python 46 lines · 7 steps

Testing FastAPI routes with dependency overrides

A pytest fixture swaps FastAPI's database and auth dependencies for test doubles so routes run against an isolated SQLite database.

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FastAPI Intermediate testing fixtures dependency-injection database-isolation authentication
1import pytest
2from fastapi.testclient import TestClient
3from sqlalchemy import create_engine
4from sqlalchemy.orm import sessionmaker
5 
6from app.main import app
7from app.database import Base, get_db
8from app.auth import get_current_user
9from app.models import User
10 
11engine = create_engine(
12 "sqlite:///./test.db",
13 connect_args={"check_same_thread": False},
14)
15TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
16 
17 
18@pytest.fixture
19def db_session():
20 Base.metadata.create_all(bind=engine)
21 session = TestingSessionLocal()
22 try:
23 yield session
24 finally:
25 session.close()
26 Base.metadata.drop_all(bind=engine)
27 
28 
29@pytest.fixture
30def client(db_session):
31 def override_get_db():
32 try:
33 yield db_session
34 finally:
35 pass
36 
37 def override_get_current_user():
38 return User(id=1, email="tester@example.com", is_active=True)
39 
40 app.dependency_overrides[get_db] = override_get_db
41 app.dependency_overrides[get_current_user] = override_get_current_user
42 
43 with TestClient(app) as test_client:
44 yield test_client
45 
46 app.dependency_overrides.clear()
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1FastAPI's dependency_overrides lets tests inject fakes without touching route code.
  2. 2Yielding fixtures give you setup-then-teardown around each test in one function.
  3. 3Overriding authentication lets you exercise protected routes as a known user.

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