Module 6 Exercises: Mastering Dependency Injection

Dependency Injection is what makes FastAPI feel like "Magic." These exercises will help you understand how to control that magic to build clean, maintainable systems.

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Exercise 1: The Request ID Logger

Design a dependency called get_request_id. It should:

1. Look for a header named X-Request-ID.
2. If the header exists, return it.
3. If it doesn't exist, return a default string "GENERIC-ID".

Goal: Show how you would inject this into a GET /status endpoint.

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Exercise 2: The Multi-Layer Chain

You are building a Payment Gateway. Design a chain of three dependencies:

1. get_user: Fetches a user model from the DB.
2. check_active_card: Depends on get_user and checks if the user has a valid credit card on file.
3. require_funds: Depends on check_active_card and checks if the user's balance is above $10.00.

How would you apply require_funds to a POST /buy-coffee endpoint?

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Exercise 3: Dependency Overrides

In your tests, you want to replace the get_db dependency (which connects to a real Postgres database) with a get_test_db function (which uses an in-memory SQLite database).

Write the single line of code needed to tell your FastAPI app to perform this override.

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Self-Correction / Discussion

Exercise 1 Answer:

from fastapi import Header, Depends

async def get_request_id(x_request_id: str | None = Header(None)):
    return x_request_id or "GENERIC-ID"

@app.get("/status")
async def check_status(rid: str = Depends(get_request_id)):
    return {"status": "ok", "request_id": rid}

Exercise 2 Answer:

@app.post("/buy-coffee")
async def buy_coffee(user: User = Depends(require_funds)):
    # Your payment logic here
    return {"message": "Success"}

Exercise 3 Answer:

app.dependency_overrides[get_db] = get_test_db

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Summary of Module 6

You have unlocked FastAPI's most powerful architectural tool.

• Separation of Concerns: Your routes only care about logic; your dependencies care about resources.
• Testability: You can swap real services for mocks in one line.
• Modularity: You can build complex checks by nesting dependencies.

In Module 7: Error Handling and Responses, we will learn how to make our API communicate failures as clearly as it communicates successes.