Why a timezone aware datetime's tzinfo does no

Question

>>> import pytz
>>> tz = pytz.timezone('America/Chicago')
>>> dt_naive = datetime(year=2017, month=6, day=6)
>>> dt_aware = tz.localize(dt_naive)
>>> dt_aware.tzinfo == tz
False

What's the reason for these to differ?

>>> dt_aware.tzinfo
<DstTzInfo 'America/Chicago' CDT-1 day, 19:00:00 DST>
>>> tz
<DstTzInfo 'America/Chicago' LMT-1 day, 18:09:00 STD>

Answer 1

The key that determines the timezone from pytz is the string you passed to create the object: 'America/Chicago'. That key is available through the .zone attribute.

>>> tz = pytz.timezone('America/Chicago')
>>> dt_naive = datetime(year=2017, month=6, day=6)
>>> dt_aware = tz.localize(dt_naive)
>>> dt_aware.tzinfo == tz
False
>>> tz.zone
'America/Chicago'
>>> dt_aware.tzinfo.zone == tz.zone
True

Answer 2

The first one has been adjusted to the date and time provided, 2016-06-06T00:00:00. Central Daylight Time (CDT) is in effect at this time. It is 5 hours behind UTC (24:00 - 05:00 = 19:00).

The second one has not been localized, so it is giving you the first offset in the available time zone data, which happens to be the Local Mean Time (LMT) entry. You can see this in the tzdata sources here. The LMT is 5 hours, 50 minutes, and 36 seconds behind UTC. The seconds of the LMT offset are rounded off somewhere in pytz, so 18:09 is reflecting this correctly (24:00 - 05:51 = 18:09)