Custom middleware creation in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in FastAPI?

Middleware is a function that runs before and/or after each request. It can modify requests, responses, or perform actions like logging or authentication.

intermediate

How do you create a custom middleware in FastAPI?

You create a class with an __init__ and async __call__ method. The __call__ method receives the request and calls the next handler, allowing you to run code before and after the request.

beginner

What is the role of the 'call_next' function in FastAPI middleware?

call_next is a function that processes the request and returns the response from the next middleware or endpoint. It lets your middleware continue the request chain.

intermediate

Why should middleware be asynchronous in FastAPI?

FastAPI is built on async Python. Middleware should be async to avoid blocking the server and to handle many requests efficiently.

beginner

Give an example use case for custom middleware in FastAPI.

Custom middleware can log request times, check authentication tokens, add headers to responses, or handle CORS policies.

What method must a custom FastAPI middleware class implement to handle requests?

A handle_request

B __init__

C __call__

D process_request

The __call__ method is used to process each request in custom middleware.

In FastAPI middleware, what does the 'call_next' function do?

A Processes the request and returns the response from the next handler

B Stops the request from continuing

C Creates a new request

D Modifies the request headers only

call_next passes the request to the next middleware or endpoint and returns the response.

Why is it important for FastAPI middleware to be asynchronous?

A To automatically cache responses

B To avoid blocking the server and handle many requests efficiently

C To make the code shorter

D Because synchronous code is not allowed

Async middleware allows FastAPI to handle multiple requests without waiting for one to finish.

Which of these is NOT a typical use case for custom middleware in FastAPI?

A Rendering HTML templates

B Logging request times

C Adding headers to responses

D Checking authentication tokens

Rendering HTML templates is usually done in route handlers, not middleware.

How do you add a custom middleware class to a FastAPI app?

A Use app.include_router with middleware

B Call middleware() function directly

C Import middleware in main.py only

D Use app.add_middleware with the middleware class

You add middleware by calling app.add_middleware and passing your middleware class.

Explain how to create and add a custom middleware in FastAPI. Include the key methods and how the request flow works.

Think about how middleware wraps around requests and responses.

Describe three practical reasons to use custom middleware in a FastAPI application.

Middleware acts like a checkpoint for every request.

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