python-peps/pep-0597.rst

PEP: 597
Title: Soft deprecation of omitting encoding
Last-Modified: 14-Apr-2020
Author: Inada Naoki  <songofacandy@gmail.com>
Discussions-To: https://discuss.python.org/t/3880
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 05-Jun-2019
Python-Version: 3.9


Abstract
========

This PEP proposes:

* ``TextIOWrapper`` raises a ``PendingDeprecationWarning`` when the
  ``encoding`` option is not specified, and dev mode is enabled.

* Add ``encoding="locale"`` option to ``TextIOWrapper``.  It behaves
  like ``encoding=None`` but don't raise a warning.


Motivation
==========

Omitting encoding is 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.

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 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_] and [4_].

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


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

We chose to use locale encoding for the default text encoding
in Python 3.0.  But UTF-8 has been adopted very widely since then.

We might change the default text encoding to UTF-8 in the future.
But this change will affect many applications and libraries.
Many ``DeprecationWarning`` will be raised if we start raising
the warning by default.  It will be too noisy.

While this PEP doesn't cover the change, this PEP will help to reduce
the number of ``DeprecationWarning`` in the future.


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

Raising a PendingDeprecationWarning
---------------------------------------

``TextIOWrapper`` raises the ``PendingDeprecationWarning`` when the
``encoding`` option is omitted, and dev mode is enabled.


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

When ``encoding="locale"`` is specified to the ``TextIOWrapper``, it
behaves same to ``encoding=None``.  In detail, the encoding is
chosen by:

1. ``os.device_encoding(buffer.fileno())``
2. ``locale.getpreferredencoding(False)``

This option can be used to suppress the ``PendingDeprecationWarning``.


``io.text_encoding``
--------------------

``TextIOWrapper`` is used indirectly in most cases.  For example, ``open``, and ``pathlib.Path.read_text()`` use it.  Warning to these
functions doesn't make sense.  Callers of these functions should be warned instead.

``io.text_encoding(encoding, stacklevel=1)`` is a helper function for it.
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 ("locale" for now),
       and raise a PendingDeprecationWarning in dev mode.

       This function can be used in APIs having encoding=None option.
       But please consider encoding="utf-8" for new APIs.
       """
       if encoding is None:
           if sys.flags.dev_mode:
               import warnings
               warnings.warn(
                       "'encoding' option is not specified. The default encoding "
                       "will be changed to 'utf-8' in the future",
                       PendingDeprecationWarning, stacklevel + 2)
           encoding = "locale"
       return encoding

``pathlib.Path.read_text()`` can use this function like this::

   def read_text(self, encoding=None, errors=None):
       """
       Open the file in text mode, read it, and close the file.
       """
       encoding = io.text_encoding(encoding)
       with self.open(mode='r', encoding=encoding, errors=errors) as f:
           return f.read()


subprocess module doesn't warn
------------------------------

While the subprocess module uses TextIOWrapper, it doesn't raise
``PendingDeprecationWarning``.  It uses the "locale" encoding
by default.


Rationale
=========

"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.


Use a PendingDeprecationWarning
-------------------------------

This PEP doesn't make decision about changing default text encoding.
So we use ``PendingDeprecationWarning`` instead of ``DeprecationWarning`` for now.


Raise warning only in dev mode
------------------------------

This PEP will produce a huge amount of ``PendingDeprecationWarning``.
It will be too noisy for most Python developers.

We need to fix warnings in standard library, pip, and major dev tools
like ``pytest`` before raise this warning by default.


subprocess module doesn't warn
------------------------------

The default encoding for PIPE is relating to the encoding of the stdio.
It should be discussed later.


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

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


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)


Copyright
=========

This document has been placed in the public domain.


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