Inconsistent behaviour in strftime

[valdemarrolfsen]

BPO	41702
Nosy	@abalkin, @ned-deily, @MojoVampire, @pganssle, @valdemarrolfsen

^{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 2021-12-10.09:51:56.998>
created_at = <Date 2020-09-03.11:45:52.911>
labels = ['3.7', 'type-bug', 'library']
title = 'Inconsistent behaviour in strftime'
updated_at = <Date 2021-12-10.09:51:56.996>
user = 'https://github.com/valdemarrolfsen'

bugs.python.org fields:

activity = <Date 2021-12-10.09:51:56.996>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-10.09:51:56.998>
closer = 'iritkatriel'
components = ['Library (Lib)']
creation = <Date 2020-09-03.11:45:52.911>
creator = 'valdemarrolfsen'
dependencies = []
files = []
hgrepos = []
issue_num = 41702
keywords = []
message_count = 3.0
messages = ['376296', '376313', '376540']
nosy_count = 5.0
nosy_names = ['belopolsky', 'ned.deily', 'josh.r', 'p-ganssle', 'valdemarrolfsen']
pr_nums = []
priority = 'normal'
resolution = 'works for me'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41702'
versions = ['Python 3.7']

[valdemarrolfsen]

Inconsistency in strftime between python 3.6 and 3.7 when parsing first-century dates.

Python 3.6
>>> datetime.datetime.strptime(d, "%Y-%m-%d").strftime("%Y-%m-%d")
'0020-10-05'

Python 3.7
>>> datetime.datetime.strptime("0020-10-05", "%Y-%m-%d").strftime("%Y-%m-%d")
'20-10-05'

This means that the following would work for 3.6 but raise a ValueError for 3.7:

>> d = "0020-10-05"
>> d = datetime.datetime.strptime(d, "%Y-%m-%d").strftime("%Y-%m-%d")
>> d = datetime.datetime.strptime(d, "%Y-%m-%d").strftime("%Y-%m-%d")

[MojoVampire]

3.8.2 (on Alpine Linux under WSL) produces '0020-10-05', just like your 3.6 example. Not seeing anything obvious in commit history that would break it for 3.7. That said, 3.7 is in security fix only mode at this point (see https://devguide.python.org/#status-of-python-branches ); as this works on the latest release, I'm thinking this won't be fixed for 3.7.

[ned-deily]

The behavior you see with a Python 3.7 is not universal. For example, on macOS:

Python 3.7.9 (v3.7.9:13c94747c7, Aug 15 2020, 01:31:08)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> datetime.datetime.strptime("0020-10-05", "%Y-%m-%d").strftime("%Y-%m-%d")
'0020-10-05'

So there is apparently something different in the environments between the Python 3.6 and 3.7 you are using, rather than an issue in Python itself. Perhaps a comparison of the outputs between:

python3.6 -m test.pythoninfo
python3.7 -m test.pythoninfo

will suggest something. In any case, as Josh notes, Python 3.7 is in the security-fix-only phase of its life cycle so even if there were an issue in Python itself it would likely not meet the criteria to be fixed in 3.7.