TDD and regression tests · Software Testing

Why you need it

The intuition

Definition

TDD

Test-driven development: write a failing test, make it pass, then tidy up, in small steps.

Regression test

A test written for a bug that was fixed, kept so the bug can't silently return.

Examples

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def word_count_old(text):
    return len(text.split(" "))

def word_count_new(text):
    return len(text.split())

def test_extra_spaces(word_count):
    got = word_count("  hello   world ")
    assert got == 2, f"expected 2, got {got}"

for version in [word_count_old, word_count_new]:
    try:
        test_extra_spaces(version)
        print(version.__name__, "PASS")
    except AssertionError as e:
        print(version.__name__, "FAIL:", e)

word_count_old FAIL: expected 2, got 7
word_count_new PASS

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def fizzbuzz(n):
    if n % 15 == 0:
        return "FizzBuzz"
    if n % 3 == 0:
        return "Fizz"
    if n % 5 == 0:
        return "Buzz"
    return str(n)

TESTS = [(1, "1"), (3, "Fizz"), (5, "Buzz"), (15, "FizzBuzz"), (30, "FizzBuzz")]
for n, want in TESTS:
    assert fizzbuzz(n) == want, (n, fizzbuzz(n))
print("all", len(TESTS), "tests green")

all 5 tests green

Common mistakes

Quick quiz

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

1. PredictWhich tests fail?

   Type exactly what this prints.

2. ModifyFix bug #42

   The regression test for bug #42 is already written and fails. Fix parse_price so it passes, and also handle spaces around the text (" $12,000 " → 12000.0). Don't change the test.

3. WriteMake the tests green

   The tests came first. Write to_roman(n) for 1 to 3999 so they all pass:

   1 → I   4 → IV   9 → IX   14 → XIV   40 → XL   90 → XC
   400 → CD   1994 → MCMXCIV   2026 → MMXXVI   3999 → MMMCMXCIX

Terms in this lesson