Python datetime not including DST when using pytz timezone

Question

If I convert a UTC datetime to swedish format, summertime is included (CEST). However, while creating a datetime with sweden as the timezone, it gets CET instead of CEST. Why is this?

>>> # Modified for readability
>>> import pytz
>>> import datetime
>>> sweden = pytz.timezone('Europe/Stockholm')
>>>
>>> datetime.datetime(2010, 4, 20, 16, 20, tzinfo=pytz.utc).astimezone(sweden)
datetime(2010, 4, 20, 18, 20, tzinfo=<... 'Europe/Stockholm' CEST+2:00:00 DST>)
>>> 
>>> datetime.datetime(2010, 4, 20, 18, 20, tzinfo=sweden)
datetime(2010, 4, 20, 18, 20, tzinfo=<... 'Europe/Stockholm' CET+1:00:00 STD>)
>>>

Answer 1

The sweden object specifies the CET time zone by default but contains enough information to know when CEST starts and stop.

In the first example, you create a datetime object and convert it to local time. The sweden object knows that the UTC time you passed occurs during daylight savings time and can convert it appropriately.

In the second example, the datetime constructor always interprets your input as not-daylight-savings-time and returns an appropriate object.

If datetime treated your input as wall-clock time and chose the appropriate daylight-savings setting for you, there would be an ambiguity during the time of year when clocks are set back. On a wall-clock the same hour occurs twice. Hence, datetime forces you to specify which timezone you're using when you create the datetime object.

Answer 2

Timezone abbreviations are not unique. For example "IST" could refer to "Irish Standard Time", "Iranian Standard Time", "Indian Standard Time" or "Isreali Standard Time". You shouldn't rely on parsing that, and instead should use zoneinfo timezones.