PEP: 597
Title: Enable UTF-8 mode by default on Windows
Author: Inada Naoki  <songofacandy@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 05-Jun-2019
Python-Version: 3.10


Abstract
========

This PEP proposes to make UTF-8 mode [#]_ enabled by default on
Windows.

The goal of this PEP is providing "UTF-8 by default" experience to
Windows users like Unix users.


Motivation
==========

UTF-8 is the best encoding nowdays
----------------------------------

Popular text editors like VS Code uses UTF-8 by default.
Even Microsoft Notepad uses UTF-8 by default since the Windows 10
May 2019 Update.
Additionally, the default encoding of Python source files is UTF-8.

We can assume that most Python programmers use UTF-8 for most text
files.

Python is one of the most popular first programming languages.
New programmers may not know about encoding.  If the default encoding
for text files is UTF-8, they can learn about encoding when they need
to handle legacy encoding.


People assume the default encoding is UTF-8 already
---------------------------------------------------

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 emoji or any other non-ASCII
character in the ``README.md`` file.

Even Python experts assume that default encoding is UTF-8.
It creates bugs that happen only on Windows. See [#]_ [#]_.

Changing the default text encoding to UTF-8 will help many Windows
users.


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

Enable UTF-8 mode on Windows unless it is disabled explicitly.

UTF-8 mode affects these areas:

* ``locale.getpreferredencoding`` returns "UTF-8".

   * ``open``, ``subprocess.Popen``, ``pathlib.Path.read_text``,
     ``ZipFile.open``, and many other functions use UTF-8 when
     the ``encoding`` option is omitted.

* The stdio uses "UTF-8" always.

   * Console I/O uses "UTF-8" already [#]_.  So this affects
     only when the stdio are redirected.

On the other hand, UTF-8 mode doesn't affect to "mbcs" encoding.
Users can still use system encoding by chosing "mbcs" encoding
explicitly.


Backwards Compatibility
=======================

Some existing applications assuming the default text encoding is the
system encoding (a.k.a. ANSI encoding) will be broken by this change.

Users can disable the UTF-8 mode by environment variable
(``PYTHONUTF8=0``) or command line option (``-Xutf8=0``) for backward
compatibility.


Rejected Ideas
===============

Change the default encoding of TextIOWrapper to "UTF-8"
-------------------------------------------------------

This idea changed the default encoding to UTF-8 always, regardless of
platform, locale, and environment variables.

While this idea looks ideal in terms of consistency, it will cause
backward compatibility problems.

Utilizing the UTF-8 mode seems better than adding one more backward
compatibility option like ``PYTHONLEGACYWINDOWSSTDIO``.


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

To be written.


References
==========

.. [#] `PEP 540 -- Add a new UTF-8 Mode <https://www.python.org/dev/peps/pep-0540/>`_
.. [#] https://github.com/pypa/packaging.python.org/pull/682
.. [#] https://bugs.python.org/issue33684
.. [#] `PEP 528 -- Change Windows console encoding to UTF-8 <https://www.python.org/dev/peps/pep-0528/>`_


Copyright
=========

This document has been placed in the public domain.

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