Python pytest: Write Tests That Actually Help You

Why Testing Matters (and Why pytest)

Tests catch bugs before users do. pytest makes writing them fast enough that you'll actually do it.

pip install pytest

Run with: pytest (auto-discovers test_*.py files)

Basic Test Structure

# test_calculator.py

def add(a: int, b: int) -> int:
    return a + b

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

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

def test_add_zero():
    assert add(0, 5) == 5

Run: pytest test_calculator.py -v

test_calculator.py::test_add_positive PASSED
test_calculator.py::test_add_negative PASSED
test_calculator.py::test_add_zero     PASSED
3 passed in 0.01s

Testing Exceptions

import pytest

def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_divide_by_zero():
    with pytest.raises(ValueError, match="Cannot divide by zero"):
        divide(10, 0)

def test_divide_normal():
    assert divide(10, 2) == 5.0

Fixtures: Reusable Setup

Fixtures provide test data or objects without repeating setup code:

import pytest

class UserDB:
    def __init__(self):
        self.users = {}

    def add_user(self, name: str, email: str):
        self.users[email] = {"name": name, "email": email}

    def get_user(self, email: str) -> dict | None:
        return self.users.get(email)

@pytest.fixture
def db():
    """Fresh database for each test."""
    return UserDB()

@pytest.fixture
def db_with_user(db):
    """Database pre-loaded with a test user."""
    db.add_user("Alice", "alice@example.com")
    return db

def test_add_user(db):
    db.add_user("Bob", "bob@example.com")
    assert db.get_user("bob@example.com")["name"] == "Bob"

def test_get_existing_user(db_with_user):
    user = db_with_user.get_user("alice@example.com")
    assert user["name"] == "Alice"

def test_get_missing_user(db):
    assert db.get_user("nobody@example.com") is None

Each test gets a fresh db — no state leaks between tests.

Parametrize: Test Multiple Inputs

import pytest

@pytest.mark.parametrize("a, b, expected", [
    (2, 3, 5),
    (-1, 1, 0),
    (0, 0, 0),
    (100, -50, 50),
])
def test_add(a, b, expected):
    assert add(a, b) == expected

One test function, four test cases. pytest runs and reports each separately.

Mocking with monkeypatch

import requests

def get_user_name(user_id: int) -> str:
    r = requests.get(f"https://api.example.com/users/{user_id}")
    return r.json()["name"]

def test_get_user_name(monkeypatch):
    class FakeResponse:
        def json(self):
            return {"name": "Alice", "id": 1}

    def fake_get(url):
        return FakeResponse()

    monkeypatch.setattr(requests, "get", fake_get)

    result = get_user_name(1)
    assert result == "Alice"

The real requests.get never fires — your test runs offline and fast.

Using pytest-mock for Cleaner Mocks

pip install pytest-mock

def test_get_user_name(mocker):
    mock_get = mocker.patch("requests.get")
    mock_get.return_value.json.return_value = {"name": "Alice"}

    result = get_user_name(1)

    assert result == "Alice"
    mock_get.assert_called_once_with("https://api.example.com/users/1")

Test Organization

project/
├── src/
│   └── myapp/
│       ├── __init__.py
│       ├── calculator.py
│       └── users.py
└── tests/
    ├── conftest.py        # Shared fixtures
    ├── test_calculator.py
    └── test_users.py

conftest.py — fixtures available to all test files without importing:

# tests/conftest.py
import pytest
from myapp.users import UserDB

@pytest.fixture
def empty_db():
    return UserDB()

@pytest.fixture
def populated_db(empty_db):
    empty_db.add_user("Alice", "alice@example.com")
    empty_db.add_user("Bob", "bob@example.com")
    return empty_db

Useful pytest Options

pytest                    # Run all tests
pytest -v                 # Verbose output
pytest -k "test_add"      # Run tests matching pattern
pytest --tb=short         # Shorter tracebacks
pytest -x                 # Stop on first failure
pytest --lf               # Run only last-failed tests
pytest -s                 # Show print() output
pytest --cov=src          # Coverage report (needs pytest-cov)

What to Test

Test these:

Happy path (valid inputs, expected output)
Edge cases (empty list, zero, None, empty string)
Error conditions (invalid input raises exception)
Boundary values (min/max for numeric inputs)

Don't test:

Python itself (assert 1 + 1 == 2)
Third-party library internals
Private implementation details that change often

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