Weird timezone issue with pytz

Question

>>> import pytz
>>> pytz.timezone('Asia/Hong_Kong')
<DstTzInfo 'Asia/Hong_Kong' LMT+7:37:00 STD>

A seven hour and 37 minute offset? This is a little strange, does anyone experience the same issue?

In fact I'm getting different behavior between

import pytz
from datetime import datetime
hk = pytz.timezone('Asia/Hong_Kong')

dt1 = datetime(2012,1,1,tzinfo=hk)
dt2 = hk.localize(datetime(2012,1,1))
if dt1 > dt2:
   print "Why?"

Answer 1

Time zones and offsets change over the years. The default zone name and offset delivered when pytz creates a timezone object are the earliest ones available for that zone, and sometimes they can seem kind of strange. When you use localize to attach the zone to a date, the proper zone name and offset are substituted. Simply using the datetime constructor to attach the zone to the date doesn't allow it to adjust properly.

Answer 2

While I'm sure historic changes in timezones are a factor, passing pytz timezone object to the DateTime constructor results in odd behavior even for timezones that have experienced no changes since their inception.

import datetime
import pytz 

dt = datetime.datetime(2020, 7, 15, 0, 0, tzinfo= pytz.timezone('US/Eastern'))

produces

2020-07-15 00:00:00-04:56

Creating the datetime object then localizing it produced expected results

import datetime
import pytz 

dt = datetime.datetime(2020, 7, 15, 0, 0)
dt_local = timezone('US/Eastern').localize(dt)

produces

2020-07-15 00:00:00-04:00

Answer 3

Coming here nearly 10 years later, I think it's worth a note that we can now exclusively utilize the Python 3.9+ standard library to handle time zones, without a "localize trap".

Use the zoneinfo module to set and replace the tzinfo however you like, ex:

from datetime import datetime
from zoneinfo import ZoneInfo

hk = ZoneInfo('Asia/Hong_Kong')
print(repr(hk))
# zoneinfo.ZoneInfo(key='Asia/Hong_Kong')

dt1 = datetime(2012,1,1,tzinfo=hk)
print(dt1)
# 2012-01-01 00:00:00+08:00

• there is a deprecation shim for pytz

Alternatives, if you're not able to use zoneinfo:

• for Python < 3.9, there's backports.zoneinfo
• you could also use dateutil, which follows the same semantics as zoneinfo

Note for pandas users:

• pandas (v1.4.1) is still using pytz internally, and seems to have some trouble with ZoneInfo timezone objects