Module 7 Exercises: Communicating with Confidence

A great API doesn't just work; it communicates. These exercises will help you build an API that provides clarity in both success and failure.

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Exercise 1: The Precision Exception

Write a FastAPI endpoint @app.get("/items/{item_id}") that:

1. Takes a string item_id.
2. If the id starts with "legacy-", raise a 410 Gone exception with the message "This resource has been retired."
3. Otherwise, return the ID.

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Exercise 2: Mapping Logic to Errors

You have a Python function in your "Services" layer that raises a standard DatabaseConnectionError.

class DatabaseConnectionError(Exception):
    pass

Write a global Exception Handler for your FastAPI app that catches this error and returns a 503 Service Unavailable status code with a JSON body saying: "The database is currently undergoing maintenance. Please try again in 5 minutes."

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Exercise 3: Designing the Paginated Contract

Design a Pydantic model for a Paginated List of Products.

• The "Envelope" should contain the list of products.
• The "Metadata" should contain total, current_page, and a boolean has_more.

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Self-Correction / Discussion

Exercise 1 Answer:

@app.get("/items/{item_id}")
async def get_item(item_id: str):
    if item_id.startswith("legacy-"):
        raise HTTPException(status_code=410, detail="This resource has been retired.")
    return {"id": item_id}

Exercise 2 Answer:

@app.exception_handler(DatabaseConnectionError)
async def db_error_handler(request: Request, exc: DatabaseConnectionError):
    return JSONResponse(
        status_code=503,
        content={"detail": "The database is currently undergoing maintenance. Please try again in 5 minutes."}
    )

Exercise 3 Answer:

class Product(BaseModel):
    name: str
    price: float

class PaginatedProducts(BaseModel):
    items: list[Product]
    total: int
    current_page: int
    has_more: bool

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Summary of Module 7

You have mastered the "Voice" of your API.

• Exceptions: You can signal errors instantly.
• Custom Handlers: You can map logic to HTTP standards globally.
• Consistent Formats: You ensure the frontend always knows what to expect.

In Module 8: Middleware and Request Lifecycle, we will look at the "Interceptors"—the code that runs before and after every single request.