[skilpat]

I've updated the post to clarify that one should not use `pytz` at all and should instead use `dateutil`. The latter has much more reasonable behavior and is more actively maintained. I also included a link to commentary about pytz by dateutil's current maintainer.

Why hasn't the bug been fixed? I'm not sure! But looking at the code it seems like this problem has existed for the past 9 years. My guess is that the developers and maintainers never saw it as an actual problem, despite its ubiquity.

[mark-r]

I believe the problem comes from a conceptual mismatch of what a timezone object should be. The datetime people envisioned a dumb object that just contains some constants, not the dynamic objects produced by pytz. Using .localize as emphasized in the pytz documentation solves the problem completely.

It would be less of a problem if pytz defaulted to the last offset in the database rather than the first.

[x1798DE]

I think your concept of "dynamic" and "static" is exactly backwards from mine. datetime specifies an API (tzinfo) for dynamic objects, so that a single object provides the time zone information for many datetimes. pytz's localize function attaches a different static object to each datetime, depending on which one is appropriate.

pytz defaulting to the last offset instead of the first would cause a lot more silent breakages, because it would probably be right about half the time, and wrong about half the time, and even when it's wrong it won't be obvious. Defaulting to the first value in the list is as close as pytz can come to failing loudly without actually raising an exception, because it's obviously wrong nearly 100% of the time.

[mkesper]

Don't guess, fail explicitly.

[buckminster]

> I believe the problem comes from a conceptual mismatch of what a timezone object should be.

There seem to be two commonly understood meanings of timezone which are confused in UIs and APIs: a geographical area which follows a particular winter and summertime regime over the year; and a particular offset from UTC like GMT or BST. I've seen a lot of bad design rooted in this confusion.