Interpret naive datetimes as local time

[adamchainz]

Description

Python defaults to interpreting naive datetimes as local time: https://blog.ganssle.io/articles/2022/04/naive-local-datetimes.html

time-machine currently turns naive datetimes into UTC, except from naive datetiems parsed from strings with datetutil, which it keeps in local times: https://twitter.com/hugovk/status/1513513262649950210

@pganssle would prefer time-machine to be consistent with Python's behaviour: https://twitter.com/pganssle/status/1513507255253184518

I'm inclined to agree. It would be good to at least be consistent between datetimes and strings. It would be a breaking change though.

We might also consider a warning for naive datetimes, as this can allow a test's success to depend on the timezone in which it is run.

[PhML]

I am not sure it is related, but this is very confusing:

In [1]: from datetime import datetime

In [2]: from freezegun import freeze_time

In [3]: import time_machine

In [4]: fake_now = datetime(2022, 7, 12)

In [5]: @freeze_time(fake_now)
   ...: def test_freeze_time():
   ...:     return datetime.now().isoformat()
   ...:

In [6]: @time_machine.travel(fake_now)
   ...: def test_time_machine():
   ...:     return datetime.now().isoformat()
   ...:

In [7]: fake_now.isoformat()
Out[7]: '2022-07-12T00:00:00'

In [8]: test_freeze_time()
Out[8]: '2022-07-12T00:00:00'

In [9]: test_time_machine()
Out[9]: '2022-07-12T02:00:00'

The time_machine traveled datetime.now is 2 hours in advance from my fake_now datetime.

[soxofaan]

Same head-scratching over here (local timezone: UTC+1):

for destination in [
    dt.datetime(2000, 1, 1),
    "2000-01-01",
    "2000-01-01 00+00",
]:
    print(f"Travel to {destination!r}")
    with time_machine.travel(destination, tick=False):
        print(f"  utcnow:   {dt.datetime.utcnow()}")
        print(f"  now:      {dt.datetime.now()}")

output = """
Travel to datetime.datetime(2000, 1, 1, 0, 0)
  utcnow:   2000-01-01 00:00:00
  now:      2000-01-01 01:00:00
Travel to '2000-01-01'
  utcnow:   1999-12-31 23:00:00
  now:      2000-01-01 00:00:00
Travel to '2000-01-01 00+00'
  utcnow:   2000-01-01 00:00:00
  now:      2000-01-01 01:00:00
"""

It's annoying that the quick and easy way of specifying a date as string (second case with "2000-01-01" in example) does not work as expected, and one should at least explicitly have a timezone in the string (00+00) or use datetime object.

I think there are multiple (related) issues raised here:

1. The current docs wrongly describe current behavior:

   destination specifies the datetime to move to. It may be: ...

   • A string, which will be parsed with dateutil.parse and converted to a timestamp. Again, if the result is naive, it will be assumed to have the UTC time zone.

   the example above (and the tweets mentioned higher) show that naive is assumed to be local time, not UTC.

2. Inconsistent behavior between handling a naive destination: a naive datetime object is assumed to be UTC, a naive string is assumed to be local time. This is counter-intuitive to say at least.

3. If I understand correctly, the OP (title "... naive ... as local time") is about changing the handling of all naive destination as local instead of UTC.

Fixing 1. (without tackling 2. and 3. yet) is easy I guess. But fixing 2. or 3. would indeed breaking changes.

Personally, I'd prefer to handle all naive destinations as UTC, because that makes most sense in a test/CI context. But I understand the desire to be consistent with Python conventions.

[soxofaan]

If you would go for fixing 3. from my previous comment (so travel(destination=...) would interpret naive as local in all cases), could it be an option to introduce a variant travel_utc(destination=...) (or a classmethod factory travel.utc(destination=...)) that would interpret all naive input as UTC instead?

[adamchainz]

could it be an option to introduce a variant travel_utc(destination=...) (or a classmethod factory travel.utc(destination=...)) that would interpret all naive input as UTC instead?

I wouldn’t like to complicate things, no. You could use a wrapper in your projcets.