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PEP 540: truncate to 72 columns

This commit is contained in:
Victor Stinner 2017-12-11 10:13:43 +01:00
parent 2f52641e11
commit 40a9e6f4b3
1 changed files with 45 additions and 34 deletions
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pep-0540.txt
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@ -18,10 +18,13 @@ Abstract
Add a new "UTF-8 Mode" to enhance Python's use of UTF-8. When UTF-8 Mode
is active, Python will:
* use the ``utf-8`` locale, irregardless of the locale currently set by the current platform, and
* change the ``stdin`` and ``stdout`` error handlers to ``surrogateescape``.
* use the ``utf-8`` locale, irregardless of the locale currently set by
the current platform, and
* change the ``stdin`` and ``stdout`` error handlers to
``surrogateescape``.
This mode is off by default, but is automatically activated when using the "POSIX" locale.
This mode is off by default, but is automatically activated when using
the "POSIX" locale.
Add the ``-X utf8`` command line option and ``PYTHONUTF8`` environment
variable to control UTF-8 Mode.
@ -42,17 +45,20 @@ locale, but are unable change the locale for various reasons. This
encoding is very limited in term of Unicode support: any non-ASCII
character is likely to cause trouble.
It isn't always easy to get an accurate locale. Locales don't get
the exact same name on different Linux distributions, FreeBSD, macOS, etc.
It isn't always easy to get an accurate locale. Locales don't get the
exact same name on different Linux distributions, FreeBSD, macOS, etc.
And some locales, like the recent ``C.UTF-8`` locale, are only supported
by a few platforms. The current locale can even vary on the *same* platform
depending on context; for example, a SSH connection can use a different
encoding than the filesystem or local terminal encoding on the same machine.
by a few platforms. The current locale can even vary on the *same*
platform depending on context; for example, a SSH connection can use a
different encoding than the filesystem or local terminal encoding on the
same machine.
On the flip side, Python 3.6 is already using UTF-8 by default on
macOS, Android and Windows (:pep:`529`) for most functions--although ``open()`` is a notable exception here. UTF-8 is also the default encoding of Python
scripts, XML and JSON file formats. The Go programming language uses
UTF-8 for all strings.
On the flip side, Python 3.6 is already using UTF-8 by default on macOS,
Android and Windows (:pep:`529`) for most functions -- although
``open()`` is a notable exception here. UTF-8 is also the default
encoding of Python scripts, XML and JSON file formats. The Go
programming language
uses UTF-8 for all strings.
UTF-8 support is nearly ubiquitous for data read and written by modern
platforms. It also has excellent support in Python. The problem is
@ -63,8 +69,9 @@ suggests itself: ignore the locale encoding and use UTF-8.
Passthough for undecodable bytes: surrogateescape
-------------------------------------------------
When decoding bytes from UTF-8 using the default ``strict`` error handler,
Python 3 raises a ``UnicodeDecodeError`` on the first undecodable byte.
When decoding bytes from UTF-8 using the default ``strict`` error
handler, 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
@ -72,18 +79,18 @@ 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 processing
data as if it were bytes, but uses Unicode in practice; undecodable bytes
are stored as surrogate characters.
data as if it were bytes, but uses Unicode in practice; undecodable
bytes are stored as surrogate characters.
UTF-8 Mode sets 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, and 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.
to be properly encoded, and 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
@ -92,14 +99,14 @@ 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
is usually equivalent to 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.
This PEP changes the behaviour for the POSIX locale since this locale is
usually equivalent to 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 for these
other locales, they are responsible for any potential mojibake issues caused
by UTF-8 Mode.
As users are responsible to enable explicitly the new UTF-8 Mode for
these other locales, they are responsible for any potential mojibake
issues caused by UTF-8 Mode.
Proposal
@ -109,11 +116,14 @@ Add a new UTF-8 Mode to use the UTF-8 encoding, ignore the locale
encoding, and change ``stdin`` and ``stdout`` error handlers to
``surrogateescape``.
Add the new ``-X utf8`` command line option and ``PYTHONUTF8`` environment
variable. Users can explicitly activate UTF-8 Mode with the command-line option ``-X utf8`` or by setting the environment variable ``PYTHONUTF8=1``.
Add the new ``-X utf8`` command line option and ``PYTHONUTF8``
environment variable. Users can explicitly activate UTF-8 Mode with the
command-line option ``-X utf8`` or by setting the environment variable
``PYTHONUTF8=1``.
This mode is disabled by default and enabled by the POSIX locale.
Users can explicitly disable UTF-8 Mode with the command-line option ``-X utf8=0`` or by setting the environment variable ``PYTHONUTF8=0``.
This mode is disabled by default and enabled by the POSIX locale. Users
can explicitly disable UTF-8 Mode with the command-line option ``-X
utf8=0`` or by setting the environment variable ``PYTHONUTF8=0``.
For standard streams, the ``PYTHONIOENCODING`` environment variable has
priority over UTF-8 Mode.
@ -142,14 +152,15 @@ Relationship with the locale coercion (PEP 538)
===============================================
The POSIX locale enables the locale coercion (:pep:`538`) and the UTF-8
mode (:pep:`540`). When the locale coercion is enabled, enabling the UTF-8
mode has no additional effect.
mode (:pep:`540`). When the locale coercion is enabled, enabling the
UTF-8 mode has no additional effect.
The UTF-8 Mode has the same effect as locale coercion:
* ``sys.getfilesystemencoding()`` returns ``'UTF-8'``,
* ``locale.getpreferredencoding()`` returns ``UTF-8``, and
* the ``sys.stdin`` and ``sys.stdout`` error handlers are set to ``surrogateescape``.
* the ``sys.stdin`` and ``sys.stdout`` error handlers are set to
``surrogateescape``.
These changes only affect Python code. But the locale coercion has
addiditonal effects: the ``LC_CTYPE`` environment variable and the