open(): remove 'U' mode, deprecated since Python 3.3

[vstinner]

BPO	37330
Nosy	@pitrou, @vstinner, @merwok, @ambv, @serhiy-storchaka, @asottile, @miss-islington, @abartlet
PRs	bpo-37330: open() no longer accept 'U' in file mode #14204 bpo-37330: open() no longer accept 'U' in file mode #16959 [3.8] bpo-37330: open(): "U" mode is removed in Python 3.9 #16972 [3.7] bpo-37330: open(): "U" mode is removed in Python 3.9 (GH-16972) #16982 bpo-39674: Revert "bpo-37330: open() no longer accept 'U' in file mode (GH-16959)" #18767 bpo-37330: open() no longer accept 'U' in file mode #28118

^{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 2021-09-02.11:16:54.115>
created_at = <Date 2019-06-18.15:48:25.669>
labels = ['library', '3.11']
title = "open(): remove 'U' mode, deprecated since Python 3.3"
updated_at = <Date 2022-01-05.17:46:12.863>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2022-01-05.17:46:12.863>
actor = 'eric.araujo'
assignee = 'none'
closed = True
closed_date = <Date 2021-09-02.11:16:54.115>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2019-06-18.15:48:25.669>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 37330
keywords = ['patch']
message_count = 25.0
messages = ['345991', '345995', '346219', '348007', '355479', '355543', '355544', '355573', '355633', '355635', '355636', '362362', '363194', '363211', '363247', '363320', '363321', '363344', '363366', '400884', '400888', '400889', '400914', '400917', '409791']
nosy_count = 8.0
nosy_names = ['pitrou', 'vstinner', 'eric.araujo', 'lukasz.langa', 'serhiy.storchaka', 'Anthony Sottile', 'miss-islington', 'abartlet']
pr_nums = ['14204', '16959', '16972', '16982', '18767', '28118']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue37330'
versions = ['Python 3.11']

[vstinner]

The builtin open() function accepts 'U' in the file mode, but it emits a DeprecationWarning since Python 3.4, change made in bpo-15204:

commit 6787a38
Author: Serhiy Storchaka <storchaka@gmail.com>
Date: Sat Nov 23 22:12:06 2013 +0200

Issue bpo-15204: Deprecated the 'U' mode in file-like objects.

$ cat x.py 
f = open(__file__, 'U')
line = f.readline()
print(type(line))
f.close()

$ python3.7 x.py 
x.py:1: DeprecationWarning: 'U' mode is deprecated
  f = open(__file__, 'U')
<class 'str'>

The 'U' mode is documented as deprecated since Python 3.3

The flag is basically only there for backward compatibility with Python 2. Python 3.9.0 final release is currently scheduled at 2020-06-08: 6 months after Python 2 end-of-life (2020-01-01).

Currently, the 'U' mode is documented as it will be removed from Python 4.0. But I dislike having a bunch of backward incompatible changes in Python 4.0. I would prefer Python 4.0 to be simply the version after Python 3.X.

Deprecation warnings are now displayed in the __main__ module by default, thanks to the PEP-565 -- Show DeprecationWarning in __main__.
https://www.python.org/dev/peps/pep-0565/

More and more projects run their test suite using -Werror and so have to fix deprecation warnings.

[vstinner]

I prefer to push backward incompatible changes at the beginning of a dev cycle (Python 3.9), so we have room to revert it later if it causes too much trouble.

[vstinner]

I reported the issue to docutils. In fact, docutils FileInput class was already fixed to prevent the deprecation warning, but there was no release yet:
https://sourceforge.net/p/docutils/bugs/363/

[vstinner]

docutils is already fixed in the development version. I requested a release:
https://sourceforge.net/p/docutils/bugs/364/

[vstinner]

I cited this change in the PEP-608 "Coordinated Python release".

[vstinner]

New changeset e471e72 by Victor Stinner in branch 'master':
bpo-37330: open() no longer accept 'U' in file mode (GH-16959)
e471e72

[vstinner]

Thanks for the review Serhiy ;-)

[asottile]

should we backport a documentation change for this? (the deprecatedremoved says 4.0 currently)

[vstinner]

New changeset 1d2862a by Victor Stinner in branch '3.8':
bpo-37330: open(): "U" mode is removed in Python 3.9 (GH-16972)
1d2862a

[miss-islington]

New changeset 4cb08b6 by Miss Skeleton (bot) in branch '3.7':
bpo-37330: open(): "U" mode is removed in Python 3.9 (GH-16972)
4cb08b6

[vstinner]

should we backport a documentation change for this? (the deprecatedremoved says 4.0 currently)

Done. Thanks for the reminder.

[abartlet]

This breaks Samba's build:

https://bugzilla.samba.org/show_bug.cgi?id=14266

While we are of course able to patch new versions of Samba, this will make it harder to bisect back though Samba history. It would be really great if this kind of change could be reverted or at least deferred so Samba can rely on a stable language platform for it's build system.