Practical Programming, 2nd Edition

Case Study: Testing running_sum

In ​Case Study: Testing above_freezing ​, we tested a program that involved only immutable types. In this section, you’ll learn how to test functions involving mutable types, like lists and dictionaries.

Suppose we need to write a function that modifies a list so that it contains a running sum of the values in it. For example, if the list is [1, 2, 3], the list should be mutated so that the first value is 1, the second value is the sum of the first two numbers, 1 + 2, and the third value is the sum of the first three numbers, 1 + 2 + 3, so we expect that the list [1, 2, 3] will be modified to be [1, 3, 6].

Following the function design recipe (see ​Designing New Functions: A Recipe​), here is a file named ...