python-tz am I wrong or it's a bug

Question

Hi Guy,

It's a bit weird it seems that when I want to get a timezone for Europe/Paris with pytz it gets me to the PMT timezone instead of GMT+1 when it seems to work for Europe/Berlin.

Not clear ? Well look at this snippet :

#!/usr/bin/python
import os
import datetime
from pytz.tzfile import build_tzinfo

base='/usr/share/zoneinfo/'
tz = build_tzinfo('Europe/Paris',
                  open(os.path.join(base,'Europe','Paris'), 'rb'))
fmt = '%Y-%m-%d %H:%M:%S %Z%z'
print datetime.datetime(2009, 01, 30, 9, 00, tzinfo=tz).strftime(fmt)

tz = build_tzinfo('Europe/Berlin',
                  open(os.path.join(base,'Europe','Berlin'), 'rb'))

print datetime.datetime(2009, 01, 30, 9, 00, tzinfo=tz).strftime(fmt)

the output is :

2009-01-30 09:00:00 PMT+0009
2009-01-30 09:00:00 CET+0100

when really paris should be as well CET+1.

Constructing from datetime.datetime.now(tz) would get the thing right no matter what.

Anybody have an idea ?

Answer 1

The docs say you can't use datetime.datetime(..., tzinfo) like you're doing:

Unfortunately using the tzinfo argument of the standard datetime constructors does not work with pytz for many timezones.

And curiously, despite all signs that the Europe/Paris timezone is wrong, when you actually use with localize as it recommends, it works nonetheless:

>>> tz= pytz.timezone('Europe/Paris')               # using built-in zoneinfo
>>> tz
<DstTzInfo 'Europe/Paris' PMT+0:09:00 STD>          # what? Pierre et Miquelon Time?
>>> datetime.datetime(2010,1,1,12,0,0, tzinfo=tz)
datetime.datetime(2010, 1, 1, 12, 0, tzinfo=<DstTzInfo 'Europe/Paris' PMT+0:09:00 STD>) # bad
>>> tz.localize(datetime.datetime(2010,1,1,12,0,0))
datetime.datetime(2010, 1, 1, 12, 0, tzinfo=<DstTzInfo 'Europe/Paris' CET+1:00:00 STD>) # OK

Note that the tzinfo property of the localized datetime references a completely different object to the tz it was created from, sharing only the name.

It's a mystery to me why this is happening. It seems to be a problem with city files that understand multiple timezone names, but why you don't get the default timezone for a city until you call localize, I've no idea.

(To be honest I've never trusted Python's datetime and tzinfo stuff. Much prefer working with int UTC timestamps.)