Modern Python Standard Library Cookbook

Mocking

When testing your code, you might face the need to replace the behavior of an existing function or class and to track whether a function was called or not with the proper arguments.

For example, say you have a function such as the following:

def print_division(x, y):
    print(x / y)

To test it, we don't want to go to the screen and check the output, but we still want to know whether the printed value was the expected one.

So a possible approach might be to replace print with something that doesn't print anything, but allows us to track the provided argument (which is the value that would be printed).

This is exactly the meaning of mocking: replacing an object or function in the code base with one that does nothing but allows us to ...