Custom middleware creation in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this FastAPI middleware log?

Consider this FastAPI middleware that logs request method and path. What will it print when a GET request is made to /items/5?

FastAPI

from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware

app = FastAPI()

class LogMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        print(f"Request: {request.method} {request.url.path}")
        response = await call_next(request)
        return response

app.add_middleware(LogMiddleware)

@app.get('/items/{item_id}')
async def read_item(item_id: int):
    return {"item_id": item_id}

A Request: GET /items/{item_id}

B Request: POST /items/5

C Request: GET /items/5

D No output printed

💡 Hint

Look at the request.method and request.url.path values used in the print statement.

The middleware prints the HTTP method and the exact path of the request. For a GET request to /items/5, it prints Request: GET /items/5.

📝 Syntax

intermediate

Identify the syntax error in this FastAPI middleware

Which option correctly identifies the syntax error in this middleware code?

FastAPI

from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware

app = FastAPI()

class ErrorMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        response = await call_next(request)
        return response

app.add_middleware(ErrorMiddleware)

@app.get('/')
async def root():
    return {"message": "Hello"}

A dispatch method missing async keyword

B Missing closing parenthesis in app.add_middleware(ErrorMiddleware

C BaseHTTPMiddleware cannot be subclassed

D Request parameter missing type annotation

💡 Hint

Check the parentheses in the app.add_middleware line.

The line app.add_middleware(ErrorMiddleware is missing a closing parenthesis, causing a syntax error.

❓ state_output

advanced

What is the response header after this middleware runs?

This middleware adds a custom header X-Custom with value 42. What will the response headers contain after a request?

FastAPI

from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware

app = FastAPI()

class HeaderMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        response = await call_next(request)
        response.headers["X-Custom"] = "42"
        return response

app.add_middleware(HeaderMiddleware)

@app.get('/')
async def root():
    return {"hello": "world"}

A Response headers include 'X-Custom': '24'

B Response headers include 'x-custom': '42' (lowercase key)

C Response headers do not include 'X-Custom'

D Response headers include 'X-Custom': '42'

💡 Hint

Look at how the header is added to the response object.

The middleware sets the header 'X-Custom' to '42' exactly as shown, so the response headers include that key and value.

🔧 Debug

advanced

Why does this middleware cause a runtime error?

Examine this middleware code. Why does it cause a runtime error when a request is made?

FastAPI

from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware

app = FastAPI()

class FaultyMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        response = call_next(request)  # Missing await
        return response

app.add_middleware(FaultyMiddleware)

@app.get('/')
async def root():
    return {"msg": "ok"}

A call_next is a coroutine and must be awaited, missing await causes runtime error

B dispatch method must not be async

C BaseHTTPMiddleware requires a sync dispatch method

D Request parameter type is incorrect

💡 Hint

Check how call_next is called inside dispatch.

call_next is an async function and must be awaited. Missing await causes a runtime error because the coroutine is not awaited.

🧠 Conceptual

expert

How does FastAPI middleware affect request processing order?

Given multiple middlewares added in this order: MiddlewareA, then MiddlewareB, which statement best describes the order of dispatch calls and response returns?

A <code>MiddlewareA.dispatch</code> runs first on request, then <code>MiddlewareB.dispatch</code>, response returns back through <code>MiddlewareB</code> then <code>MiddlewareA</code>

B <code>MiddlewareB.dispatch</code> runs first on request, then <code>MiddlewareA.dispatch</code>, response returns back through <code>MiddlewareA</code> then <code>MiddlewareB</code>

C Both middlewares run their dispatch methods simultaneously

D Middleware order does not affect dispatch call order

💡 Hint

Think of middleware as layers wrapping the app, like nested boxes.

FastAPI calls middleware dispatch methods in the order they are added for the request. The response flows back in reverse order, so MiddlewareA dispatch runs first on request and last on response.