Skip to main content

Introduction to pytest (Part 2): Fixtures, parameterized tests, and API clients

Part 1 covered plain tests. Now you add fixtures for reusable setup, parametrize for tabular cases, and FastAPI’s TestClient for HTTP-level assertions without binding ports.

📚 Prerequisites​

  • Pytest basics.
  • FastAPI routing from Chapter 4 is helpful for the final section.

🎯 What you'll learn​

  • Declare fixtures with scopes (function, module, session).
  • Generate combinatorial test matrices with @pytest.mark.parametrize.
  • Exercise ASGI apps in-process using TestClient.

Fixtures​

import pytest


@pytest.fixture
def sample_user():
    return {"name": "Ada", "id": 1}


def test_user_id(sample_user):
    assert sample_user["id"] == 1

Promote expensive resources to scope="module" or scope="session" only when isolation still holds.

Parametrization​

import pytest
from math_utils import add


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

Each tuple becomes an independent row in pytest’s report—ideal for validation tables supplied by product teams.

FastAPI TestClient​

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()


@app.get("/health")
def health():
    return {"status": "ok"}


client = TestClient(app)


def test_health():
    response = client.get("/health")
    assert response.status_code == 200
    assert response.json() == {"status": "ok"}

Use the context manager form when startup/shutdown lifecycles must fire:

with TestClient(app) as c:
    assert c.get("/needs_startup").status_code == 200

Override dependencies to inject fake DB sessions:

from app import app, get_session

app.dependency_overrides[get_session] = lambda: fake_session
try:
    ...
finally:
    app.dependency_overrides.clear()

💡 Key takeaways​

  • Fixtures + parametrization remove copy/paste setup while keeping tests explicit about dependencies.
  • HTTP tests belong at the boundary; still unit-test pure functions deeply underneath.

➡️ Next steps​

Practice red/green cycles in Test-driven development basics.