_FixedOffset.normalize() doesn't correctly change the timezone

[Dale Hui]

In _FixedOffset:

    def normalize(self, dt, is_dst=False):
        '''Correct the timezone information on the given datetime'''
        if dt.tzinfo is None:
            raise ValueError('Naive time - no tzinfo set')
        return dt.replace(tzinfo=self)

should be:

    def normalize(self, dt, is_dst=False):
        '''Correct the timezone information on the given datetime'''
        if dt.tzinfo is self:
            return dt
        if dt.tzinfo is None:
            raise ValueError('Naive time - no tzinfo set')
        return dt.astimezone(self)

[Stuart Bishop (stub)]

In what circumstances does the current implementation fail? datetimes with a FixedOffset tzinfo should never need adjustment by normalize(),

[Dale Hui (dalehui)]

We have some datetimes that stored in a custom format where the timezone offset is stored separately from the date and time. We're converting that timezone offset to a pytz compatible tzinfo using FixedOffset and constructing a native Python datetime object. We need a pytz compatible tzinfo so we can use normalize() and work with our datetime objects in UTC. We'd also like to be consistent with our usage of pytz compatible tzinfos and always use localize() and normalize().

Is there a better approach to this problem than using FixedOffset? Thanks!

[Stuart Bishop (stub)]

When calling mytzinfo.normalize(dt), if mytzinfo is a _FixedOffset then dt.tzinfo should always be the same _FixedOffset instance. normalize() is there to correct drift after datetime calculations, and not convert to a different timezone.

>>> from datetime import datetime
>>> from pytz import FixedOffset, timezone, UTC
>>> dt = datetime.now()
>>> offset = 7*60
>>> loc_tz = FixedOffset(offset)
>>> loc_dt = dt.replace(tzinfo=loc_tz))
>>> utc_dt = loc_dt.astimezone(UTC)
>>> loc_tz.normalize(loc_dt + timedelta(days=1))

[Stuart Bishop (stub)]

However, looking at the docstring for StaticTzInfo:

    def normalize(self, dt, is_dst=False):
       '''
       [...]
       The supported method of converting between timezones is to use
       datetime.astimezone(). Currently normalize() also works:
       [...]

So yes, I should use your implementation for consistency and to support normalize converting between timezones.

[Dale Hui (dalehui)]

Thanks for the quick fix!