python - pytz localize vs datetime replace

Question

I'm having some weird issues with pytz's .localize() function. Sometimes it wouldn't make adjustments to the localized datetime:

.localize behaviour:

>>> tz
<DstTzInfo 'Africa/Abidjan' LMT-1 day, 23:44:00 STD> 
>>> d
datetime.datetime(2009, 9, 2, 14, 45, 42, 91421)

>>> tz.localize(d)
datetime.datetime(2009, 9, 2, 14, 45, 42, 91421, 
                  tzinfo=<DstTzInfo 'Africa/Abidjan' GMT0:00:00 STD>)
>>> tz.normalize(tz.localize(d))
datetime.datetime(2009, 9, 2, 14, 45, 42, 91421,
                  tzinfo=<DstTzInfo 'Africa/Abidjan' GMT0:00:00 STD>)

As you can see, time has not been changed as a result of localize/normalize operations. However, if .replace is used:

>>> d.replace(tzinfo=tz)
datetime.datetime(2009, 9, 2, 14, 45, 42, 91421, 
                  tzinfo=<DstTzInfo 'Africa/Abidjan' LMT-1 day, 23:44:00 STD>)
>>> tz.normalize(d.replace(tzinfo=tz))
datetime.datetime(2009, 9, 2, 15, 1, 42, 91421,
                  tzinfo=<DstTzInfo 'Africa/Abidjan' GMT0:00:00 STD>)

Which seems to make adjustments into datetime.

Question is - which is correct and why other's wrong?

Answer 1

localize just assumes that the naive datetime you pass it is "right" (except for not knowing about the timezone!) and so just sets the timezone, no other adjustments.

You can (and it's advisable...) internally work in UTC (rather than with naive datetimes) and use replace when you need to perform I/O of datetimes in a localized way (normalize will handle DST and the like).