[Python-Dev] PEP 540: Add a new UTF-8 mode (v3)

Chris Barker - NOAA Federal chris.barker at noaa.gov
Thu Dec 7 21:12:19 EST 2017 

I made the following two changes to the PEP 540:

* open() error handler remains "strict"
* remove the "Strict UTF8 mode" which doesn't make much sense anymore


+1 — ignore my previous note.

-CHB


I wrote the Strict UTF-8 mode when open() used surrogateescape error
handler in the UTF-8 mode. I don't think that a Strict UTF-8 mode is
required just to change the error handler of stdin and stdout. Well,
read the "Passthough undecodable bytes: surrogateescape" section of
the PEP rationale :-)


https://www.python.org/dev/peps/pep-0540/

Victor


PEP: 540
Title: Add a new UTF-8 mode
Version: $Revision$
Last-Modified: $Date$
Author: Victor Stinner <victor.stinner at gmail.com>
BDFL-Delegate: INADA Naoki
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 5-January-2016
Python-Version: 3.7


Abstract
========

Add a new UTF-8 mode to ignore the locale, use the UTF-8 encoding, and
change ``stdin`` and ``stdout`` error handlers to ``surrogateescape``.
This mode is enabled by default in the POSIX locale, but otherwise
disabled by default.

The new ``-X utf8`` command line option and ``PYTHONUTF8`` environment
variable are added to control the UTF-8 mode.


Rationale
=========

Locale encoding and UTF-8
-------------------------

Python 3.6 uses the locale encoding for filenames, environment
variables, standard streams, etc. The locale encoding is inherited from
the locale; the encoding and the locale are tightly coupled.

Many users inherit the ASCII encoding from the POSIX locale, aka the "C"
locale, but are unable change the locale for different reasons. This
encoding is very limited in term of Unicode support: any non-ASCII
character is likely to cause troubles.

It is not easy to get the expected locale. Locales don't get the exact
same name on all Linux distributions, FreeBSD, macOS, etc. Some
locales, like the recent ``C.UTF-8`` locale, are only supported by a few
platforms. For example, a SSH connection can use a different encoding
than the filesystem or terminal encoding of the local host.

On the other side, Python 3.6 is already using UTF-8 by default on
macOS, Android and Windows (PEP 529) for most functions, except of
``open()``. UTF-8 is also the default encoding of Python scripts, XML
and JSON file formats. The Go programming language uses UTF-8 for
strings.

When all data are stored as UTF-8 but the locale is often misconfigured,
an obvious solution is to ignore the locale and use UTF-8.

PEP 538 attempts to mitigate this problem by coercing the C locale
to a UTF-8 based locale when one is available, but that isn't a
universal solution. For example, CentOS 7's container images default
to the POSIX locale, and don't include the C.UTF-8 locale, so PEP 538's
locale coercion is ineffective.


Passthough undecodable bytes: surrogateescape
---------------------------------------------

When decoding bytes from UTF-8 using the ``strict`` error handler, which
is the default, Python 3 raises a ``UnicodeDecodeError`` on the first
undecodable byte.

Unix command line tools like ``cat`` or ``grep`` and most Python 2
applications simply do not have this class of bugs: they don't decode
data, but process data as a raw bytes sequence.

Python 3 already has a solution to behave like Unix tools and Python 2:
the ``surrogateescape`` error handler (:pep:`383`). It allows to process
data "as bytes" but uses Unicode in practice (undecodable bytes are
stored as surrogate characters).

The UTF-8 mode uses the ``surrogateescape`` error handler for ``stdin``
and ``stdout`` since these streams as commonly associated to Unix
command line tools.

However, users have a different expectation on files. Files are expected
to be properly encoded. Python is expected to fail early when ``open()``
is called with the wrong options, like opening a JPEG picture in text
mode. The ``open()`` default error handler remains ``strict`` for these
reasons.


No change by default for best backward compatibility
----------------------------------------------------

While UTF-8 is perfect in most cases, sometimes the locale encoding is
actually the best encoding.

This PEP changes the behaviour for the POSIX locale since this locale
usually gives the ASCII encoding, whereas UTF-8 is a much better choice.
It does not change the behaviour for other locales to prevent any risk
or regression.

As users are responsible to enable explicitly the new UTF-8 mode, they
are responsible for any potential mojibake issues caused by this mode.


Proposal
========

Add a new UTF-8 mode to ignore the locale, use the UTF-8 encoding, and
change ``stdin`` and ``stdout`` error handlers to ``surrogateescape``.
This mode is enabled by default in the POSIX locale, but otherwise
disabled by default.

The new ``-X utf8`` command line option and ``PYTHONUTF8`` environment
variable are added. The UTF-8 mode is enabled by ``-X utf8`` or
``PYTHONUTF8=1``.

The POSIX locale enables the UTF-8 mode. In this case, the UTF-8 mode
can be explicitly disabled by ``-X utf8=0`` or ``PYTHONUTF8=0``.

For standard streams, the ``PYTHONIOENCODING`` environment variable has
priority over the UTF-8 mode.

On Windows, the ``PYTHONLEGACYWINDOWSFSENCODING`` environment variable
(:pep:`529`) has the priority over the UTF-8 mode.


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

The only backward incompatible change is that the UTF-8 encoding is now
used for the POSIX locale.


Annex: Encodings And Error Handlers
===================================

The UTF-8 mode changes the default encoding and error handler used by
``open()``, ``os.fsdecode()``, ``os.fsencode()``, ``sys.stdin``,
``sys.stdout`` and ``sys.stderr``.

Encoding and error handler
--------------------------

============================  =======================
==========================
Function                      Default                  UTF-8 mode or
POSIX locale
============================  =======================
==========================
open()                        locale/strict            **UTF-8**/strict
os.fsdecode(), os.fsencode()  locale/surrogateescape
  **UTF-8**/surrogateescape
sys.stdin, sys.stdout         locale/strict
           **UTF-8/surrogateescape**
sys.stderr                    locale/backslashreplace
**UTF-8**/backslashreplace
============================  =======================
==========================

By comparison, Python 3.6 uses:

============================  =======================
==========================
Function                      Default                  POSIX locale
============================  =======================
==========================
open()                        locale/strict            locale/strict
os.fsdecode(), os.fsencode()  locale/surrogateescape
  locale/surrogateescape
sys.stdin, sys.stdout         locale/strict
locale/**surrogateescape**
sys.stderr                    locale/backslashreplace
 locale/backslashreplace
============================  =======================
==========================

Encoding and error handler on Windows
-------------------------------------

On Windows, the encodings and error handlers are different:

============================  =======================
==========================  ==========================
Function                      Default                  Legacy Windows
FS encoding  UTF-8 mode
============================  =======================
==========================  ==========================
open()                        mbcs/strict              mbcs/strict
           **UTF-8**/strict
os.fsdecode(), os.fsencode()  UTF-8/surrogatepass
**mbcs/replace**            UTF-8/surrogatepass
sys.stdin, sys.stdout         UTF-8/surrogateescape
UTF-8/surrogateescape       UTF-8/surrogateescape
sys.stderr                    UTF-8/backslashreplace
UTF-8/backslashreplace      UTF-8/backslashreplace
============================  =======================
==========================  ==========================

By comparison, Python 3.6 uses:

============================  =======================
==========================
Function                      Default                  Legacy Windows
FS encoding
============================  =======================
==========================
open()                        mbcs/strict              mbcs/strict
os.fsdecode(), os.fsencode()  UTF-8/surrogatepass      **mbcs/replace**
sys.stdin, sys.stdout         UTF-8/surrogateescape    UTF-8/surrogateescape
sys.stderr                    UTF-8/backslashreplace
  UTF-8/backslashreplace
============================  =======================
==========================

The "Legacy Windows FS encoding" is enabled by the
``PYTHONLEGACYWINDOWSFSENCODING`` environment variable.

If stdin and/or stdout is redirected to a pipe, ``sys.stdin`` and/or
``sys.output`` use ``mbcs`` encoding by default rather than UTF-8. But
in the UTF-8 mode, ``sys.stdin`` and ``sys.stdout`` always use the UTF-8
encoding.

.. note:
  There is no POSIX locale on Windows. The ANSI code page is used to the
  locale encoding, and this code page never uses the ASCII encoding.


Annex: Differences between PEP 538 and PEP 540
==============================================

PEP 538's locale coercion is only effective if a suitable UTF-8
based locale is available as a coercion target. PEP 540's
UTF-8 mode can be enabled even for operating systems that don't
provide a suitable platform locale (such as CentOS 7).

PEP 538 only changes the interpreter's behaviour for the C locale. While the
new UTF-8 mode of this PEP is only enabled by default in the C locale, it
can
also be enabled manually for any other locale.

PEP 538 is implemented with ``setlocale(LC_CTYPE, "<coercion target>")`` and
``setenv("LC_CTYPE", "<coercion target>")``, so any non-Python code running
in the process and any subprocesses that inherit the environment is impacted
by the change. PEP 540 is implemented in Python internals and ignores the
locale: non-Python running in the same process is not aware of the
"Python UTF-8 mode". The benefit of the PEP 538 approach is that it helps
ensure that encoding handling in binary extension modules and subprocesses
is consistent with CPython's encoding handling. The upside of the PEP 540
approach is that it allows an embedding application to change the
interpreter's behaviour without having to change the process global
locale settings.


Links
=====

* `bpo-29240: Implementation of the PEP 540: Add a new UTF-8 mode
 <http://bugs.python.org/issue29240>`_
* `PEP 538 <https://www.python.org/dev/peps/pep-0538/>`_:
 "Coercing the legacy C locale to C.UTF-8"
* `PEP 529 <https://www.python.org/dev/peps/pep-0529/>`_:
 "Change Windows filesystem encoding to UTF-8"
* `PEP 528 <https://www.python.org/dev/peps/pep-0528/>`_:
 "Change Windows console encoding to UTF-8"
* `PEP 383 <https://www.python.org/dev/peps/pep-0383/>`_:
 "Non-decodable Bytes in System Character Interfaces"


Post History
============

* 2017-12: `[Python-Dev] PEP 540: Add a new UTF-8 mode
 <https://mail.python.org/pipermail/python-dev/2017-December/151054.html>`_
* 2017-04: `[Python-Dev] Proposed BDFL Delegate update for PEPs 538 &
 540 (assuming UTF-8 for *nix system boundaries)
 <https://mail.python.org/pipermail/python-dev/2017-April/147795.html>`_
* 2017-01: `[Python-ideas] PEP 540: Add a new UTF-8 mode
 <https://mail.python.org/pipermail/python-ideas/2017-January/044089.html>`_
* 2017-01: `bpo-28180: Implementation of the PEP 538: coerce C locale to
 C.utf-8 (msg284764) <https://bugs.python.org/issue28180#msg284764>`_
* 2016-08-17: `bpo-27781: Change sys.getfilesystemencoding() on Windows
 to UTF-8 (msg272916) <https://bugs.python.org/issue27781#msg272916>`_
 -- Victor proposed ``-X utf8`` for the :pep:`529` (Change Windows
 filesystem encoding to UTF-8)


Copyright
=========

This document has been placed in the public domain.
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