Documentation for sys.stdout encoding does not reflect the new Windows behavior in Python 3.6+

@pfmoore

BPO	30410
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba, @Mariatta, @lysnikolaou, @miss-islington
PRs	bpo-30410: Documentation of sys.stdin/out/err update to reflect change in 3.6 #10264 [3.7] bpo-30410: Documentation of sys.stdin/out/err update to reflect change in 3.6 (GH-10264) #11860

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-02-14.23:36:54.782>
created_at = <Date 2017-05-20.08:58:49.633>
labels = ['easy', 'type-bug', '3.8', '3.7', 'OS-windows', 'docs']
title = 'Documentation for sys.stdout encoding does not reflect the new Windows behavior in Python 3.6+'
updated_at = <Date 2019-02-14.23:45:23.357>
user = 'https://github.com/pfmoore'

bugs.python.org fields:

activity = <Date 2019-02-14.23:45:23.357>
actor = 'miss-islington'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-02-14.23:36:54.782>
closer = 'Mariatta'
components = ['Documentation', 'Windows']
creation = <Date 2017-05-20.08:58:49.633>
creator = 'paul.moore'
dependencies = []
files = []
hgrepos = []
issue_num = 30410
keywords = ['patch', 'easy']
message_count = 12.0
messages = ['294020', '294046', '294061', '294063', '328766', '328798', '330764', '330765', '330901', '335573', '335574', '335575']
nosy_count = 9.0
nosy_names = ['paul.moore', 'tim.golden', 'docs@python', 'zach.ware', 'eryksun', 'steve.dower', 'Mariatta', 'lys.nikolaou', 'miss-islington']
pr_nums = ['10264', '11860']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30410'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

The documentation for the encoding of sys.stdin/out/err (see https://docs.python.org/3.6/library/sys.html#sys.stdout) does not reflect the change in Python 3.6 on Windows to use the console Unicode APIs, and hence UTF-8 for the encoding.

How about this?

The character encoding is platform-dependent. Non-Windows 
platforms use the locale encoding (see 
locale.getpreferredencoding()).

On Windows, UTF-8 is used for console character 
devices (i.e. CON, CONIN$, and CONOUT$). However, this
can be overridden to use the console as a generic 
character device by setting the environment variable 
PYTHONLEGACYWINDOWSSTDIO before starting Python. Non-
character devices such as disk files and pipes use the 
system locale encoding (i.e. the ANSI codepage). 
Character devices such as NUL (i.e. isatty() returns 
True) use the value of the console input and output
codepages at startup, respectively for stdin and
stdout/stderr. This defaults to the system locale
encoding if the process is not initially attached to a
console.

Under all platforms, you can override this value by
setting the PYTHONIOENCODING environment variable before
starting Python. However, for the Windows console, this
only applies when PYTHONLEGACYWINDOWSSTDIO is also set.

Looks great, though I wonder whether the rest of the paragraph after "Character devices such as NUL" would be more confusing than it's worth?

Can you create a PR? (And having links to the environment variable docs would be great.)

I discussed character devices mostly because of the NUL device. It could be surprising that Python dies on an encoding error when output is redirected to NUL:

C:\>chcp 1252
Active code page: 1252

C:\>python -c "print('\u20ac')" > nul
C:\>chcp 437
Active code page: 437

    C:\>python -c "print('\u20ac')" > nul
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "C:\Program Files\Python36\lib\encodings\cp437.py", line 19, in encode
        return codecs.charmap_encode(input,self.errors,encoding_map)[0]
    UnicodeEncodeError: 'charmap' codec can't encode character '\u20ac' in position 0:
    character maps to <undefined>

Unix has a similar problem:

    $ LANG=C python3 -c 'print("\u20ac")' > /dev/null
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    UnicodeEncodeError: 'ascii' codec can't encode character '\u20ac' in position 0:
    ordinal not in range(128)

Except /dev/null isn't a TTY. Also, it's rare nowadays for the locale encoding in Unix systems to be something other than UTF-8.

It would be useful if we special-cased NUL like we do for the Windows console, but just to make it use the backslashreplace error handler. Unfortunately I don't know how to do that without calling NtQueryObject, for which ObjectNameInformation (1) can't be used because it's undocumented 1. GetFinalPathNameByHandle also can't be used because it requires file-system devices. As a crude workaround, we could lump together all non-console character devices (i.e. isatty() but not a console). That will affect serial devices, too, but I can't think of a good reason someone would redirect stdout or stderr to a COM port.

Shall I create a PR for this?

Please do!

Ping.

The proposed wording seems a bit over-complex to me. Maybe the following re-wording would be easier to understand?

The character encoding is platform-dependent. Non-Windows 
platforms use the locale encoding (see 
locale.getpreferredencoding()).

On Windows, UTF-8 is used for the console device.  Non-character
devices such as disk files and pipes use the system locale
encoding (i.e. the ANSI codepage).  Non-console character
devices such as NUL (i.e. where isatty() returns True) use the
value of the console input and output codepages at startup,
respectively for stdin and stdout/stderr. This defaults to the
system locale encoding if the process is not initially attached
to a console.

The special behaviour of the console can be overridden
by setting the environment variable PYTHONLEGACYWINDOWSSTDIO
before starting Python. In that case, the console codepages are
used as for any other character device.

Under all platforms, you can override this value by
setting the PYTHONIOENCODING environment variable before
starting Python. However, for the Windows console, this
only applies when PYTHONLEGACYWINDOWSSTDIO is also set.

I updated the PR with the new wording by Paul, since I found it easier to understand as well.

New changeset 5723263 by Miss Islington (bot) (Lysandros Nikolaou) in branch 'master':
bpo-30410: Documentation of sys.stdin/out/err update to reflect change in 3.6 (GH-10264)
5723263

Fixed in 3.8 and 3.7.
Thanks!

New changeset b8bcec3 by Miss Islington (bot) in branch '3.7':
bpo-30410: Documentation of sys.stdin/out/err update to reflect change in 3.6 (GH-10264)
b8bcec3

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Documentation for sys.stdout encoding does not reflect the new Windows behavior in Python 3.6+ #74595

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Documentation for sys.stdout encoding does not reflect the new Windows behavior in Python 3.6+ #74595

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pfmoore commented on May 20, 2017

eryksun commented on May 20, 2017

zooba commented on May 20, 2017

eryksun commented on May 21, 2017

lysnikolaou commented on Oct 28, 2018

zooba commented on Oct 29, 2018

lysnikolaou commented on Nov 30, 2018

pfmoore commented on Nov 30, 2018

lysnikolaou commented on Dec 2, 2018

miss-islington commented on Feb 14, 2019

Mariatta commented on Feb 14, 2019

miss-islington commented on Feb 14, 2019

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