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python datetime timezone python-datetime pytz
Updated Sat, 23 Jul 2022 02:17:45 GMT

How to make a timezone aware datetime object in Python?


What I need to do

I have a timezone-unaware datetime object, to which I need to add a time zone in order to be able to compare it with other timezone-aware datetime objects. I do not want to convert my entire application to timezone unaware for this one legacy case.

What I've Tried

First, to demonstrate the problem:

Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> import pytz
>>> unaware = datetime.datetime(2011,8,15,8,15,12,0)
>>> unaware
datetime.datetime(2011, 8, 15, 8, 15, 12)
>>> aware = datetime.datetime(2011,8,15,8,15,12,0,pytz.UTC)
>>> aware
datetime.datetime(2011, 8, 15, 8, 15, 12, tzinfo=<UTC>)
>>> aware == unaware
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes

First, I tried astimezone:

>>> unaware.astimezone(pytz.UTC)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: astimezone() cannot be applied to a naive datetime
>>>

It's not terribly surprising this failed, since it's actually trying to do a conversion. Replace seemed like a better choice (as per How do I get a value of datetime.today() in Python that is "timezone aware"?):

>>> unaware.replace(tzinfo=pytz.UTC)
datetime.datetime(2011, 8, 15, 8, 15, 12, tzinfo=<UTC>)
>>> unaware == aware
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes
>>>

But as you can see, replace seems to set the tzinfo, but not make the object aware. I'm getting ready to fall back to doctoring the input string to have a timezone before parsing it (I'm using dateutil for parsing, if that matters), but that seems incredibly kludgy.

Also, I've tried this in both Python 2.6 and Python 2.7, with the same results.

Context

I am writing a parser for some data files. There is an old format I need to support where the date string does not have a timezone indicator. I've already fixed the data source, but I still need to support the legacy data format. A one time conversion of the legacy data is not an option for various business BS reasons. While in general, I do not like the idea of hard-coding a default timezone, in this case it seems like the best option. I know with reasonable confidence that all the legacy data in question is in UTC, so I'm prepared to accept the risk of defaulting to that in this case.

Solution

In general, to make a naive datetime timezone-aware, use the localize method:

import datetime
import pytz
unaware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0)
aware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0, pytz.UTC)
now_aware = pytz.utc.localize(unaware)
assert aware == now_aware

For the UTC timezone, it is not really necessary to use localize since there is no daylight savings time calculation to handle:

now_aware = unaware.replace(tzinfo=pytz.UTC)

works. (.replace returns a new datetime; it does not modify unaware.)

Comments (5)

  • +0 – Well, I feel silly. Replace returns a new datetime. It says that right there in the docs too, and I completely missed that. Thanks, that's exactly what I was looking for. — Aug 15, 2011 at 14:24  
  • +2 – "Replace returns a new datetime." Yep. The hint that the REPL gives you is that it's showing you the returned value. :) — Aug 15, 2011 at 14:33  
  • +5 – if the timezone is not UTC then don't use the constructor directly: aware = datetime(..., tz), use .localize() instead. — Mar 08, 2014 at 21:07  
  • +2 – It is worth mentioning that local time may be ambiguous. tz.localize(..., is_dst=None) asserts that it is not . — Mar 08, 2014 at 21:10  
  • +0 – putz has a bug that sets Amsterdam timezone to + 20 minutes to UTC, some archaic timezone from 1937. You had one job pytz. — Feb 18, 2020 at 16:09  

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