datetime, date and time: strftime method takes different keyword argument: fmt (pure) or format (C)

[asottile]

BPO	41260
Nosy	@srittau, @abalkin, @asottile, @pganssle, @ZackerySpytz, @AlexWaygood
PRs	bpo-41260: C impl of datetime.date.strftime() takes different keyword arg #21712

^{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 2020-07-09.17:01:48.789>
labels = ['type-bug', '3.9', '3.10', '3.11', 'extension-modules', 'library']
title = 'datetime, date and time: strftime method takes different keyword argument: fmt (pure) or format (C)'
updated_at = <Date 2021-11-16.19:27:41.778>
user = 'https://github.com/asottile'

bugs.python.org fields:

activity = <Date 2021-11-16.19:27:41.778>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'Library (Lib)']
creation = <Date 2020-07-09.17:01:48.789>
creator = 'Anthony Sottile'
dependencies = []
files = []
hgrepos = []
issue_num = 41260
keywords = ['patch']
message_count = 6.0
messages = ['373407', '373421', '373424', '406418', '406421', '406424']
nosy_count = 6.0
nosy_names = ['srittau', 'belopolsky', 'Anthony Sottile', 'p-ganssle', 'ZackerySpytz', 'AlexWaygood']
pr_nums = ['21712']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41260'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-85432: Harmonise parameter names between C and pure-Python implementations of datetime.time.strftime, datetime.datetime.fromtimestamp #99993

[asottile]

C:

cpython/Modules/_datetimemodule.c

Line 3183 in 8b33961

static char *keywords[] ={"format", NULL};

pure python:

cpython/Lib/datetime.py

Line 927 in 8b33961

def strftime(self, fmt):

this makes it difficult to properly type in mypy:

https://github.com/python/typeshed/blob/209b6bb127f61fe173a60776e23883ac450cf1c8/stdlib/2and3/datetime.pyi#L55

and calling with .strftime(fmt=...) or .strftime(format=...) is inconsistent

(that said, it should _probably_ be a positional-only argument)

[pganssle]

I think a positional-only argument would be the best option if we could do it, but it would be a backwards-incompatible change and it's probably not worth the hassle.

If anyone is using the keyword argument, they're probably using format= rather than fmt=, since it's pretty rare for anyone to use the pure python version of datetime. PyPy uses it, but I think they tend to aim for consistency with the C API of CPython, and it seems like they already patch s/fmt/format/ themselves: https://foss.heptapod.net/pypy/pypy/-/blob/branch/default/lib_pypy/datetime.py#L781

If anyone wants to make a PR I think we can fix this for 3.10, though unfortunately because it is an API change it can't be backported. I think in typeshed they can safely change from fmt to format even today (which would almost certainly be more accurate to end user use cases).

[asottile]

awesome, I'm going to work through this with someone in my discord as a demo / mentorship opportunity -- hope that's ok!

[pganssle]

Updating this issue to cover the problem in date, time and datetime.