Full set of format codes applies to strftime only

[vkatsikaros]

BPO	34249
Nosy	@pganssle, @vkatsikaros
PRs	bpo-34249: Full set of format codes applies to strftime only #8502

^{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 = None
created_at = <Date 2018-07-27.15:07:15.820>
labels = ['3.7', '3.8', '3.9', '3.10', 'type-feature', 'docs']
title = 'Full set of format codes applies to strftime only'
updated_at = <Date 2020-05-31.13:44:57.308>
user = 'https://github.com/vkatsikaros'

bugs.python.org fields:

activity = <Date 2020-05-31.13:44:57.308>
actor = 'serhiy.storchaka'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-07-27.15:07:15.820>
creator = 'v kats'
dependencies = []
files = []
hgrepos = []
issue_num = 34249
keywords = ['patch']
message_count = 3.0
messages = ['322498', '322499', '339993']
nosy_count = 3.0
nosy_names = ['docs@python', 'p-ganssle', 'v kats']
pr_nums = ['8502']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue34249'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[vkatsikaros]

Section "strftime() and strptime() Behavior" says:

"The full set of format codes supported varies across platforms, because Python calls the platform C library’s strftime() function, and platform variations are common. To see the full set of format codes supported on your platform, consult the strftime(3) documentation."

It implies that this applies for strftime only, and indeed it refers to C library’s strftime().

However, it's easy to miss (at least I got confused and lost some time), so I propose to clarify the paragraph.

strptime and %s

# python
Python 2.7.6 (default, Nov 23 2017, 15:49:48) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from datetime import datetime
>>> datetime.strptime('1532689414','%s')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

strftime and %s

python
Python 2.7.6 (default, Nov 23 2017, 15:49:48) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from datetime import datetime
>>> datetime.now().strftime('%s')
'1532702817'

File "/usr/lib/python2.7/_strptime.py", line 317, in _strptime
(bad_directive, format))
ValueError: 's' is a bad directive in format '%s'

[vkatsikaros]

Note, the last 3 lines of the previous comment seem to be a rogue copy-paste

[pganssle]

To clarify, as far as I know, this note *does* apply to both strftime and strptime, as CPython doesn't have its own implementation of either. I'm not sure if any guarantees are made that the supported formatting codes will be the same between the two.

[StanFromIreland]

%s is not listed in the documentation, therefore its unsupported, only %S is. It is also not supported in the C strptime.

I don't know what to do here, I guess a note could be possibly added, but then again, why?

The version labels can be removed, its still relevant, or it can just be closed really.