Comparing a time in UTC with a time in Eastern tim

问题:

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?

回答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

回答2:

You receive the error because you are trying to measure the difference between one time object that can be tied to a particular UTC instant and another time object that is "naive" and cannot be tied to a particular UTC instant. The fix is either to make both comparands offset-aware, or both naive.

The below uses datetime objects but it's basically the same idea.

import datetime, time, pytz

EST = pytz.timezone('America/New_York')
UTC = pytz.timezone('Etc/UTC')
dt1 = datetime.datetime.fromtimestamp(time.time(), EST)
# ... time passes
dt2 = datetime.datetime.fromtimestamp(time.time(), UTC)
elapsed = dt2 - dt1

回答3:

I'm guessing that the problem is that UTC is considered to be not-any-timezone, or "offset-naive", perhaps? I'd recommend converting everything to UTC before doing any comparisons.

You need to know timezones for inputs and outputs, obviously, but you should try to keep your internal representations all in UTC, and maybe just store the timezone of each user and convert when you need to. It will save a lot of headache in the long run.

Also, you shouldn't do it this way. It's better to use

timezone.localize(dt)

as explained here: http://pytz.sourceforge.net/#localized-times-and-date-arithmetic