Transition from pytz to zoneinfo

[niccokunzmann]

In EOL of Python 3.8 is 31st October 2024. Then, zoneinfo is the library that is shipped with Python. There are problems using pytz as timezone implementations have evolved. My proposal to solve this issue is the following:

• create icalendar.set_default_timezone_implementation(TimeZone) where TimeZone can be pytz.Timezone or zoneinfo.ZoneInfo or others, taking a string like "UTC" or Europe/Berlin.
• Allow a switch from zoneinfo to pytz by testing all timezone dependent tests with both implementations
• a version 6 release after October will then switch from pytz to zoneinfo by setting another default and removing pytz altogether as a dependency. Tracked here: Version 6 - roadmap #630

What are your thoughts? Who thinks this will break existing code? What is needed for a transition?

See also:

• Funding from NLnet Foundation :tada: #595
• Making icalendar.from_ical independent of pytz and usable with all timezone implementations #532

Possibly related:

• Datetimes with timezones from dateutil.tz badly supported #336
• dst_offset is not False #321

Code that needs touching:
The Timezone class contains logic to parse and create timezones from icalendar specifications:

icalendar/src/icalendar/cal.py

Line 545 in 2b1cd4e

class Timezone(Component):

[niccokunzmann]

Also:

• Pytz has certain timezone names. We should support all of them with zoneinfo., see Zoneinfo #623 (comment)
• Empty the timezone cache when switching timezone implementations.
• Maybe, we can do a pre-release and ask people to test it and come back with ideas. - tracked in Version 6 - roadmap #630
• make tzdata a dependency
• document how to switch the timezone provider
• run the tests with pytz as default and with zoneinfo as default (set at the start before any test is run) - we have global state. This is how we test that it really works with both versions.

For a new release:

• remove pytz as a dependecy - Remove pytz dependency #650
• create release v6 tracked in Version 6 - roadmap #630
• switch over to zoneinfo (default now)
• generate a warning if pytz is installed and how to make icalendar use pytz. - tracked in Version 6 - roadmap #630

[niccokunzmann]

I created #623 to work on some improvements.

[niccokunzmann]

This is complete. We do not depend on pytz any more.

🥳