Module 14 Exercises: Building a Safety Net

Code that isn't tested is code that is already broken. These exercises will help you build the "Safety Net" that allows you to move fast and break nothing.

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Exercise 1: The First Test

Write a test for a POST /items/ endpoint.

• The endpoint expects a JSON body: {"name": "Widget", "price": 10}.
• The endpoint returns the same body plus an id: 1.
• Goal: Write the full Pytest function using TestClient.

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Exercise 2: The DB Override Challenge

You have a dependency get_database_session. Write the code to override this dependency in your test file so it returns a local dictionary {} instead of a real database connection.

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Exercise 3: Validating Failure

You have an endpoint that requires a token. If the token is missing, it returns a 401 Unauthorized. Write a test that:

1. Calls the endpoint without the Authorization header.
2. Asserts that the status code is 401.

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Self-Correction / Discussion

Exercise 1 Answer:

def test_create_item():
    payload = {"name": "Widget", "price": 10}
    response = client.post("/items/", json=payload)
    assert response.status_code == 200 # or 201
    data = response.json()
    assert data["name"] == "Widget"
    assert "id" in data

Exercise 2 Answer:

def fake_db():
    return {}

app.dependency_overrides[get_database_session] = fake_db

Exercise 3 Answer:

def test_auth_failure():
    response = client.get("/protected")
    assert response.status_code == 401

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Summary of Module 14

You have moved from being a "Coder" to an "Engineer."

• Automation: You can verify your logic in milliseconds.
• Isolation: You can test your app without paying for external services.
• Trust: You can refactor your code knowing that your tests will catch any mistakes.

In Module 15: Performance and Scalability, we will look at how to handle millions of requests and how to optimize FastAPI's async core for maximum throughput.