
other modules using them will emit ``EncodingWarning`` when the
``encoding`` is omitted.

Since ``EncodingWarning`` is a subclass of ``Warning``, they are
shown by default, unlike ``DeprecationWarning``.


``encoding="locale"`` option
----------------------------

``io.TextIOWrapper`` accepts ``encoding="locale"`` option. It means
same to current ``encoding=None``. But ``io.TextIOWrapper`` doesn't
emit ``EncodingWarning`` when ``encoding="locale"`` is specified.


``io.text_encoding()``
-----------------------

``io.text_encoding()`` is a helper function for functions having
``encoding=None`` option and passing it to ``io.TextIOWrapper()`` or
``open()``.

Pure Python implementation will be like this::

   def text_encoding(encoding, stacklevel=1):
       """Helper function to choose the text encoding.

       When *encoding* is not None, just return it.
       Otherwise, return the default text encoding (i.e., "locale").

       This function emits EncodingWarning if *encoding* is None and
       sys.flags.encoding_warning is true.

       This function can be used in APIs having encoding=None option
       and pass it to TextIOWrapper or open.
       But please consider using encoding="utf-8" for new APIs.
       """
       if encoding is None:
           if sys.flags.encoding_warning:
               import warnings
               warnings.warn("'encoding' option is omitted",
                            EncodingWarning, stacklevel + 2)
           encoding = "locale"
       return encoding

For example, ``pathlib.Path.read_text()`` can use the function like:

.. code-block::

   def read_text(self, encoding=None, errors=None):
       encoding = io.text_encoding(encoding)
       with self.open(mode='r', encoding=encoding, errors=errors) as f:
           return f.read()

By using ``io.text_encoding()``, ``EncodingWarning`` is emitted for
the caller of ``read_text()`` instead of ``read_text()`` itself.


Affected stdlibs
-----------------

Many stdlibs will be affected by this change.

Most APIs accepting ``encoding=None`` will use ``io.text_encoding()``
as written in the previous section.

Where using locale encoding as the default encoding is reasonable,
``encoding="locale"`` will be used instead. For example,
the ``subprocess`` module will use locale encoding for the default
encoding of the pipes.

Many tests use ``open()`` without ``encoding`` specified to read
ASCII text files. They should be rewritten with ``encoding="ascii"``.


Rationale
=========

Opt-in warning
---------------

Although ``DeprecationWarning`` is suppressed by default, emitting
``DeprecationWarning`` always when the ``encoding`` option is omitted
would be too noisy.

Noisy warnings may lead developers to dismiss the
``DeprecationWarning``.


"locale" is not a codec alias
-----------------------------

We don't add the "locale" to the codec alias because locale can be
changed in runtime.

Additionally, ``TextIOWrapper`` checks ``os.device_encoding()``
when ``encoding=None``. This behavior can not be implemented in
the codec.


Backward Compatibility
======================

The new warning is not emitted by default. So this PEP is 100%
backward compatible.


Forward Compatibility
=====================

``encoding="locale"`` option is not forward compatible. Codes
using the option will not work on Python older than 3.10. It will
raise ``LookupError: unknown encoding: locale``.

Until developers can drop Python 3.9 support, ``EncodingWarning``
can be used only for finding missing ``encoding="utf-8"`` options.


How to teach this
=================

For new users
-------------

Since ``EncodingWarning`` is used to write a cross-platform code,
no need to teach it to new users.

We can just recommend using UTF-8 for text files and use
``encoding="utf-8"`` when opening test files.


For experienced users
---------------------

Using ``open(filename)`` to read text files encoded in UTF-8 is a
common mistake. It may not work on Windows because UTF-8 is not the
default encoding.

You can use ``-X warn_default_encoding`` or
``PYTHONWARNDEFAULTENCODING=1`` to find this type of mistake.

Omitting ``encoding`` option is not a bug when opening text files
encoded in locale encoding. But ``encoding="locale"`` is recommended
after Python 3.10 because it is more explicit.


Reference Implementation
========================

https://github.com/python/cpython/pull/19481


Discussions
===========

The latest discussion thread is:
https://mail.python.org/archives/list/python-dev@python.org/thread/SFYUP2TWD5JZ5KDLVSTZ44GWKVY4YNCV/


* Why not implement this in linters?

  * ``encoding="locale"`` and ``io.text_encoding()`` must be in
    Python.

  * It is difficult to find all caller of functions wrapping
    ``open()`` or ``TextIOWrapper()``. (See ``io.text_encoding()``
    section.)

* Many developers will not use the option.

  * Some developers use the option and report the warnings to
    libraries they use. So the option is worth enough even though
    many developers won't use it.

  * For example, I find [7]_ and [8]_ by running
    ``pip install -U pip`` and find [9]_ by running ``tox``
    with the reference implementation. It demonstrates how this
    option can be used to find potential issues.


References
==========

.. [1] "Packages can't be installed when encoding is not UTF-8"
       (https://github.com/methane/pep597-pypi-ascii)

.. [2] "Logging - Inconsistent behaviour when handling unicode"
       (https://bugs.python.org/issue37111)

.. [3] Packaging tutorial in packaging.python.org didn't specify
       encoding to read a ``README.md``
       (https://github.com/pypa/packaging.python.org/pull/682)

.. [4] ``json.tool`` had used locale encoding to read JSON files.
       (https://bugs.python.org/issue33684)

.. [5] site: Potential UnicodeDecodeError when handling pth file
       (https://bugs.python.org/issue33684)

.. [6] pypa/pip: "Installing packages fails if Python 3 installed
       into path with non-ASCII characters"
       (https://github.com/pypa/pip/issues/9054)

.. [7] "site: Potential UnicodeDecodeError when handling pth file"
       (https://bugs.python.org/issue43214)

.. [8] "[pypa/pip] Use ``encoding`` option or binary mode for open()"
       (https://github.com/pypa/pip/pull/9608)

.. [9] "Possible UnicodeError caused by missing encoding="utf-8""
       (https://github.com/tox-dev/tox/issues/1908)


Copyright
=========

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

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