Subclassing datetime.datetime

Question

I'm looking into creating a python datetime-subclass which provides a default timezone when created.

For the sake of keeping this question simple, let's assume I always want to hard-code my datetimes to be in UTC.

I can't figure out why the following works:

import datetime, dateutil.tz

def foo(*args, **kwargs):
    kwargs['tzinfo'] = dateutil.tz.tzutc()
    return datetime.datetime(*args, **kwargs)

But the following doesn't work:

class Foo(datetime.datetime):
    def __init__(self, *args, **kwargs):
        kwargs['tzinfo'] = dateutil.tz.tzutc()
        super(Foo, self).__init__(*args, **kwargs)

Running the first method gives me the datetime object I expect:

>>> foo(2012, 11, 10)
datetime.datetime(2012, 11, 10, 0, 0, tzinfo=tzutc())
>>> foo(2012, 11, 10).tzinfo is None
False

But creating an instance of the Foo class doesn't seem to set the tzinfo object.

>>> Foo(2012, 11, 10)
Foo(2012, 11, 10, 0, 0)
>>> Foo(2012, 11, 10).tzinfo is None
True

Any ideas of what's going on here?

Thanks!

Thank you sir, you are correct. Since datetimes are immutable, the extra logic should be placed in __new__ rather than __init__. — Vilhelm, Jul 26 '14 at 02:38

score 3 · Answer 1 · edited May 23 '17 at 10:25

Providing the solution, as explained in Why can't I subclass datetime.date? for the sake of completeness, thanks to dano for the link.

The extra functionality has to be implemented in new since the datetime object is immutable:

class Foo(datetime.datetime):
    def __new__(cls, *args, **kwargs):
        kwargs['tzinfo'] = dateutil.tz.tzutc()
        return datetime.datetime.__new__(cls, *args, **kwargs)

Subclassing datetime.datetime

1 Answers1