Unclear datetime.date type when using isinstance

Chris Rebert clp2 at rebertia.com
Sat Oct 2 14:26:20 CEST 2010


On Sat, Oct 2, 2010 at 5:12 AM, Mailing List <lists at fastmail.net> wrote:
> Was including a input check on a function argument which is expecting a
> datetime.date. When running unittest no exception was raised when a
> datetime.datetime instance was used as argument. Some playing with the
> console lead to this:
>
>>>> import datetime
>
>>>> dt1 = datetime.datetime(2010, 10, 2)
>>>> type(dt1)
> <type 'datetime.datetime'>
>>>> isinstance(dt1, datetime.datetime)
> True
>>>> isinstance(dt1, datetime.date)
> True
>
>>>> dt2 = datetime.date(2010, 10, 2)
>>>> type(dt2)
> <type 'datetime.date'>
>>>> isinstance(dt2, datetime.datetime)
> False
>>>> isinstance(dt2, datetime.date)
> True
>
> My issue (or misunderstanding) is in the first part, while dt1 is a
> datetime.date object (confirmed by type), the isinstance(dt1,
> datetime.datetime) returns True. Is this correct?

I think you accidentally swapped "date" and "datetime" in this
paragraph (otherwise, it's rather nonsensical).
Anyway, to answer what I think you were trying to ask:
>>> from datetime import date, datetime
>>> issubclass(datetime, date)
True
>>> datetime.__bases__
(<type 'datetime.date'>,)

Recall how isinstance() works when sub/superclasses are involved:
>>> class A(object):
...     pass
...
>>> isinstance(object(), object)
True
>>> isinstance(A(), object)
True

> If so, is it possible
> to verify whether an object is indeed a datetime.date and not a
> datetime.datetime object?

Of course:
>>> this_moment = datetime.now()
>>> today = date.today()
>>> type(today) is date
True
>>> type(this_moment) is date
False

Cheers,
Chris
--
http://blog.rebertia.com



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