How should I check that a given argument is a datetime.date object?

Question

I'm currently using an assert statement with isinstance. Because datetime is a subclass of date, I also need to check that it isn't an instance of datetime. Surely there's a better way?

from datetime import date, datetime

def some_func(arg):
    assert isinstance(arg, date) and not isinstance(arg, datetime),\
        'arg must be a datetime.date object'
    # ...

Answer 1

The Python way is not to check it, just go ahead and let your code do what it needs to do, and if the object doesn't have a required method or something, the code will fail with an exception at the point where that method will be called. This is called duck typing and it is part of what makes Python so flexible.

Now, if you really can't accept a datetime.datetime object... well, why can't you? A datetime can do anything a date can do, so I can't imagine what reason you'd have for disallowing a datetime, or indeed any subclass of date.

If you really really have a good reason for doing this (I guess maybe as a debugging thing, but even then, I don't get it...):

assert type(arg) == datetime.date

Answer 2

I don't understand your motivation for rejecting instances of subclasses (given that by definition they support all the behavior the superclass supports!), but if that's really what you insist on doing, then:

if type(arg) is not datetime.date:
    raise TypeError('arg must be a datetime.date, not a %s' % type(arg))

Don't use assert except for sanity check during development (it gets turned to a no-op when you run with python -o), and don't raise the wrong kind of exception (such as, an AssertionError when a TypeError is clearly what you mean here).

Using isinstance and then excluding one specific subclass is not a sound way to get a rigidly specified exact type with subclasses excluded: after all, the user might perfectly well subclass datetime.date and add whatever it is you're so keep to avoid by rejecting instances of datetime.datetime specifically!-)

Answer 3

If your problem is that the graph goes wonky because it is using fractions of a day, you can test for that in other ways e.g. hasattr(arg, 'hour') distinguishes between a datetime instance and a date instance.