python - datetime and timezone conversion with pytz - mind blowing behaviour

Question

I'm trying to convert timezone aware datetime object to UTC and then back to it's original timezone. I have a following snippet

t = datetime(
    2013, 11, 22, hour=11, minute=0,
    tzinfo=pytz.timezone('Europe/Warsaw')
)

now in ipython:

In [18]: t
Out[18]: datetime.datetime(
    2013, 11, 22, 11, 0, tzinfo=<DstTzInfo 'Europe/Warsaw' WMT+1:24:00 STD>
)

and now let's try to do conversion to UTC and back. I would expect to have the same representation as:

In [19]: t.astimezone(pytz.utc).astimezone(pytz.timezone('Europe/Warsaw'))
Out[19]: datetime.datetime(
    2013, 11, 22, 10, 36, tzinfo=<DstTzInfo 'Europe/Warsaw' CET+1:00:00 STD>
)

Yet we see that Out[18] and Out[19] differ. What's going on?

Django stores datetimes naive database wise. If you handle that case, have a look here to convert dates to the timezone you need: docs.djangoproject.com/en/dev/topics/i18n/timezones/#usage — Jingo
whether Django stores naive datetimes of not is controlled by the USE_TZ setting. If that is True then Django treats datetimes in forms etc as being in the current timezone and stores them in the db converted to UTC. — Anentropic
Could you specify the "mind blowing behaviour" further? I don't like clickbait on StackExchange — Martin Thoma

James K James K · Accepted Answer · 2013-08-30T20:55:07

The documentation http://pytz.sourceforge.net/ states "Unfortunately using the tzinfo argument of the standard datetime constructors 'does not work' with pytz for many timezones." The code:

t = datetime(
    2013, 5, 11, hour=11, minute=0,
    tzinfo=pytz.timezone('Europe/Warsaw')
)

doesn't work according to this, instead you should use the localize method:

t = pytz.timezone('Europe/Warsaw').localize(
        datetime(2013, 5, 11, hour=11, minute=0))

python - datetime and timezone conversion with pytz - mind blowing behaviour

1 Answers