Document removal of os.errno

[hroncok]

BPO	33666
Nosy	@vstinner, @encukou, @methane, @serhiy-storchaka, @hroncok
PRs	bpo-33666: Add what's new entry for os.errno removal #8497 [3.7] bpo-33666: Add what's new entry for os.errno removal (GH-8497) #8526

^{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 2018-07-29.11:08:23.230>
created_at = <Date 2018-05-28.13:06:57.881>
labels = ['3.7', '3.8', 'type-bug', 'library', 'docs']
title = 'Document removal of os.errno'
updated_at = <Date 2018-07-29.11:08:23.229>
user = 'https://github.com/hroncok'

bugs.python.org fields:

activity = <Date 2018-07-29.11:08:23.229>
actor = 'methane'
assignee = 'docs@python'
closed = True
closed_date = <Date 2018-07-29.11:08:23.230>
closer = 'methane'
components = ['Documentation', 'Library (Lib)']
creation = <Date 2018-05-28.13:06:57.881>
creator = 'hroncok'
dependencies = []
files = []
hgrepos = []
issue_num = 33666
keywords = ['patch']
message_count = 18.0
messages = ['317847', '317850', '317855', '317856', '317857', '317860', '317861', '317876', '317881', '317993', '317996', '317998', '317999', '318000', '318005', '318006', '322553', '322624']
nosy_count = 6.0
nosy_names = ['vstinner', 'petr.viktorin', 'methane', 'docs@python', 'serhiy.storchaka', 'hroncok']
pr_nums = ['8497', '8526']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33666'
versions = ['Python 3.7', 'Python 3.8']

[hroncok]

In 3.7.0b4 I see the following traceback:

>>> import os
>>> os.errno
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'os' has no attribute 'errno'

This was not the case for Python 3.6:

>>> import os
>>> os.errno
<module 'errno' (built-in)>

os.errno might not have been documented, however there are projects out there that use it:

intel/bmap-tools#34
https://bugzilla.redhat.com/show_bug.cgi?id=1583196

So I suggest the removal of os.errno is something worth documenting at https://docs.python.org/3.7/whatsnew/3.7.html

The root of the change is in https://github.com/python/cpython/pull/1269/files#diff-405b29928f2a3ae216e45afe9b5d0c60

[serhiy-storchaka]

os.errno as well as os.sys or os.abc is an implementation detail. It can be changed without notice even in a bugfix release. Projects that depend on it are incorrect and should be fixed.

[encukou]

I think this was closed prematurely.

Projects that depend on it are incorrect and should be fixed.

That's definitely correct; no one here is arguing against it.
However, there are projects out there depending on it -- it worked since around Python 2.5, and is readily available in os. tab completion.
I think that documenting the removal would be good for our users.

Furthermore, not all of its uses can be fixed. I've seen it printed in books. There are many answers/guides/tutorials around mentioning os.errno, for example:
http://nullege.com/codes/search/os.errno.EACCES
https://ubuntuforums.org/showthread.php?t=1459923
http://code.activestate.com/recipes/580759-show-os-error-codes-and-messages-from-the-oserrno-/
https://www.georgevreilly.com/blog/2016/03/24/RaisingIOErrorForFileNotFound.html

... and people tend based their code on such recipes, rather than checking what's documented.

And, of course, projects run into this:
intel/bmap-tools#34
https://bugzilla.redhat.com/show_bug.cgi?id=1583196
python/typeshed#1646

[vstinner]

os.errno as well as os.sys or os.abc is an implementation detail.

I agree but...

It can be changed without notice even in a bugfix release. Projects that depend on it are incorrect and should be fixed.

I don't think that it would hurt to document the removal of "os.errno" in "Porting to Python 3.7":
https://docs.python.org/dev/whatsnew/3.7.html#changes-in-python-behavior

Miro: are you volunteer to write such change?

[hroncok]

I can document this, yes. I've opened this issue so I could prep a PR.