v Mocking Functions - Python

Mocking Functions

Interesting in learning more? Here are some good books on unit testing in Python: Python Testing: Beginner's Guide and Python Testing Cookbook.

Preliminaries

import unittest
import mock
from math import exp

The Scenario

Imagine we have a function that takes in some external API or database and we want to test that function, but with fake (or mocked) inputs. The Python mock library lets us do that.

For this tutorial pretend that math.exp is some expensive operation (e.g. database query, API call, etc) that costs \\(10,000 every time we use it. To test it without paying \\)10,000, we can create mock_function which imitates the behavior of math.exp and allows us to test it.

Create The Mock Function

# Create a function,
def mock_function(x):
    # That returns a string.
    return 'This is not exp, but rather mock_function.'

Create A Unit Test

# Create a test case,
class TestRandom(unittest.TestCase):
    # where math.exp (__main__.exp is because we imported the exp module from math)
    # math.exp is mocked (replaced) by mock_function,
    @mock.patch('__main__.exp', side_effect=mock_function)
    # now create a unit test that would only be true IF the exp(4) was being mocked
    # (so we can prove that math.exp is actually being mocked)
    def test_math_exp(self, mock_function):
        # assert that math.exp(4) is actually a string, which would only be the case
        # if math.exp was being mocked by mock_function
        assert exp(4) == 'This is not exp, but rather mock_function.'

Run Unit Test

unittest.main(argv=['ignored', '-v'], exit=False)
test_math_exp (__main__.TestRandom) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.002s

OK





<unittest.main.TestProgram at 0x104945358>