A test is a function that sets something up, calls your code, and asserts on the result. pytest finds functions named test_* and runs them. When an assert fails, it shows the values involved.

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

def test_add_negative():
    assert add(2, -3) == -1

• One behaviour per test, named after that behaviour.
• Boundaries: empty input, one item, the largest allowed value.
• Each bug you fix, so it stays fixed.

import pytest

def test_bad_input():
    with pytest.raises(ValueError):
        int("nope")

A good test fails when the code is wrong. A test that passes no matter what gives false comfort, so break the code on purpose once and watch the test catch it.

slugify is written for you. Write test_lowercases, test_spaces_become_dashes and test_empty_string, each using a plain assert on a slugify call. Each test must fail if that behaviour breaks.

The explanation opens here once every check passes.