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

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 2

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.