tzinfo and pytz

Python provides the powerful library to work with date and time: datetime. The interesting part is datetime objects have the special interface for timezone support (namely tzinfo attribute), but this module only has limited support of its interface, leaving the rest of the job to different modules.

The most popular module for this job is pytz. The tricky part pytz don't fully satisfy tzinfo interface. The pytz documentation states this at one the first lines: “This library differs from the documented Python API for tzinfo implementations.”

You can't use pytz timezone objects as a tzinfo attribute. If you try, you may get the absolute insane results:

In : paris = pytz.timezone('Europe/Paris')
In : str(datetime(2017, 1, 1, tzinfo=paris))
Out: '2017-01-01 00:00:00+00:09'

Look at this +00:09 offset. The proper use of pytz is following:

In : str(paris.localize(datetime(2017, 1, 1)))
Out: '2017-01-01 00:00:00+01:00'

Also, after any arithmetic operations, you should normalize your datetime object in case of offset changes (on the edge of DST period for instance).

In : new_time = time + timedelta(days=2)
In : str(new_time)
Out: '2018-03-27 00:00:00+01:00'
In : str(paris.normalize(new_time))
Out: '2018-03-27 01:00:00+02:00'

Since Python 3.6 it's recommended to use dateutil.tz instead of pytz. It's fully compatible with tzinfo, can be passed as an attribute, doesn't require normalize, though works a bit slower.

If you are interested why pytz doesn't support datetime API, or you wish to see more examples, consider reading the decent article on the topic.