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PEP: 597
Title: Add optional EncodingWarning
Author: Inada Naoki <songofacandy@gmail.com>
Status: Final
Type: Standards Track
Created: 05-Jun-2019
Python-Version: 3.10


Abstract
========

Add a new warning category ``EncodingWarning``. It is emitted when the
``encoding`` argument to ``open()`` is omitted and the default
locale-specific encoding is used.

The warning is disabled by default. A new ``-X warn_default_encoding``
command-line option and a new ``PYTHONWARNDEFAULTENCODING`` environment
variable can be used to enable it.

A ``"locale"`` argument value for ``encoding`` is added too. It
explicitly specifies that the locale encoding should be used, silencing
the warning.


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, using ``long_description = open("README.md").read()`` in
``setup.py`` is a common mistake. Many Windows users cannot install
such packages if there is at least one non-ASCII character
(e.g. emoji, author names, copyright symbols, and the like)
in their UTF-8-encoded ``README.md`` file.

Of the 4000 most downloaded packages from PyPI, 489 use non-ASCII
characters in their README, and 82 fail to install from source on
non-UTF-8 locales due to not specifying an encoding for a non-ASCII
file. [1]_

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

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

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


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

``open(filename)`` isn't explicit about which encoding is expected:
* If ASCII is assumed, this isn't a bug, but may result in decreased
  performance on Windows, particularly with non-Latin-1 locale encodings
* If UTF-8 is assumed, this may be a bug or a platform-specific script
* If the locale encoding is assumed, the behavior is as expected
  (but could change if future versions of Python modify the default)

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

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

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


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

Since UTF-8 has become the de-facto standard text encoding,
we might default to it for opening files in the future.

However, such a change will affect many applications and libraries.
If we start emitting ``DeprecationWarning`` everywhere the ``encoding``
argument is omitted, it will be too noisy and painful.

Although this PEP doesn't propose changing the default encoding,
it will help enable that change by:
* Reducing the number of omitted ``encoding`` arguments in libraries
  before we start emitting a ``DeprecationWarning`` by default.
* Allowing users to pass ``encoding="locale"`` to suppress
  the current warning and any ``DeprecationWarning`` added in the future,
  as well as retaining consistent behavior if later Python versions
  change the default, ensuring support for any Python version >=3.10.
Specification
=============
``EncodingWarning``
-------------------
Add a new ``EncodingWarning`` warning class as a subclass of
``Warning``. It is emitted when the ``encoding`` argument is omitted and
the default locale-specific encoding is used.
Options to enable the warning
-----------------------------
The ``-X warn_default_encoding`` option and the
``PYTHONWARNDEFAULTENCODING`` environment variable are added. They
are used to enable ``EncodingWarning``.
``sys.flags.warn_default_encoding`` is also added. The flag is true when
``EncodingWarning`` is enabled.
When the flag is set, ``io.TextIOWrapper()``, ``open()`` and other
modules using them will emit ``EncodingWarning`` when the ``encoding``
argument is omitted.
Since ``EncodingWarning`` is a subclass of ``Warning``, they are
shown by default (if the ``warn_default_encoding`` flag is set), unlike
``DeprecationWarning``.
``encoding="locale"``
---------------------
``io.TextIOWrapper`` will accept ``"locale"`` as a valid argument to
``encoding``. It has the same meaning as the current ``encoding=None``,
except that ``io.TextIOWrapper`` doesn't emit ``EncodingWarning`` when
``encoding="locale"`` is specified.


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

``io.text_encoding()`` is a helper for functions with an
``encoding=None`` parameter that pass it to ``io.TextIOWrapper()`` or
``open()``.

A pure Python implementation will look like this::

   def text_encoding(encoding, stacklevel=1):
       """A 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 an EncodingWarning if *encoding* is None and
       sys.flags.warn_default_encoding is true.

       This function can be used in APIs with an encoding=None parameter
       that pass it to TextIOWrapper or open.
       However, please consider using encoding="utf-8" for new APIs.
       """
       if encoding is None:
           if sys.flags.warn_default_encoding:
               import warnings
               warnings.warn(
                   "'encoding' argument not specified.",
                   EncodingWarning, stacklevel + 2)
           encoding = "locale"
       return encoding

For example, ``pathlib.Path.read_text()`` can use it like this:

.. 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 standard library modules
---------------------------------

Many standard library modules will be affected by this change.

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

Where using the locale encoding as the default encoding is reasonable,
``encoding="locale"`` will be used instead. For example,
the ``subprocess`` module will use the locale encoding as the default
for 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, always
emitting ``DeprecationWarning`` when the ``encoding`` argument 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 "locale" as a codec alias because the locale can be
changed at runtime.
Additionally, ``TextIOWrapper`` checks ``os.device_encoding()``
when ``encoding=None``. This behavior cannot be implemented in
a codec.
Backward Compatibility
======================
The new warning is not emitted by default, so this PEP is 100%
backwards-compatible.
Forward Compatibility
=====================
Passing ``"locale"`` as the argument to ``encoding`` is not
forward-compatible. Code using it will not work on Python older than
3.10, and will instead raise ``LookupError: unknown encoding: locale``.
Until developers can drop Python 3.9 support, ``EncodingWarning``
can only be used for finding missing ``encoding="utf-8"`` arguments.
How to Teach This
=================
For new users
-------------
Since ``EncodingWarning`` is used to write cross-platform code,
there is no need to teach it to new users.
We can just recommend using UTF-8 for text files and using
``encoding="utf-8"`` when opening them.
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 the ``encoding`` argument is not a bug when opening text files
encoded in the locale encoding, but ``encoding="locale"`` is recommended
in Python 3.10 and later 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 implemented
    in Python.
  * It is difficult to find all callers of functions wrapping
    ``open()`` or ``TextIOWrapper()`` (see the ``io.text_encoding()``
    section).
* Many developers will not use the option.
  * Some will, and report the warnings to libraries they use,
    so the option is worth it even if many developers don't enable it.

  * For example, I found [7]_ and [8]_ by running
    ``pip install -U pip``, and [9]_ by running ``tox``
    with the reference implementation. This 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.

	PEP: 597
	Title: Add optional EncodingWarning
	Author: Inada Naoki <songofacandy@gmail.com>
	Status: Final
	Type: Standards Track
	Created: 05-Jun-2019
	Python-Version: 3.10


	Abstract
	========

	Add a new warning category ``EncodingWarning``. It is emitted when the
	``encoding`` argument to ``open()`` is omitted and the default
	locale-specific encoding is used.

	The warning is disabled by default. A new ``-X warn_default_encoding``
	command-line option and a new ``PYTHONWARNDEFAULTENCODING`` environment
	variable can be used to enable it.

	A ``"locale"`` argument value for ``encoding`` is added too. It
	explicitly specifies that the locale encoding should be used, silencing
	the warning.


	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, using ``long_description = open("README.md").read()`` in
	``setup.py`` is a common mistake. Many Windows users cannot install
	such packages if there is at least one non-ASCII character
	(e.g. emoji, author names, copyright symbols, and the like)
	in their UTF-8-encoded ``README.md`` file.

	Of the 4000 most downloaded packages from PyPI, 489 use non-ASCII
	characters in their README, and 82 fail to install from source on
	non-UTF-8 locales due to not specifying an encoding for a non-ASCII
	file. [1]_

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

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

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


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

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

	* If ASCII is assumed, this isn't a bug, but may result in decreased
	performance on Windows, particularly with non-Latin-1 locale encodings
	* If UTF-8 is assumed, this may be a bug or a platform-specific script
	* If the locale encoding is assumed, the behavior is as expected
	(but could change if future versions of Python modify the default)

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

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

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


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

	Since UTF-8 has become the de-facto standard text encoding,
	we might default to it for opening files in the future.

	However, such a change will affect many applications and libraries.
	If we start emitting ``DeprecationWarning`` everywhere the ``encoding``
	argument is omitted, it will be too noisy and painful.

	Although this PEP doesn't propose changing the default encoding,
	it will help enable that change by:

	* Reducing the number of omitted ``encoding`` arguments in libraries
	before we start emitting a ``DeprecationWarning`` by default.

	* Allowing users to pass ``encoding="locale"`` to suppress
	the current warning and any ``DeprecationWarning`` added in the future,
	as well as retaining consistent behavior if later Python versions
	change the default, ensuring support for any Python version >=3.10.


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

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

	Add a new ``EncodingWarning`` warning class as a subclass of
	``Warning``. It is emitted when the ``encoding`` argument is omitted and
	the default locale-specific encoding is used.


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

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

	``sys.flags.warn_default_encoding`` is also added. The flag is true when
	``EncodingWarning`` is enabled.

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

	Since ``EncodingWarning`` is a subclass of ``Warning``, they are
	shown by default (if the ``warn_default_encoding`` flag is set), unlike
	``DeprecationWarning``.


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

	``io.TextIOWrapper`` will accept ``"locale"`` as a valid argument to
	``encoding``. It has the same meaning as the current ``encoding=None``,
	except that ``io.TextIOWrapper`` doesn't emit ``EncodingWarning`` when
	``encoding="locale"`` is specified.


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

	``io.text_encoding()`` is a helper for functions with an
	``encoding=None`` parameter that pass it to ``io.TextIOWrapper()`` or
	``open()``.

	A pure Python implementation will look like this::

	def text_encoding(encoding, stacklevel=1):
	"""A 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 an EncodingWarning if encoding is None and
	sys.flags.warn_default_encoding is true.

	This function can be used in APIs with an encoding=None parameter
	that pass it to TextIOWrapper or open.
	However, please consider using encoding="utf-8" for new APIs.
	"""
	if encoding is None:
	if sys.flags.warn_default_encoding:
	import warnings
	warnings.warn(
	"'encoding' argument not specified.",
	EncodingWarning, stacklevel + 2)
	encoding = "locale"
	return encoding

	For example, ``pathlib.Path.read_text()`` can use it like this:

	.. 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 standard library modules
	---------------------------------

	Many standard library modules will be affected by this change.

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

	Where using the locale encoding as the default encoding is reasonable,
	``encoding="locale"`` will be used instead. For example,
	the ``subprocess`` module will use the locale encoding as the default
	for 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, always
	emitting ``DeprecationWarning`` when the ``encoding`` argument 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 "locale" as a codec alias because the locale can be
	changed at runtime.

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


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

	The new warning is not emitted by default, so this PEP is 100%
	backwards-compatible.


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

	Passing ``"locale"`` as the argument to ``encoding`` is not
	forward-compatible. Code using it will not work on Python older than
	3.10, and will instead raise ``LookupError: unknown encoding: locale``.

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


	How to Teach This
	=================

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

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

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


	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 the ``encoding`` argument is not a bug when opening text files
	encoded in the locale encoding, but ``encoding="locale"`` is recommended
	in Python 3.10 and later 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 implemented
	in Python.

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

	* Many developers will not use the option.

	* Some will, and report the warnings to libraries they use,
	so the option is worth it even if many developers don't enable it.

	* For example, I found [7]_ and [8]_ by running
	``pip install -U pip``, and [9]_ by running ``tox``
	with the reference implementation. This 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.