Tests should contain as little logic as possible. This helps eliminate the test itself as the culprit when a test fails. As a rule of thumb, avoid control flow or conditional logic inside tests.

As a motivating example, consider the following, buggy Fizzbuzz implementation:

def fizzbuzz(n):
    if n % 3 == 0:
        return "fizz"
    if n % 5 == 0:
        return "buzz"
    if n % 15 == 0:
        return "fizzbuzz"
    return str(n)

fizzbuzz(3)
# Correct, returns "fizz"
fizzbuzz(5)
# Correct, returns "buzz"
fizzbuzz(15)
# Incorrect, returns "fizz"

Let’s say we want to test the fizzbuz function with a number of inputs, but we think writing out each assertion by hand is too tedious; instead we write a test like this:

def test_fizzbuzz():
    for n in range(1,20):
        if n % 3 == 0:
             expected = "fizz"
        elif n % 5 == 0:
            expected = "buzz"
        elif n % 15 == 0:
           expected = "fizzbuzz"
        else:
            expected = str(n)
        assert fizzbuzz(n) = expected  

This test repeats the logic error from the implementation, so although the implementation is buggy the test will succeed.

Had we written the “obvious” tests, the test for fi