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Before shipping your API, make sure it works. Testing with FastAPI's TestClient lets you simulate real HTTP requests without running a server.

Tip

FastAPI's TestClient is built on httpx. You write tests with pytest, call your endpoints just like a real client would, and assert on status codes and JSON responses. No server needed.

How Testing Works

Step 1: Write Test

Define a test function using pytest conventions (def test_*).

Testing with pytest
Write and run tests for your FastAPI endpoints with TestClient and pytest.

These are the patterns that trip up developers most often. Switch between Wrong and Fixed to compare the code side by side.

1
Testing against a real database
Not using dependency overrides for database sessions
Don't do this
test_main.py
def test_create_item():
    client = TestClient(app)
    # Writes to your production database!
    response = client.post("/items", json={"name": "Test"})
    assert response.status_code == 200
Always override database dependencies in tests. Without this, tests run against your real database — corrupting data, creating flaky tests, and making cleanup a nightmare.
2
Forgetting to clear overrides
Dependency overrides leaking between test files
Don't do this
test_main.py
# test_items.py
app.dependency_overrides[get_db] = get_test_db

def test_items():
    ...
# Overrides leak to other test files!
Use a pytest fixture with autouse=True and clear overrides in the teardown (after yield). This ensures each test file starts with a clean slate.