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.