[issue46169] Same-moment datetimes with different ZoneInfo timezones are not considered ==

[Huon Wilson]

New submission from Huon Wilson :

The documentation suggests that two datetimes that represent the same moment 
should compare equal, even if they have different timezones: "If both 
comparands are aware and have different tzinfo attributes, the comparands are 
first adjusted by subtracting their UTC offsets (obtained from 
self.utcoffset())." (below 
https://docs.python.org/3/library/datetime.html#datetime.datetime.fold)

This doesn't seem to be true for == with ZoneInfo timezones, even though it is 
true for <= and >=, and -, meaning these seem to violate mathematical 
laws/expectations:

from zoneinfo import ZoneInfo
from datetime import datetime, timezone

dt_utc = datetime(2020, 11, 1, 8, tzinfo=timezone.utc)
dt_local = dt_utc.astimezone(ZoneInfo("America/Los_Angeles"))
print(f"{dt_local == dt_utc = }")
print(f"{dt_local <= dt_utc = }")
print(f"{dt_local >= dt_utc = }")
print(f"{dt_local - dt_utc = }")

Output:

dt_local == dt_utc = False
dt_local <= dt_utc = True
dt_local >= dt_utc = True
dt_local - dt_utc = datetime.timedelta(0)

Tested with:

- macOS 11.4; Python 3.9.7, 3.10.1, 3.11.0a3
- Linux; via docker image python:3.9.7

Full test including comparisons to python-dateutil (same behaviour as ZoneInfo) 
and pytz (== gives True as expected) is attached.

Comparing timestamps for exact equality is potentially unusual, but we use it 
extensively in tests for some time-handling functions.

--
components: Library (Lib)
files: tz.py
messages: 409113
nosy: huonw
priority: normal
severity: normal
status: open
title: Same-moment datetimes with different ZoneInfo timezones are not 
considered ==
type: behavior
versions: Python 3.10, Python 3.11, Python 3.9
Added file: https://bugs.python.org/file50516/tz.py

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[issue46169] Same-moment datetimes with different ZoneInfo timezones are not considered ==

[Daniel Diniz]

Daniel Diniz  added the comment:

Confirmed for 3.11 in Windows. 

The C datetime code can be trivially fixed so your equality test returns True, 
but there are two Python tests that depend on current behavior so it might not 
be so easy. They were added with current code in issue 24773, to implement PEP 
495 -- Local Time Disambiguation.

Also, using a timezone implementation from the test suite makes the equality 
work, so maybe the bug depends on zoneinfo.

If you change datetimemodule.c line 5761 from "diff = 1;" to "diff = 0;", two 
tests fail and your code works.

See attached file for the tests that fail and using the timezone implementation 
mentioned above.

--
nosy: +ajaksu2
Added file: https://bugs.python.org/file50517/dt_equality.py

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[issue46169] Same-moment datetimes with different ZoneInfo timezones are not considered ==

[Daniel Diniz]

Change by Daniel Diniz :


--
nosy: +belopolsky

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