6.3The Mental Mutation Test

For every assertion, ask yourself: "If I removed or inverted this assertion, would a real bug go undetected?"

This is the fastest way to evaluate whether a test has actual value.

# Test with LOW mutation score
def test_get_user():
    response = client.get("/api/users/1")
    assert response.status_code == 200
    # If the user's name field is empty, this test still passes.
    # If the user's email is wrong, this test still passes.
    # This test only verifies the endpoint exists and returns 200.

# Same test with HIGH mutation score
def test_get_user():
    response = client.get("/api/users/1")
    assert response.status_code == 200
    user = response.json()
    assert user["name"] == "Alice"        # Catches name corruption
    assert user["email"] == "alice@x.com" # Catches email corruption
    assert user["role"] == "admin"        # Catches role assignment bug
    assert user["created_at"] is not None # Catches missing timestamp

Every additional assertion is a mutation that the test would catch. Aim for tests where removing any single assertion would leave a realistic bug undetected.