1. Setting up the Environment#

We use pytest as our testing framework and httpx (required by FastAPI’s TestClient).

uv add pytest httpx --dev

(Note: I used --dev because testing tools shouldn’t be bundled in your final production build.)

2. Your First Test Case#

We create a new file called test_main.py. FastAPI’s TestClient allows us to simulate a user visiting our API.

from fastapi.testclient import TestClient
from main import app # Import your FastAPI instance

client = TestClient(app)

def test_read_main():
    # 1. Simulate a GET request
    response = client.get("/")
    
    # 2. Assert the results
    assert response.status_code == 200
    # Updated to match our API versioning
    assert response.json() == {"message": "Hello World", "version": "2.0"}

3. Testing Protected Routes#

Since we implemented OAuth2 and JWT in Phase 4, we need to test how our API handles missing tokens.

def test_read_item_no_token():
    # We use our protected route /items/secret
    response = client.get("/items/secret")
    # We expect a 401 because we haven't logged in in this test!
    assert response.status_code == 401
    # Our custom exception handler returns "message", not "detail"
    assert "Not authenticated" in response.json()["message"] or "Invalid authentication credentials" in response.json()["message"]

4. Why This Matters#

• Regression Testing: Every time you run pytest, you are verifying that your old code still works.
• Documentation: Tests serve as an example of how your API is supposed to be used.
• Fast Feedback: You know within seconds if your logic is flawed, rather than waiting for a user to find a bug in production.

🛠️ Implementation Checklist#

• Installed pytest and httpx using uv.
• Created test_main.py.
• Wrote a test for the root (/) endpoint.
• Wrote a test to verify that the Global Exception Handler (from Day 8) returns the correct JSON format for a 404.
• Ran pytest in the terminal and saw the satisfying green dots! ✅

📚 Resources#

1. Official Docs: FastAPI Testing Tutorial
2. Pytest Docs: Getting Started with Pytest
3. Book: FastAPI: Modern Python Web Development (Chapter 10: Testing and Debugging).