Custom exception handlers in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a custom exception is raised in FastAPI?

Consider this FastAPI app with a custom exception and handler. What will the client receive when the endpoint is called?

FastAPI

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

app = FastAPI()

class MyCustomError(Exception):
    def __init__(self, name: str):
        self.name = name

@app.exception_handler(MyCustomError)
async def custom_exception_handler(request: Request, exc: MyCustomError):
    return JSONResponse(
        status_code=418,
        content={"message": f"Oops! {exc.name} caused an error."}
    )

@app.get("/error")
async def error_endpoint():
    raise MyCustomError("TestError")

A {"detail": "MyCustomError"} with status code 500

B A JSON response with message "Internal Server Error" and status code 500

C A plain text response with status code 200

D {"message": "Oops! TestError caused an error."} with status code 418

💡 Hint

Look at the exception handler's return value and status code.

The custom exception handler catches MyCustomError and returns a JSON response with status 418 and a message including the error name.

📝 Syntax

intermediate

Which option correctly registers a custom exception handler in FastAPI?

You want to handle a custom exception MyError globally in FastAPI. Which code snippet correctly registers the handler?

FastAPI

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

app = FastAPI()

class MyError(Exception):
    pass

async def my_error_handler(request: Request, exc: MyError):
    return JSONResponse({"error": "MyError occurred"}, status_code=400)

# Which line correctly registers the handler?

A app.add_exception_handler(MyError, my_error_handler)

B app.register_exception_handler(MyError, my_error_handler)

C app.exception_handler(MyError)(my_error_handler)

D app.handle_exception(MyError, my_error_handler)

💡 Hint

Check FastAPI's method to add exception handlers programmatically.

The correct method to register an exception handler is add_exception_handler(exception_class, handler_function).

🔧 Debug

advanced

Why does this custom exception handler not work as expected?

This FastAPI app defines a custom exception and handler but the handler is never called. Why?

FastAPI

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

app = FastAPI()

class CustomError(Exception):
    pass

@app.exception_handler(Exception)
async def generic_handler(request: Request, exc: Exception):
    return JSONResponse({"error": "Generic error"}, status_code=500)

@app.get("/fail")
async def fail():
    raise CustomError()

A CustomError is not derived from Exception, so handler is not called.

B The handler is missing the @app.exception_handler(CustomError) decorator.

C The raise statement is incorrect syntax and never triggers the handler.

D The generic handler for Exception overrides the CustomError handler, so CustomError handler is ignored.

💡 Hint

Check which exception the handler is registered for.

Only a handler registered for CustomError will catch it. The generic Exception handler does not replace specific handlers but here no CustomError handler is defined.

❓ state_output

advanced

What is the HTTP status code returned when a custom handler returns a JSONResponse with status 404?

Given this handler, what status code does the client receive when the exception is raised?

FastAPI

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

app = FastAPI()

class NotFoundError(Exception):
    pass

@app.exception_handler(NotFoundError)
async def not_found_handler(request: Request, exc: NotFoundError):
    return JSONResponse({"detail": "Item not found"}, status_code=404)

@app.get("/item")
async def get_item():
    raise NotFoundError()

A 404

B 500

C 200

D 400

💡 Hint

Look at the status_code argument in JSONResponse.

The handler returns a JSONResponse with status_code=404, so the client receives HTTP 404.

🧠 Conceptual

expert

Which statement about FastAPI custom exception handlers is true?

Select the correct statement about how FastAPI handles custom exceptions and their handlers.

A Custom exception handlers must return plain text responses, JSONResponse is not supported.

B FastAPI only allows one global exception handler for all exceptions and ignores specific ones.

C If multiple handlers match an exception, FastAPI uses the most specific handler registered for the exception's class.

D Exception handlers must be synchronous functions; async handlers cause runtime errors.

💡 Hint

Think about inheritance and handler specificity.

FastAPI chooses the handler registered for the most specific exception class matching the raised exception.