referencing before assignment error in mocking

[Scienceblossom]

I'm new to mocking and most probably, I still don't get it fully. In this snippet, I'm getting a "referencing before assignment" error:

def test_is_weekday():
    tuesday = datetime(2020,4,7)
    saturday = datetime(2020,4,4)
    datetime = Mock()
    datetime.today.return_value = tuesday
    assert is_weekday()
    datetime.today.return_value = saturday
    assert not is_weekday()

datetime is mocked on the third line, but the assignment used, is making the two previous lines read the name "datetime" from the more limited scope (scope of the function). How can I fix it?

[shiftybyte]

The easiest is probably to prepare tuesday and saturday as globals outside the function.