Fixture organization in FastAPI

Introduction

Fixtures help set up things you need for tests. Organizing them well keeps tests clean and easy to understand.

When you have multiple tests needing the same setup, like a test database or user login.

When you want to reuse setup code across many test files.

When you want to keep your test code tidy and avoid repeating yourself.

When you want to control how long a setup lasts, like per test or per session.

Syntax

FastAPI

import pytest

@pytest.fixture(scope='function')
def my_fixture():
    # setup code
    yield resource
    # teardown code

Use @pytest.fixture to create a fixture function.

The scope controls how often the fixture runs: 'function', 'module', 'session', etc.

Examples

A simple fixture returning data for tests.

FastAPI

import pytest

@pytest.fixture
def sample_data():
    return {'name': 'Alice', 'age': 30}

Fixture with module scope sets up a database connection once per module.

FastAPI

import pytest

@pytest.fixture(scope='module')
def db_connection():
    conn = connect_to_db()
    yield conn
    conn.close()

Fixture for FastAPI test client shared across all tests in the session.

FastAPI

import pytest

@pytest.fixture(scope='session')
def app_client():
    from fastapi.testclient import TestClient
    from myapp import app
    client = TestClient(app)
    yield client

Sample Program

This example shows a fixture named client that creates a FastAPI test client once per module. The test uses this fixture to call the /hello endpoint and check the response.

FastAPI

import pytest
from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get('/hello')
def hello():
    return {'message': 'Hello World'}

@pytest.fixture(scope='module')
def client():
    with TestClient(app) as c:
        yield c

def test_hello(client):
    response = client.get('/hello')
    assert response.status_code == 200
    assert response.json() == {'message': 'Hello World'}

Important Notes

Keep fixtures in a separate file like conftest.py to share them easily.

Use yield in fixtures to run cleanup code after tests finish.

Choose fixture scope carefully to balance speed and isolation.

Summary

Fixtures help prepare and clean up test environments.

Organize fixtures by scope and reuse to keep tests simple.

Use conftest.py to share fixtures across test files.