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?

Answer

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)

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