Identifying and fixing bugs

A program may work for certain values but crash for others. Therefore, you test with different types of input: a standard case, a small value, a large value, and an unusual case (zero, empty string, negative number).

Once you have identified the error: change as little as possible and run all tests again. If you fix one thing and break another, you will spot it immediately.

Test the average function:

def average(lst):
    return sum(lst) / len(lst)

average([2, 4]) → 3 ✔. average([5]) → 5 ✔. average([]) → crash (division by zero). The error is only visible with an empty list.

Fix: if len(lst) == 0: return 0. Run all three tests again — now they all pass. Document your tests so you can run them again next time you change the code.