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This rule raises an issue when passing a pytz.timezone to the datetime.datetime constructor.
Why is this an issue?
When working with timezones in Python, it’s important to understand that the datetime.datetime constructor and pytz
timezone objects handle timezones differently. This difference can lead to unexpected results if a pytz object is used as the
tzinfo argument in the datetime.datetime constructor.
The datetime.datetime constructor expects a tzinfo object that is a subclass of the datetime.tzinfo base
class. pytz timezone objects do provide this interface, but they implement it in a way that’s not fully compatible with
datetime.datetime.
One key difference is how they handle historical changes in timezone offsets. The datetime module uses the IANA time zone database,
which includes historical changes.
When you create a datetime object with a pytz timezone object as the tzinfo argument, it uses the earliest
known offset for that timezone. This can lead to unexpected offsets, as the earliest known offset may not match the current or most commonly used
offset for that timezone.
For example, if you were to use 'US/Eastern' as your timezone, you might expect the offset to be either -5 hours (Eastern Standard Time) or -4
hours (Eastern Daylight Time), depending on the time of the year. However, due to historical changes, the actual offset might be something different,
like -4 hours and 56 minutes. This can lead to subtle bugs in your code, especially if you’re doing calculations with datetime objects.
Note that, when using Python 3.9 and later, it is recommended to use the zoneinfo package from the standard library over
pytz.
How to fix it
To avoid these issues, it’s recommended to use the localize method of pytz timezone objects to attach a timezone to a
datetime object. This method correctly handles historical changes and other timezone-related issues.
Code examples
Noncompliant code example
import datetime
import pytz
dt = datetime.datetime(2022, 1, 1, tzinfo=pytz.timezone('US/Eastern')) # Noncompliant: 2022-01-01 00:00:00-04:56
Compliant solution
import datetime
import pytz
dt = pytz.timezone('US/Eastern').localize(datetime.datetime(2022, 1, 1)) # OK: 2022-01-01 00:00:00-05:00
Resources
Documentation