Why Tests Matter

Scenario: A one-character arithmetic fix silently breaks a different customer path.

New words, in plain English

• Test - code that checks other code behaves as expected
• Regression - a previously working behavior that breaks
• Assertion - a statement that must be true
• Test case - one named behavior and expectation

Tests turn assumptions into repeatable evidence. Good tests are small, deterministic, independent, and describe behavior. Use Arrange-Act-Assert: prepare inputs, call code, verify output.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

def add(a, b): return a + b

def test_adds_negative_number():
    result = add(5, -2)
    assert result == 3

Tip: Test boundaries and failure paths, not only the happy path.

Goal: Practice this idea in the py-pytest-suite challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.