I have an issue comparing outputs with dateutil
and pytz
. I'm creating a aware datetime object (UTC) and then converting to a given time zone, but I get different answers. I suspect that dateutil sometimes gives wrong results because it has problems taking into account daylight saving time (at least, I read a comment about it) but I can't find confirmation or a fix to that issue. This is the code:
import dateutil
u = dateutil.tz.tzutc()
date1 = datetime.datetime(2010, 5, 2, 11, 10, tzinfo=u)
# 2010-05-02 11:10:00+00:00
u2 = dateutil.tz.gettz('America/Chicago')
date2 = datetime.datetime(2010, 5, 2, 11, 10, tzinfo=u2)
# 2010-05-02 11:10:00-05:00
import pytz
u = pytz.timezone('UTC')
date1 = datetime.datetime(2010, 5, 2, 11, 10, tzinfo=u)
# 2010-05-02 11:10:00+00:00
u2 = pytz.timezone('America/Chicago')
date2 = datetime.datetime(2010, 5, 2, 11, 10, tzinfo=u2)
# 2010-05-02 11:10:00-06:00
So, what could be the problem here?
I just tried this:
print u2.normalize(date1.astimezone(u2))
# 2010-05-02 06:10:00-05:00
So pytz needs normalize
to consider DST?
It seemed as if pytz and dateutil don't give the answer for America/Argentina/San_Luis but this works:
import pytz, dateutil, datetime
now = datetime.datetime.now()
for zone in pytz.all_timezones:
utc_dateutil = dateutil.tz.tzutc()
utcdate_dateutil = datetime.datetime(now.year, now.month, now.day, now.hour, now.minute, tzinfo=utc_dateutil)
zone_dateutil = dateutil.tz.gettz(zone)
newzone_dateutil = utcdate_dateutil.astimezone(zone_dateutil)
utc_pytz = pytz.timezone('UTC')
utcdate_pytz = datetime.datetime(now.year, now.month, now.day, now.hour, now.minute, tzinfo=utc_pytz)
zone_pytz = pytz.timezone(zone)
newzone_pytz = utcdate_pytz.astimezone(zone_pytz)
assert newzone_dateutil == newzone_pytz
Am I missing something?
Thanks
Pytz brings the Olson tz database into Python and thus supports almost all time zones. This module serves the date-time conversion functionalities and helps user serving international client's base.
Functions. dateutil.tz. gettz (name=None) Retrieve a time zone object from a string representation. This function is intended to retrieve the tzinfo subclass that best represents the time zone that would be used if a POSIX TZ variable were set to the same value.
The pytz package encourages using UTC for internal timezone representation by including a special UTC implementation based on the standard Python reference implementation in the Python documentation. The UTC timezone unpickles to be the same instance, and pickles to a smaller size than other pytz tzinfo instances.
Edit: The discrepancy discussed below no longer exists when using
>>> dateutil.__version__
'1.5'
>>> pytz.__version__
'2012c'
The pytz module warns,
this library differs from the documented Python API for tzinfo implementations; if you want to create local wallclock times you need to use the localize() method
and further on
This library only supports two ways of building a localized time. The first is to use the localize() method provided by the pytz library.
In [61]: u4 = pytz.timezone('America/Chicago')
In [62]: print(u4.localize(datetime.datetime(2010, 5, 2, 11, 10)))
2010-05-02 11:10:00-05:00
The other way is to use the astimezone
method, which is used to convert a timezone-aware datetime into another timezone-aware datetime.
And to be completely explicit, it warns against constructing a timezone-aware datetime using the tzinfo
argument:
Unfortunately using the tzinfo argument of the standard datetime constructors ‘’does not work’’ with pytz for many timezones.
Let's test the hypothesis that
datetime.datetime(year, month, day, hour, minute, tzinfo = dateutil_tz)
equals
pytz_tz.localize(datetime.datetime(year, month, day, hour, minute))
with this code:
import dateutil.tz
import datetime
import pytz
now = datetime.datetime.now()
for name in pytz.all_timezones:
dateutil_tz = dateutil.tz.gettz(name)
pytz_tz = pytz.timezone(name)
dateutil_date = datetime.datetime(
now.year, now.month, now.day, now.hour, now.minute, tzinfo = dateutil_tz)
pytz_date = pytz_tz.localize(datetime.datetime(
now.year, now.month, now.day, now.hour, now.minute))
try:
assert dateutil_date.isoformat() == pytz_date.isoformat()
except AssertionError:
print(name)
print(dateutil_date.isoformat())
print(pytz_date.isoformat())
The code yields:
America/Argentina/San_Luis
2012-12-18T22:32:00-04:00 <-- dateutil datetime
2012-12-18T22:32:00-03:00 <-- pytz's datetime
So my hypothesis was wrong: dateutil and pytz return different results.
So which one is correct? I'm not really sure, but according to this website, currently,
America/Argentina/San_Luis time zone offset is:
UTC / GMT -03:00 hours
so it appears pytz is correct.
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