gh-148085: datetime cache time module lookups

[maurycy]

See gh-148085 for more details.

This takes a lazy approach, in the spirit of lazy, instead of preloading in init_state(). The trade-off is decreased readability.

Please see Links to previous discussion of this feature section in the gh-148085. My understanding is that only @ericsnowcurrently's work in gh-119810 made this possible and clean.

Benchmark

Benchmark	main-c50d6cd	datetime-cache-time-module-lookups-ae32bfd
datetime.timetuple	260 ns	124 ns: 2.10x faster
datetime.strftime	784 ns	517 ns: 1.52x faster
date.timetuple	258 ns	124 ns: 2.08x faster
Geometric mean	(ref)	1.88x faster

For:

import pyperf, datetime

r = pyperf.Runner()
dt = datetime.datetime(2026, 4, 4, 12, 30, 45)
d = datetime.date(2026, 4, 4)

r.bench_func("datetime.timetuple", dt.timetuple)
r.bench_func("datetime.strftime", dt.strftime, "%Y-%m-%d")
r.bench_func("date.timetuple", d.timetuple)

For:

2026-04-04T14:17:02.794335000+0200 maurycy@gimel /Users/maurycy/work/cpython-main (main c50d6cd) % ./python.exe -c "import sysconfig; print(sysconfig.get_config_var('CONFIG_ARGS'))"
'--enable-optimizations' '--with-lto'
[127] 2026-04-04T14:17:24.011667000+0200 maurycy@gimel /Users/maurycy/src/github.com/maurycy/cpython (datetime-cache-time-module-lookups a49f862) % ./python.exe -c "import sysconfig; print(sysconfig.get_config_var('CONFIG_ARGS'))"
'--enable-optimizations' '--with-lto'

[StanFromIreland]

We don't have a precedent for lazy caching, I think it's always eager? I'm worried this might cause issues, historically datetime and caching hasn't worked out.

[maurycy]

@StanFromIreland We do:

https://github.com/python/cpython/blob/dea4083aa95/Modules/arraymodule.c#L2561

https://github.com/python/cpython/blob/dea4083aa95/Modules/_decimal/_decimal.c#L3699

I copied my approach from there. I'm OK with moving to init_state(), somehow PEP 810 set the tone.

I believe the reason why it didn't work is that it relied on a global variable, instead of per-interpreter module. I left a note in #148085 in the previous discussions feature section.

[StanFromIreland]

Thinking more about this, I think we could just replace this with a call to PyTime_Time directly, I assume this predates the existence of the C-API.

[picnixz]

I remember there were issues with datetime and the capsule API so be careful there. Likewise, if you do per-state caching, it may break existing hooks (removing the module from sys.modules will not make it reload I think)