How to test that a function is called within a function with nosetests

Question

I'm trying to set up some automatic unit testing for a project. I have some functions which, as a side effect occasionally call another function. I want to write a unit test which tests that the second function gets called but I'm stumped. Below is pseudocode example:

def a(self):
    data = self.get()
    if len(data) > 3500:
        self.b()

    # Bunch of other magic, which is easy to test.

def b(self):
    serial.write("\x00\x01\x02")

How do I test that b()-gets called?

score 22 · Accepted Answer · edited May 23 '17 at 12:10

22

You can mock the function b using mock module and check if it was called. Here's an example:

import unittest
from mock import patch


def a(n):
    if n > 10:
        b()

def b():
    print "test"


class MyTestCase(unittest.TestCase):
    @patch('__main__.b')
    def test_b_called(self, mock):
        a(11)
        self.assertTrue(mock.called)

    @patch('__main__.b')
    def test_b_not_called(self, mock):
        a(10)
        self.assertFalse(mock.called)

if __name__ == "__main__":
    unittest.main()

Also see:

Hope that helps.

edited May 23 '17 at 12:10

Community

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1

answered Sep 12 '13 at 11:04

alecxe

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Indeed, if `b` is writing stuff to the serial port, I'd say that you *must* mock it in a unit test. – Daniel Roseman Sep 12 '13 at 11:37
Thank you, this is *precisely* what I need to do. – msvalkon Sep 12 '13 at 12:14
The order of a, and b seems to matter here. If you switch their order, the tests fail. – TheJKFever Feb 05 '20 at 19:53
Fantastic answer, simple and easy to understand for a topic I found confusing. – a2f0 Apr 13 '21 at 17:02

score 1 · Answer 2 · answered Sep 12 '13 at 11:17

Various solutions are possible for this thing. But first, I have to point out that unit testing is meant for testing units; what a unit is can be a matter of viewing things. In your case I'd say a unit is the function a() because you would like to test that your unit behaves correctly, meaning it calls function b() at the right spot. A different view would be to say that functions a() and b() are a unit. Then you do not like to check whether function b() gets called, you just want to test the result of the call of function a(). So make up your mind which suits your case best.

In case you really want to test function a() as a unit, you should prepare your unit to being tested. In your case that could be done by adding to it an additional argument (which defaults to function b) and which will be used instead of the hard-coded function b() call in a():

def a(self, subfunction=None):
    is subfunction is None:  # for testing
        subfunction = self.b
    data = self.get()
    if len(data) > 3500:
        subfunction()

Now you can inject (during testing) a helper function which just informs the test that it got called:

store = None
def injected(*args, **kwargs):
    global store
    store = args, kwargs

obj.a(subfunction=injected)
# now check if global store is set properly

How to test that a function is called within a function with nosetests

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