Testing Python Code with pytest: A Beginner's Guide

Part 13 of the Python for Everyone track. Last updated: September 2026.

pytest is the de facto standard for testing Python — simpler than unittest, no boilerplate classes required. If you write code that matters, test it; pytest makes the habit painless.

Your first test

# test_math_utils.py
from math_utils import add

def test_add_positive():
    assert add(2, 3) == 5

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

Run with pytest in the terminal. Test functions just use plain assert — pytest rewrites assertions to show beautiful failure diffs.

Testing exceptions with pytest.raises

import pytest
from accounts import withdraw

def test_withdraw_too_much():
    with pytest.raises(ValueError, match="insufficient funds"):
        withdraw(100, 200)

Fixtures: reusable setup

import pytest

@pytest.fixture
def sample_user():
    return {"name": "Ada", "age": 36}   # fresh dict for EVERY test

def test_user_name(sample_user):
    assert sample_user["name"] == "Ada"  # just name the fixture as a parameter!

def test_user_age(sample_user):
    assert sample_user["age"] == 36

Fixtures are pytest's superpower: declare setup once with @pytest.fixture, inject it by naming it as a parameter. Each test gets a fresh copy.

Parametrize: one test, many cases

import pytest
from math_utils import is_even

@pytest.mark.parametrize("n,expected", [
    (2, True), (3, False), (0, True), (-4, True),
])
def test_is_even(n, expected):
    assert is_even(n) == expected

@pytest.mark.parametrize runs the same test with different inputs — four test cases from one function.

Key takeaways

• Test files named test_*.py, test functions named test_* — pytest discovers them automatically.
• Plain assert is all you need; pytest gives rich failure output.
• pytest.raises tests exceptions; @pytest.fixture shares setup; @pytest.mark.parametrize multiplies cases.
• Run pytest -v for verbose output, pytest -x to stop at the first failure.

Next in this series: Async Python: asyncio Crash Course — the final post in the core track.