Custom exception handlers in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom exception handler in FastAPI?

A custom exception handler in FastAPI is a function you write to catch specific errors and return a friendly response instead of the default error message.

beginner

How do you register a custom exception handler in FastAPI?

You use the @app.exception_handler decorator with the exception class you want to handle, then define a function that takes the request and exception as parameters and returns a response.

intermediate

Why use custom exception handlers instead of default error responses?

Custom handlers let you control the message, status code, and format sent to users, making errors clearer and improving user experience.

beginner

What parameters does a FastAPI exception handler function receive?

It receives two parameters: the request object and the exception instance that was raised.

beginner

Show a simple example of a custom exception handler for a ValueError in FastAPI.

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse

app = FastAPI()

@app.exception_handler(ValueError)
async def value_error_handler(request: Request, exc: ValueError):
    return JSONResponse(
        status_code=400,
        content={"message": f"Value error occurred: {exc}"}
    )

What decorator is used to register a custom exception handler in FastAPI?

A @app.catch_error

B @app.error_handler

C @app.handle_exception

D @app.exception_handler

The correct decorator is @app.exception_handler to register custom exception handlers.

Which two parameters does a FastAPI exception handler function receive?

A request and exception

B request and response

C exception and status_code

D response and status_code

The handler function receives the request object and the exception instance.

What type of response is commonly returned from a custom exception handler?

A Plain text string

B HTMLResponse

C JSONResponse

D RedirectResponse

JSONResponse is commonly used to send structured error messages in JSON format.

Why might you create a custom exception handler in FastAPI?

A To customize error messages and status codes

B To speed up the app

C To change database connections

D To add new routes

Custom handlers let you control error messages and HTTP status codes for better user experience.

If you want to handle a KeyError with a custom message, what do you pass to @app.exception_handler?

A HTTPException

B KeyError

C ValueError

D Exception

You pass the specific exception class you want to handle, here KeyError.

Explain how to create and register a custom exception handler in FastAPI.

Think about the decorator and function signature.

Describe why custom exception handlers improve user experience in FastAPI applications.

Consider how users see error messages.