Mocking is a great way to isolate your code from distracting dependencies while testing. But, it can be an arcane art unto itself. Today I wrote a test for code that uses the current date. While testing, it can’t use the actual current date, because the test will produce different results on different days.

A solution is to mock out the datetime.datetime.today() function to return a known fixed date. But a few factors complicate matters. First, datetime.datetime is written in C, so Mock can’t replace attributes on the class, so you can’t simply mock out just the today() function.

Second, I want to mock just one function in the module, not the whole module. There are a few suggestions of how to do this out there: Michael Foord wrote about Partial mocking in the Mock docs, William John Bert showed another way, and of course, a Stack Overflow question about it.

None of these worked for me, perhaps because of subtle differences between my code under test and theirs. When mocking, it’s critical to mock at the appropriate place. If your module has “import datetime”, then you need to mock “mymodule.datetime” as a module. If instead you have “from datetime import datetime”, then you need to mock “mymodule.datetime” as a class. Datetime’s eponymous class structure only adds to the confusion.

I ended up with help from tos9 (Julian Berman) on the #python-testing IRC channel, and used this code in my test class:

def setUp(self):
    datetime_patcher = mock.patch.object(
        my_module.datetime, 'datetime', 
        mock.Mock(wraps=datetime.datetime)
    )
    mocked_datetime = datetime_patcher.start()
    mocked_datetime.today.return_value = datetime.datetime(2012, 6, 16)
    self.addCleanup(datetime_patcher.stop)

Here, mock.patch.object is being used to patch the datetime attribute (the class) of the datetime import in my module. It replaces it with a mock, one that wraps the real datetime class. Here, “wraps” means that anything not explicitly changed on the mock is proxied to the real datetime class, so most of our functionality is in place. We change the return value of today() to be a specific date, accomplishing our goal.

If you haven’t seen it before, addCleanup() is a new feature of unittest in 2.7. Instead of writing a tearDown method in which you clean up all the stuff you did in setUp, you can register callables with addCleanup, and they will be called to clean up at the end of tests. Because you can register as many as you like, it’s easier to modularize your setup and teardown code.

BTW, Julian also has a clever decorator to auto-register the cleanup functions for patches, and has packaged it into a mixin: ivoire/tests/util.py. Check it out.