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gh-148603: Update docs now UTF-8 is default (PEP 686) #157372

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02ad240
Update docs now UTF-8 is default (PEP 686)
hugovk Sep 12, 2026
043b203
Sentence case for headers
hugovk Sep 12, 2026
c3986bc
Clarify for Windows with Python UTF-8 Mode disabled
hugovk Sep 13, 2026
0823961
Merge remote-tracking branch 'upstream/main' into 3.15-utf8-docs
hugovk Sep 13, 2026
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19 changes: 13 additions & 6 deletions Doc/builtins/functions.rst
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@@ -1,7 +1,7 @@
.. XXX document all delegations to __special__ methods
.. _built-in-funcs:

Built-in Functions
Built-in functions
==================

The Python interpreter has a number of functions and types built into it that
Expand Down Expand Up @@ -1459,7 +1459,8 @@ are always available. They are listed here in alphabetical order.
already exists), ``'x'`` for exclusive creation, and ``'a'`` for appending
(which on *some* Unix systems, means that *all* writes append to the end of
the file regardless of the current seek position). In text mode, if
*encoding* is not specified the encoding used is platform-dependent:
*encoding* is not specified, UTF-8 is used by default; if
:ref:`Python UTF-8 Mode <utf8-mode>` is disabled,
:func:`locale.getencoding` is called to get the current locale encoding.
(For reading and writing raw bytes use binary mode and leave
*encoding* unspecified.) The available modes are:
Expand Down Expand Up @@ -1490,7 +1491,7 @@ are always available. They are listed here in alphabetical order.
argument) return contents as :class:`bytes` objects without any decoding. In
text mode (the default, or when ``'t'`` is included in the *mode* argument),
the contents of the file are returned as :class:`str`, the bytes having been
first decoded using a platform-dependent encoding or using the specified
first decoded using the default encoding or using the specified
*encoding* if given.

.. note::
Expand Down Expand Up @@ -1519,9 +1520,11 @@ are always available. They are listed here in alphabetical order.
described above for binary files.

*encoding* is the name of the encoding used to decode or encode the file.
This should only be used in text mode. The default encoding is platform
dependent (whatever :func:`locale.getencoding` returns), but any
:term:`text encoding` supported by Python can be used.
This should only be used in text mode. The default encoding is UTF-8;
if :ref:`Python UTF-8 Mode <utf8-mode>` is disabled, the default is
platform-dependent (whatever :func:`locale.getencoding` returns).
Any :term:`text encoding` supported by Python can be used, and
``encoding="locale"`` specifies the current locale encoding explicitly.
See the :mod:`codecs` module for the list of supported encodings.

*errors* is an optional string that specifies how encoding and decoding
Expand Down Expand Up @@ -1638,6 +1641,10 @@ are always available. They are listed here in alphabetical order.
.. versionchanged:: 3.11
The ``'U'`` mode has been removed.

.. versionchanged:: 3.15
UTF-8 is now the default encoding, instead of the
platform-dependent locale encoding (:pep:`686`).

.. function:: ord(character, /)

Return the ordinal value of a character.
Expand Down
17 changes: 8 additions & 9 deletions Doc/library/csv.rst
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:mod:`!csv` --- CSV File Reading and Writing
:mod:`!csv` --- CSV file reading and writing
============================================

.. module:: csv
Expand Down Expand Up @@ -42,7 +42,7 @@ using the :class:`DictReader` and :class:`DictWriter` classes.

.. _csv-contents:

Module Contents
Module contents
---------------

The :mod:`!csv` module defines the following functions:
Expand Down Expand Up @@ -451,7 +451,7 @@ The :mod:`!csv` module defines the following exception:

.. _csv-fmt-params:

Dialects and Formatting Parameters
Dialects and formatting parameters
----------------------------------

To make it easier to specify the format of input and output records, specific
Expand Down Expand Up @@ -557,7 +557,7 @@ with the specified formatting parameters replaced.

.. _reader-objects:

Reader Objects
Reader objects
--------------

Reader objects (:class:`DictReader` instances and objects returned by the
Expand Down Expand Up @@ -594,7 +594,7 @@ DictReader objects have the following public attribute:



Writer Objects
Writer objects
--------------

:class:`writer` objects (:class:`DictWriter` instances and objects returned by
Expand Down Expand Up @@ -673,17 +673,16 @@ The corresponding simplest possible writing example is::
writer.writerows(someiterable)

Since :func:`open` is used to open a CSV file for reading, the file
will by default be decoded into unicode using the system default
encoding (see :func:`locale.getencoding`). To decode a file
will by default be decoded into Unicode using UTF-8. To decode a file
Comment thread
hugovk marked this conversation as resolved.
using a different encoding, use the ``encoding`` argument of open::

import csv
with open('some.csv', newline='', encoding='utf-8') as f:
with open('some.csv', newline='', encoding='latin-1') as f:
reader = csv.reader(f)
for row in reader:
print(row)

The same applies to writing in something other than the system default
The same applies to writing in something other than the default
encoding: specify the encoding argument when opening the output file.

Registering a new dialect::
Expand Down
43 changes: 15 additions & 28 deletions Doc/library/io.rst
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Expand Up @@ -115,34 +115,21 @@ The raw stream API is described in detail in the docs of :class:`RawIOBase`.

.. _io-text-encoding:

Text Encoding
Text encoding
-------------

The default encoding of :class:`TextIOWrapper` and :func:`open` is
locale-specific (:func:`locale.getencoding`).

However, many developers forget to specify the encoding when opening text files
encoded in UTF-8 (e.g. JSON, TOML, Markdown, etc...) since most Unix
platforms use UTF-8 locale by default. This causes bugs because the locale
encoding is not UTF-8 for most Windows users. For example::

# May not work on Windows when non-ASCII characters in the file.
with open("README.md") as f:
long_description = f.read()

Accordingly, it is highly recommended that you specify the encoding
explicitly when opening text files. If you want to use UTF-8, pass
``encoding="utf-8"``. To use the current locale encoding,
``encoding="locale"`` is supported since Python 3.10.
The default encoding of :class:`TextIOWrapper` and :func:`open` is UTF-8.
If :ref:`Python UTF-8 Mode <utf8-mode>` is disabled, the default encoding
is locale-specific (:func:`locale.getencoding`).

.. seealso::

:ref:`utf8-mode`
Python UTF-8 Mode can be used to change the default encoding to
UTF-8 from locale-specific encoding.
Python UTF-8 Mode ignores the locale encoding and forces the use
of UTF-8.

:pep:`686`
Python 3.15 will make :ref:`utf8-mode` default.
Python 3.15 made :ref:`utf8-mode` the default.

.. _io-encoding-warning:

Expand All @@ -152,7 +139,7 @@ Opt-in EncodingWarning
.. versionadded:: 3.10
See :pep:`597` for more details.

To find where the default locale encoding is used, you can enable
To find where the default encoding is used, you can enable
the :option:`-X warn_default_encoding <-X>` command line option or set the

@vstinner vstinner Sep 13, 2026

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@methane: It now looks wrong to emit EncodingWarning if open() is called with no encoding argument, since it defaults to UTF-8 which is fine, no? For me, the purpose of PEP 597 "Add optional EncodingWarning" is to detect when platform dependent locale encoding is used. Am I wrong?

At least, I see one advantage of the current behavior, it helps writing new code which is not platform dependent when running on older Python versions.

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EncodingWarning is a tool for people who want to identify all the places that will be affected by the transition to UTF-8 Mode and verify that the transition will not cause any problems.

Considering that some people may not start thinking about enabling UTF-8 Mode until Python 3.15, I think removing EncodingWarning immediately in Python 3.15 would be premature. How about removing it in Python 3.17 instead?

@vstinner vstinner Sep 13, 2026

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Since -X warn_default_encoding is an opt-in option to emit EncodingWarning, I'm fine with emitting EncodingWarning on Python 3.15 when the encoding parameter is omitted. As I wrote, it should help writing new code on Python 3.15 which is compatible (behave as expected) on older Python versions.

:envvar:`PYTHONWARNDEFAULTENCODING` environment variable, which will
emit an :exc:`EncodingWarning` when the default encoding is used.
Expand All @@ -165,7 +152,7 @@ please consider using UTF-8 by default (i.e. ``encoding="utf-8"``) for
new APIs.


High-level Module Interface
High-level module interface
---------------------------

.. data:: DEFAULT_BUFFER_SIZE
Expand Down Expand Up @@ -315,7 +302,7 @@ ABC Inherits Stub Methods Mixin M
========================= ================== ======================== ==================================================


I/O Base Classes
I/O base classes
^^^^^^^^^^^^^^^^

.. class:: IOBase
Expand Down Expand Up @@ -660,7 +647,7 @@ I/O Base Classes
so the implementation should only access *b* during the method call.


Raw File I/O
Raw file I/O
^^^^^^^^^^^^

.. class:: FileIO(name, mode='r', closefd=True, opener=None)
Expand Down Expand Up @@ -728,7 +715,7 @@ Raw File I/O
given in the constructor.


Buffered Streams
Buffered streams
^^^^^^^^^^^^^^^^

Buffered I/O streams provide a higher-level interface to an I/O device
Expand Down Expand Up @@ -1004,8 +991,8 @@ Text I/O
:class:`TextIOBase`.

*encoding* gives the name of the encoding that the stream will be decoded or
encoded with. In :ref:`UTF-8 Mode <utf8-mode>`, this defaults to UTF-8.
Otherwise, it defaults to :func:`locale.getencoding`.
encoded with. This defaults to UTF-8; if :ref:`UTF-8 Mode <utf8-mode>` is
disabled, it defaults to :func:`locale.getencoding`.
``encoding="locale"`` can be used to specify the current locale's encoding
explicitly. See :ref:`io-text-encoding` for more information.

Expand Down Expand Up @@ -1187,7 +1174,7 @@ Text I/O
It inherits from :class:`codecs.IncrementalDecoder`.


Static Typing
Static typing
-------------

The following protocols can be used for annotating function and method
Expand Down
30 changes: 13 additions & 17 deletions Doc/tutorial/inputoutput.rst
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.. _tut-io:

****************
Input and Output
Input and output
****************

There are several ways to present the output of a program; data can be printed
Expand All @@ -11,7 +11,7 @@ discuss some of the possibilities.

.. _tut-formatting:

Fancier Output Formatting
Fancier output formatting
=========================

So far we've encountered two ways of writing values: *expression statements* and
Expand Down Expand Up @@ -111,7 +111,7 @@ This syntax is easy to use, although it offers much less control for formatting.

.. _tut-f-strings:

Formatted String Literals
Formatted string literals
-------------------------

:ref:`Formatted string literals <f-strings>` (also called f-strings for
Expand Down Expand Up @@ -163,7 +163,7 @@ the reference guide for the :ref:`formatspec`.

.. _tut-string-format:

The String format() Method
The string format() method
--------------------------

Basic usage of the :meth:`str.format` method looks like this::
Expand Down Expand Up @@ -240,7 +240,7 @@ For a complete overview of string formatting with :meth:`str.format`, see
:ref:`formatstrings`.


Manual String Formatting
Manual string formatting
------------------------

Here's the same table of squares and cubes, formatted manually::
Expand Down Expand Up @@ -303,20 +303,19 @@ More information can be found in the :ref:`old-string-formatting` section.

.. _tut-files:

Reading and Writing Files
Reading and writing files
=========================

.. index::
pair: built-in function; open
pair: object; file

:func:`open` returns a :term:`file object`, and is most commonly used with
two positional arguments and one keyword argument:
``open(filename, mode, encoding=None)``
two positional arguments: ``open(filename, mode)``

::

>>> f = open('workfile', 'w', encoding="utf-8")
>>> f = open('workfile', 'w')

.. XXX str(f) is <io.TextIOWrapper object at 0x82e8dc4>

Expand All @@ -334,10 +333,7 @@ omitted.

Normally, files are opened in :dfn:`text mode`, that means, you read and write
strings from and to the file, which are encoded in a specific *encoding*.
If *encoding* is not specified, the default is platform dependent
(see :func:`open`).
Because UTF-8 is the modern de-facto standard, ``encoding="utf-8"`` is
recommended unless you know that you need to use a different encoding.
If *encoding* is not specified, the default is UTF-8 (see :func:`open`).
Appending a ``'b'`` to the mode opens the file in :dfn:`binary mode`.
Binary mode data is read and written as :class:`bytes` objects.
You can not specify *encoding* when opening file in binary mode.
Expand All @@ -356,7 +352,7 @@ after its suite finishes, even if an exception is raised at some
point. Using :keyword:`!with` is also much shorter than writing
equivalent :keyword:`try`\ -\ :keyword:`finally` blocks::

>>> with open('workfile', encoding="utf-8") as f:
>>> with open('workfile') as f:
... read_data = f.read()

>>> # We can check that the file has been automatically closed.
Expand Down Expand Up @@ -389,7 +385,7 @@ automatically fail. ::

.. _tut-filemethods:

Methods of File Objects
Methods of file objects
-----------------------

The rest of the examples in this section will assume that a file object called
Expand Down Expand Up @@ -532,8 +528,8 @@ To decode the object again, if ``f`` is a :term:`binary file` or
x = json.load(f)

.. note::
JSON files must be encoded in UTF-8. Use ``encoding="utf-8"`` when opening
JSON file as a :term:`text file` for both of reading and writing.
JSON files must be encoded in UTF-8, the default encoding for
:term:`text files <text file>`.

This simple serialization technique can handle lists and dictionaries, but
serializing arbitrary class instances in JSON requires a bit of extra effort.
Expand Down
11 changes: 6 additions & 5 deletions Doc/using/windows.rst
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Expand Up @@ -943,7 +943,7 @@ checking for that feed.
}


Proxy Settings
Proxy settings
--------------

.. versionadded:: 26.4
Expand Down Expand Up @@ -1345,14 +1345,15 @@ UTF-8 mode
Python UTF-8 mode is now enabled by default (:pep:`686`).

Windows still uses legacy encodings for the system encoding (the ANSI Code
Page). Python uses it for the default encoding of text files (e.g.
:func:`locale.getencoding`).
Page). When the :ref:`Python UTF-8 Mode <utf8-mode>` is disabled, Python
uses the ANSI Code Page as the default encoding of text files, as
returned by :func:`locale.getencoding`.

This may cause issues because UTF-8 is widely used on the internet
and most Unix systems, including WSL (Windows Subsystem for Linux).

The :ref:`Python UTF-8 Mode <utf8-mode>`, enabled by default, can help by
changing the default text encoding to UTF-8.
The :ref:`Python UTF-8 Mode <utf8-mode>`, enabled by default, ignores the
system encoding and uses UTF-8 as the default text encoding.
When the :ref:`UTF-8 mode <utf8-mode>` is enabled, you can still use the
system encoding (the ANSI Code Page) via the "mbcs" codec.

Expand Down
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