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

Question

Another option is to use https://github.com/spulec/freezegun/

Install it:

pip install freezegun

And use it:

from freezegun import freeze_time

@freeze_time("2012-01-01")
def test_something():

    from datetime import datetime
    print(datetime.now()) #  2012-01-01 00:00:00

    from datetime import date
    print(date.today()) #  2012-01-01

It also affects other datetime calls in method calls from other modules:

other_module.py:

from datetime import datetime

def other_method():
    print(datetime.now())    

main.py:

from freezegun import freeze_time

@freeze_time("2012-01-01")
def test_something():

    import other_module
    other_module.other_method()

And finally:

$ python main.py
# 2012-01-01

For what it's worth, the Mock docs talk about datetime.date.today specifically, and it's possible to do this without having to create a dummy class:

https://docs.python.org/3/library/unittest.mock-examples.html#partial-mocking

>>> from datetime import date
>>> with patch('mymodule.date') as mock_date:
...     mock_date.today.return_value = date(2010, 10, 8)
...     mock_date.side_effect = lambda *args, **kw: date(*args, **kw)
...
...     assert mymodule.date.today() == date(2010, 10, 8)
...     assert mymodule.date(2009, 6, 8) == date(2009, 6, 8)
...

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)

I guess I came a little late for this but I think the main problem here is that you're patching datetime.date.today directly and, according to the documentation, this is wrong.

You should patch the reference imported in the file where the tested function is, for example.

Let's say you have a functions.py file where you have the following:

import datetime

def get_today():
    return datetime.date.today()

then, in your test, you should have something like this

import datetime
import unittest

from functions import get_today
from mock import patch, Mock

class GetTodayTest(unittest.TestCase):

    @patch('functions.datetime')
    def test_get_today(self, datetime_mock):
        datetime_mock.date.today = Mock(return_value=datetime.strptime('Jun 1 2005', '%b %d %Y'))
        value = get_today()
        # then assert your thing...

Hope this helps a little bit.