override python function-local variable in unittest

Question

I have a method in python (2.7) that does foo, and gives up after 5 minutes if foo didn\'t work.

def keep_trying(self):
    timeout = 300  #empirically derived,          


        

Answer 1

Rather than trying to mock the value if timeout, you'll want to mock the return value of time.time().

e.g.

@patch.object(time, 'time')
def test_keep_trying(self, mock_time):
    mock_time.side_effect = iter([100, 200, 300, 400, 500, 600, 700, 800])
    ...

Now the first time time.time() is called, you'll get the value of 100, so it should timeout when after a few turns of your while loop. You can also mock time.sleep and just count how many times it gets called to make sure that part of the code is working properly.

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Another approach (which isn't completely orthogonal to the one above) is to allow the user to pass an optional timeout keyword to the function:

def keep_trying(self, timeout=300):
    ...

This allows you to specify whatever timeout you want in the tests (and in future code which doesn't want to wait 5 minutes ;-).

Answer 2

You can't mock a function's local variable. To make your code easier to test, change it to, e.g:

def keep_trying(self, timeout=300):
    end_time = time.time() + timeout
    # etc, as above

so it becomes trivial for tests to run it with a shorter timeout!