Testing Basics

A first pytest test

pytest finds any function starting with test_ and runs it. Plain assert statements express what should be true.

# calc.py
def add(a, b):
    return a + b

# test_calc.py
from calc import add

def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0

Output:

$ pytest
1 passed in 0.01s

What makes a good test

Good tests are small, check one behaviour, and use clear names like test_add_negative_numbers so a failure tells you exactly what broke.