datetime.combine with timezone is different from datetime.now with timezone

Question

In the below code:

from datetime import datetime
import pytz

EDT = pytz.timezone('US/Eastern')

d1 = datetime.now(tz=EDT)

d2 = datetime.combine(d1.date(), d1.time(), tzinfo=EDT)

Why are d1 and d2 showing different timezone information?

>> d1
datetime.datetime(2021, 4, 8, 7, 0, 44, 316514, tzinfo=<DstTzInfo 'US/Eastern' EDT-1 day, 20:00:00 DST>)

>> d2
datetime.datetime(2021, 4, 8, 7, 0, 44, 316514, tzinfo=<DstTzInfo 'US/Eastern' LMT-1 day, 19:04:00 STD>)

How do I get the same datetime as datetime.now when using datetime.combine?

FObersteiner · Accepted Answer · 2021-04-08T12:22:07.350

datetime.now effectively converts (localizes) your datetime with the pytz timezone object - from the docs:

If tz is not None, it must be an instance of a tzinfo subclass, and the current date and time are converted to tz’s time zone.

datetime.combine does not do that. It is as if you would write something like datetime(2020,1,1, tzinfo=pytz.timezone('US/Eastern')) - effectively not adjusting the time zone to the provided date/time. See also e.g. pytz localize vs datetime replace and pytz: The Fastest Footgun in the West for more background info.

The correct way to get d2 with pytz would be

d2 = EDT.localize(datetime.combine(d1.date(), d1.time()))

No such issues if using timezone objects from dateutil or zoneinfo (Python 3.9+):

from datetime import datetime
from zoneinfo import ZoneInfo

EDT = ZoneInfo('US/Eastern')
d1 = datetime.now(tz=EDT)
d2 = datetime.combine(d1.date(), d1.time(), tzinfo=EDT)

# d1
# Out[75]: datetime.datetime(2021, 4, 8, 7, 57, 18, 309209, tzinfo=zoneinfo.ZoneInfo(key='US/Eastern'))

# d2
# Out[76]: datetime.datetime(2021, 4, 8, 7, 57, 18, 309209, tzinfo=zoneinfo.ZoneInfo(key='US/Eastern'))

# d1 == d2
# Out[77]: True

datetime.combine with timezone is different from datetime.now with timezone

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