In this video, we'll explore how to test a function that mutates a parameter. We'll start with a doctest example, and then we'll translate it to unittest. Here is a function that removes items from one list that appear in another list. The algorithm is straightforward for each item in the second list. If that item appears in the first list, remove it. In the doctest, we create two lists. We have intentionally not used l1 and l2 as variable names in order to remind everyone that parameters are local variables, and we can use any name we like for the two lists outside of the function. Now, we call function RemoveShared. The function has no return statement. It produces none. This means that when we call the function there is no useful return value to examine in our test. Instead, we examine list 1 and then list 2. We want to make sure that list 1 has been mutated properly, and we want to make sure that list 2 has not been mutated. For thorough testing, it's important to check this. If you pass a mutable value to a function that isn't supposed to mutate that value, make sure to check that it hasn't been mutated. Let's write the unittest version. Every unittest starts out by importing unittest as well as the, the module containing the functions that we're testing. We need a subclass of test case. We'll name it based on the function we're testing. And we'll write a brief doc string for our new class. We'll write only one method, because there is only one call on function the move shared in the dot test. Remember that every unit has test method, has to start with the letters, test, because our test involves two lists where there are some shared items and some non-shared items. We'll choose test_general_case for our name. The doc string should describe the test. Remember that if the test fails, then this doc string is part of the output, so try to make it clear and helpful. Now, let's write the actual testing code. We'll Copy and Paste from the doctest to initialize our two lists. Once we call RemoveShared, we expect the lists to look like this. We need to call function remove_shared, and then we need to assert that the two lists are equal to what we expected. And now, with variable underscores name refers to the string underscores main, then we'll run our unittests. Let's see how we did. We'll run our doctests. Notice that there's no output. That's a good thing. And now, we'll run our unittest. The single dot indicates that our only test method passed. And that's how you test a function that mutates a parameter. If you wanted to thoroughly test function RemoveShared, you would have to decide what your tests should be and then write one test method for each test you came up with. We'll leave that, as a practice exercise.
