datetime - Comparing a time in UTC with a time in Eastern time using Python

Question

I'm trying to compare two times using the Python datetime module, but I can't seem to create a timezone-aware time object in UTC.

>>> import pytz, datetime
>>> UTC_TZ = pytz.utc
>>> EASTERN_TZ = pytz.timezone('America/New_York')
>>> d1 = datetime.time(10, tzinfo = UTC_TZ)
>>> d1
datetime.time(10, 0, tzinfo=<UTC>)
>>> d2 = datetime.time(10, tzinfo = EASTERN_TZ)
>>> d2
datetime.time(10, 0, tzinfo=<DstTzInfo 'America/New_York' EST-1 day, 19:00:00 STD>)
>>> d1 < d2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware times

Is this a bug? Is there a special UTC timezone I need to use? What's going on?

Answer 1

All credit to wberry for puzzling this out, but in the interest of having a concise answer, I'll summarize it here.

According to the datetime docs, when comparing two datetime.time objects: "If both comparands are aware and have different tzinfo attributes, the comparands are first adjusted by subtracting their UTC offsets (obtained from self.utcoffset())"

In the example you gave, the comparison throws the TypeError because EASTERN_TZ.utcoffset() returns None. utcoffset is None because the eastern US observes Daylight Savings Time and so the time offset from UTC depends on the date which isn't available in datetime.time.

You should use datetime.datetime objects for cross-timezone comparisons:

>>> import pytz, datetime
>>> UTC_TZ = pytz.utc
>>> EASTERN_TZ = pytz.timezone('America/New_York')
>>> d1 = datetime.datetime(2012, 1, 1, 10, 0, tzinfo=UTC_TZ)
>>> d2 = datetime.datetime(2012, 1, 1, 10, 0, tzinfo=EASTERN_TZ)
>>> d1 < d2
True