Mocking classes and overriding methods in Python |

Mocking classes and overriding methods in Python

I have this class Waterfall:

class Waterfall:
  def height_in_feet(self):
    return requests.get("").content

It's making web service calls to Nicaragua all the time, requiring me to be online whenever I run my tests:

import unittest

class ExploringMocking(unittest.TestCase):
    def test_using_the_real_class(self):
        waterfall = Waterfall()
        assert waterfall.height_in_feet() == 164

For this reason, I'd like to mock the Waterfall class (as it has so many methods and I want the method signatures to all be correct, I use unittest.mock.create_autospec) and provide my own implementation of the height_in_feet() method:

import types
import unittest.mock

class ExploringMocking(unittest.TestCase):
    def test_mocking_a_class(self):
        waterfall = unittest.mock.create_autospec(Waterfall)
        waterfall.height_in_feet = types.MethodType(
            self.my_height_func, self
        assert waterfall.height_in_feet() == 10

    def my_height_func(self, ref):
        return 10

The essential bit here is the types.MethodType which is what you need to do when overriding the method on a class instance. Just doing waterfall.hight_in_feet = self.my_height_func will not cut it.

With this in place, I can implement any logic I want in my_height_func which will get executed whenever Waterfall#height_in_feet() would have been.

If you strive to see the point of this, consider the case where you have a method that uses an instance Waterfall. When passing in an instance of Waterfall, we want to control what the method is returning depending on the input. Imagine that we have this silly method:

def get_colour_of_waterfall(waterfall: Waterfall) -> str:
    if waterfall.height_in_feet() > 10:
       return "blue"
       return "true"

By using the original implementation of Waterfall, we not only have to be online all the time (and thus writing an integration test instead of a unit test), but even more importantly, we cannot test what this get_colour_of_waterfall method returns given different situations and return values from the Nicaragua web service.

Overriding the Waterfall#height_in_feet() method solves both of these problems. With it, we not only can write a pure unit test (no need to interact with the other system, no need to be online) and we can easily simulate different values, or exceptions, returned from Waterfall#height_in_feet().

Happy testing!

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