pythontestingdatetimemocking

Trying to mock datetime.date.today(), but not working


Can anyone tell me why this isn't working?

>>> import mock
>>> @mock.patch('datetime.date.today')
... def today(cls):
...  return date(2010, 1, 1)
...
>>> from datetime import date
>>> date.today()
datetime.date(2010, 12, 19)

Perhaps someone could suggest a better way?


Solution

  • There are a few problems.

    First of all, the way you're using mock.patch isn't quite right. When used as a decorator, it replaces the given function/class (in this case, datetime.date.today) with a Mock object only within the decorated function. So, only within your today() will datetime.date.today be a different function, which doesn't appear to be what you want.

    What you really want seems to be more like this:

    @mock.patch('datetime.date.today')
    def test():
        datetime.date.today.return_value = date(2010, 1, 1)
        print datetime.date.today()
    

    Unfortunately, this won't work:

    >>> test()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "build/bdist.macosx-10.6-universal/egg/mock.py", line 557, in patched
      File "build/bdist.macosx-10.6-universal/egg/mock.py", line 620, in __enter__
    TypeError: can't set attributes of built-in/extension type 'datetime.date'
    

    This fails because Python built-in types are immutable - see this answer for more details.

    In this case, I would subclass datetime.date myself and create the right function:

    import datetime
    class NewDate(datetime.date):
        @classmethod
        def today(cls):
            return cls(2010, 1, 1)
    datetime.date = NewDate
    

    And now you could do:

    >>> datetime.date.today()
    NewDate(2010, 1, 1)