Some timezones breaks after migrating from pytz to zoneinfo

[tobixen]

I just encountered a strange issue when running tests in my calendar-cli project. In particular, it broke with the DeNoronha timestamp (according to a contributor of test code to the caldav library, this place is well-known for being a "remote place", so it's used as a "remote time zone" in the tests). After migrating from pytz to zoneinfo the test code started breaking.

I wrote up the following code to highlight the breakage:

import datetime
import pytz
import zoneinfo
import icalendar

utc1 = pytz.utc
utc2 = zoneinfo.ZoneInfo('UTC')
oslo1 = pytz.timezone('Europe/Oslo')
oslo2 = zoneinfo.ZoneInfo('Europe/Oslo')
remote1 = pytz.timezone('Brazil/DeNoronha')
remote2 = zoneinfo.ZoneInfo('Brazil/DeNoronha')

for tz in (utc1, utc2, oslo1, oslo2, remote1, remote2):
    myevent = icalendar.Event()
    myevent.add('dtstart', datetime.datetime.now().astimezone(tz))
    print(myevent.to_ical().decode('ASCII'))

When running it, I get the following output:

BEGIN:VEVENT
DTSTART;VALUE=DATE-TIME:20211116T232629Z
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=UTC;VALUE=DATE-TIME:20211116T232629Z
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=Europe/Oslo;VALUE=DATE-TIME:20211117T002629
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=CET;VALUE=DATE-TIME:20211117T002629
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=Brazil/DeNoronha;VALUE=DATE-TIME:20211116T212629
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=-02;VALUE=DATE-TIME:20211116T212629
END:VEVENT

Note that for every time zone above, the output has changed simply from moving from pytz to zoneinfo. I would expect there to be no changes in the output, and I think the pytz behaviour is correct:

• for timestamp suffixed with Zulu, no point to specify UTC.
• The full name of the timezone should be given, and not the short name
• In particular, "-02" is simply not an acceptable TZID.

[tobixen]

I entered debug mode and found a function tzid_from_dt that returns the wrong thing

(Pdb) tzid_from_dt(pytz.timezone('Europe/Oslo').localize(datetime.now()))
'Europe/Oslo'
(Pdb) tzid_from_dt(datetime.now().astimezone(zoneinfo.ZoneInfo('Europe/Oslo')))
'CET'

[tobixen]

I found the tzid_from_dt and managed to mend it so the new test code passes.

However, it will still be broken for time zones created through dateutil.tz. There seems to be no trivial way to find the full name of the time zone from a given timezone object produced by dateutil.tz.

Timestamps and time zones in Python is such a mess ...

[tobixen]

New test run of the code above:

$ python ./test-ical.py
BEGIN:VEVENT
DTSTART;VALUE=DATE-TIME:20211117T150915Z
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=UTC;VALUE=DATE-TIME:20211117T150915Z
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=Europe/Oslo;VALUE=DATE-TIME:20211117T160915
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=Europe/Oslo;VALUE=DATE-TIME:20211117T160915
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=Brazil/DeNoronha;VALUE=DATE-TIME:20211117T130915
END:VEVENT

BEGIN:VEVENT
DTSTART;TZID=Brazil/DeNoronha;VALUE=DATE-TIME:20211117T130915
END:VEVENT

Still one issue here, the UTC time zone should not be displayed. I'll create a separate issue for that.

Replacing zoneinfo.ZoneInfo with dateutil.tz.gettz, the original issues still remain. I'll create a separate issue for that, too.

[NicoHood]

Does this library use pytz or zoneinfo? Because in the readme it states pytz and I was about to ask, if we can replace it with zoneinfo nowadays?

[tobixen]

Zoneinfo is the way ahead, pytz is legacy.

[tobixen]

I believe this bug can be closed, as #339 has been merged. Or should the issue stay open until a proper release has been done?

[niccokunzmann]

Hi, thanks again! I will close this when PRs get merged because issues are for development and the changelog for the releases. Thanks for asking!