Using naive object to get timezone out of a timestamp should not raise an error

[LefterisJP]

I am not sure if this is the right way to do it. but if you want to get a datetime from a timestamp and be aware of the timestamp's timezone (especially whether it's DST or not) you need to do the following (if there is a better/smarter way please let me know because this feels weird). This is in my locale time CET/CEST.

>>> formatstr = '%d/%m/%Y %H:%M:%S %Z'
>>> ts = 1675261083
>>> datetime.datetime.fromtimestamp(ts, tz=datetime.datetime.fromtimestamp(ts).astimezone().tzinfo).strftime(formatstr)
'01/02/2023 15:18:03 CET'
>>> ts = 1686065064
>>> datetime.datetime.fromtimestamp(ts, tz=datetime.datetime.fromtimestamp(ts).astimezone().tzinfo).strftime(formatstr)
'06/06/2023 17:24:24 CEST

This works fine and is correct but flake8-datetimez complains DTZ006 The use ofdatetime.datetime.fromtimestamp()withouttz argument is not allowed.

I think this is a false positive in this case and it should not complain. I am going to add noqa ignore but perhaps there is a way to code this into the logic of the tool?

[kbehlers]

@LefterisJP I believe the warning is legitimate. The call to .astimezone() uses your local system timezone:

If called without arguments (or with tz=None) the system local timezone is assumed for the target timezone.

As the warning suggests, it would be best to pass the tz argument to fromtimestamp so that the output doesn't change based on the local system timezone of the execution environment.

If you are on Python 3.9+, you can use zoneinfo.Zoneinfo to do that, otherwise you will need to use a third-party package like dateutil.

For your case, that would look something like:

>>> from datetime import datetime
>>> import zoneinfo
>>> cet_tz = zoneinfo.ZoneInfo("CET")
>>> datetime.fromtimestamp(ts, tz=cet_tz).strftime(formatstr)
'06/06/2023 17:24:24 CEST'

Longer Example:

>>> import os
>>> import time
>>> from datetime import datetime
>>> os.environ["TZ"] = "CET"  # python respects the TZ env var
>>> time.tzset()  # https://docs.python.org/3/library/time.html#time.tzset
>>> formatstr = '%d/%m/%Y %H:%M:%S %Z'
>>> ts = 1675261083
>>> datetime.fromtimestamp(ts).astimezone().strftime(formatstr)  # simplified version of original
'01/02/2023 15:18:03 CET'
>>> ts = 1686065064
>>> datetime.fromtimestamp(ts).astimezone().strftime(formatstr)  # simplified version of original
'06/06/2023 17:24:24 CEST'
>>> os.environ["TZ"] = "UTC"
>>> time.tzset()  # https://docs.python.org/3/library/time.html#time.tzset
>>> datetime.fromtimestamp(ts).astimezone().strftime(formatstr)
'06/06/2023 15:24:24 UTC'
>>> import zoneinfo
>>> cet_tz = zoneinfo.ZoneInfo("CET")
>>> datetime.fromtimestamp(ts, tz=cet_tz).strftime(formatstr)
'06/06/2023 17:24:24 CEST'  # Despite TZ being UTC, passing tz=cet_tz forces CET time