Custom exception handlers in FastAPI - Mini Project: Build & Apply

Custom Exception Handlers in FastAPI

📖 Scenario: You are building a simple web API using FastAPI. You want to handle errors gracefully by creating custom exception handlers that return friendly error messages to users.

🎯 Goal: Build a FastAPI app that defines a custom exception and a handler for it. When the exception is raised, the app should return a JSON response with a clear error message and status code.

📋 What You'll Learn

Create a custom exception class called MyCustomException.

Add a FastAPI app instance called app.

Write a custom exception handler function called my_custom_exception_handler that returns a JSON response with status_code=418 and a message.

Register the custom exception handler with the FastAPI app using app.exception_handler(MyCustomException).

Create a GET route /raise-error that raises MyCustomException.

💡 Why This Matters

🌍 Real World

Custom exception handlers help APIs respond with clear, user-friendly error messages instead of generic server errors.

💼 Career

Knowing how to create and register custom exception handlers is essential for backend developers building robust and maintainable APIs.

1

Create the custom exception class

Create a custom exception class called MyCustomException that inherits from Exception.

FastAPI

# Your code here

Need a hint?

Define a class named MyCustomException that inherits from Exception. Use pass inside the class.

2

Create the FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

class MyCustomException(Exception):
    pass

# Your code here

Need a hint?

Use from fastapi import FastAPI and then create app = FastAPI().

3

Write the custom exception handler function

Import Request from fastapi and JSONResponse from fastapi.responses. Write a function called my_custom_exception_handler that takes request: Request and exc: MyCustomException as parameters. Return a JSONResponse with status_code=418 and a JSON content with {"message": "This is a custom error"}.

FastAPI

from fastapi import FastAPI

class MyCustomException(Exception):
    pass

app = FastAPI()

# Your code here

Need a hint?

Define a function with parameters request and exc. Return JSONResponse with the specified status code and message.

4

Register the handler and create the route

Register the custom exception handler with app using @app.exception_handler(MyCustomException). Then create a GET route /raise-error that raises MyCustomException.

FastAPI

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

class MyCustomException(Exception):
    pass

app = FastAPI()

def my_custom_exception_handler(request: Request, exc: MyCustomException):
    return JSONResponse(
        status_code=418,
        content={"message": "This is a custom error"}
    )

# Your code here

Need a hint?

Use the decorator @app.exception_handler(MyCustomException) above the handler function. Then add a GET route /raise-error that raises the exception.