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.

Practice

1. What is the main purpose of creating custom middleware in FastAPI?

easy

A. To handle user authentication only

B. To define database models

C. To create HTML templates

D. To run code before and after each request is processed

Solution

1. Step 1: Understand middleware role

   Middleware runs code around request processing, before and after the main handler.

2. Step 2: Identify correct purpose

   Custom middleware is not for database or templates but for request/response handling.

3. Final Answer:

   To run code before and after each request is processed -> Option D

4. Quick Check:

   Middleware = pre/post request code [OK]

Hint: Middleware wraps requests to add extra processing [OK]

Common Mistakes:

• Confusing middleware with database or template code
• Thinking middleware only handles authentication
• Believing middleware runs only after requests

2. Which method must be overridden when creating a custom middleware class in FastAPI?

easy

A. dispatch

B. execute

C. process

D. handle_request

Solution

1. Step 1: Recall FastAPI middleware structure

   Custom middleware classes override the dispatch method to handle requests.

2. Step 2: Match method names

   Only dispatch is the correct method name; others are invalid in FastAPI middleware.

3. Final Answer:

   dispatch -> Option A

4. Quick Check:

   dispatch method overrides middleware behavior [OK]

Hint: dispatch handles request and response in middleware [OK]

Common Mistakes:

• Using incorrect method names like handle_request
• Confusing middleware methods with route handlers
• Forgetting to override dispatch method

3. Given this middleware code snippet, what will be printed when a request is processed?

class LogMiddleware:
    async def dispatch(self, request, call_next):
        print('Before request')
        response = await call_next(request)
        print('After request')
        return response

medium

A. Only Before request printed

B. Before request printed, then After request printed after response

C. Only After request printed

D. No output printed

Solution

1. Step 1: Analyze dispatch method flow

   Print 'Before request' runs before calling next handler, 'After request' runs after awaiting response.

2. Step 2: Understand async call_next behavior

   call_next processes the request and returns response; code after await runs after response is ready.

3. Final Answer:

   Before request printed, then After request printed after response -> Option B

4. Quick Check:

   Print before and after call_next = B [OK]

Hint: Code before await runs first, after await runs last [OK]

Common Mistakes:

• Thinking 'After request' prints before response
• Ignoring async await order
• Assuming only one print runs

4. Identify the error in this custom middleware code:

class MyMiddleware:
    async def dispatch(self, request):
        response = await call_next(request)
        return response

medium

A. Response must be created manually, not awaited

B. dispatch method should not be async

C. Missing call_next parameter in dispatch method

D. Middleware class must inherit from BaseHTTPMiddleware

Solution

1. Step 1: Check dispatch method signature

   dispatch must accept both request and call_next parameters to call next handler.

2. Step 2: Identify missing parameter

   Code lacks call_next parameter, causing runtime error when calling call_next(request).

3. Final Answer:

   Missing call_next parameter in dispatch method -> Option C

4. Quick Check:

   dispatch(request, call_next) required [OK]

Hint: dispatch needs call_next argument to forward requests [OK]

Common Mistakes:

• Omitting call_next parameter
• Making dispatch synchronous
• Not inheriting middleware base class (optional but recommended)

5. You want to create a middleware that adds a custom header 'X-Process-Time' showing how long the request took. Which code snippet correctly implements this?

hard

A. class TimerMiddleware:
async def dispatch(self, request, call_next):
start = time.time()
response = await call_next(request)
process_time = time.time() - start
response.headers['X-Process-Time'] = str(process_time)
return response

B. class TimerMiddleware:
async def dispatch(self, request):
start = time.time()
response = await call_next(request)
response.headers['X-Process-Time'] = str(time.time() - start)
return response

C. class TimerMiddleware:
def dispatch(self, request, call_next):
start = time.time()
response = call_next(request)
response.headers['X-Process-Time'] = str(time.time() - start)
return response

D. class TimerMiddleware:
async def dispatch(self, request, call_next):
response = await call_next(request)
response.headers['X-Process-Time'] = time.time()
return response

Solution

1. Step 1: Check dispatch method signature and async usage

   dispatch must be async and accept request and call_next parameters.

2. Step 2: Verify timing and header addition

   Start time before await call_next, calculate duration after, add header as string.

3. Step 3: Identify correct code snippet

   class TimerMiddleware: async def dispatch(self, request, call_next): start = time.time() response = await call_next(request) process_time = time.time() - start response.headers['X-Process-Time'] = str(process_time) return response correctly implements timing, async, and header addition.

4. Final Answer:

   The code snippet that correctly adds X-Process-Time header -> Option A

5. Quick Check:

   Async dispatch with timing and header = A [OK]

Hint: Measure time before and after await call_next, add header [OK]

Common Mistakes:

• Missing call_next parameter
• Not awaiting call_next
• Adding header before timing calculation
• Using synchronous dispatch method