Global exception middleware in FastAPI - Mini Project: Build & Apply

Global Exception Middleware in FastAPI

📖 Scenario: You are building a simple web API using FastAPI. You want to handle unexpected errors globally so that your API always returns a friendly error message instead of crashing.

🎯 Goal: Create a global exception middleware in FastAPI that catches all exceptions and returns a JSON response with an error message and status code 500.

📋 What You'll Learn

Create a FastAPI app instance named app

Define a global exception middleware function named global_exception_handler

Register the middleware with the FastAPI app using app.middleware

Return a JSON response with {"detail": "Internal Server Error"} and status code 500 when an exception occurs

💡 Why This Matters

🌍 Real World

Global exception middleware helps keep APIs stable and user-friendly by catching unexpected errors and returning consistent error messages.

💼 Career

Understanding middleware and error handling is essential for backend developers building reliable web APIs with FastAPI.

1

Create the FastAPI app instance

Create a FastAPI app instance called app by importing FastAPI from fastapi and initializing it.

FastAPI

from fastapi import FastAPI

# Create the FastAPI app instance
# Your code here

Need a hint?

Use app = FastAPI() to create the app instance.

2

Define the global exception middleware function

Define an async function called global_exception_handler that takes request and call_next as parameters. Inside, use a try block to await call_next(request) and return the response. Use an except Exception as e block to catch all exceptions.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Define the global exception middleware function
# Your code here

Need a hint?

Use async def and await call_next(request) inside a try-except block.

3

Return JSON response on exception

Inside the except Exception as e block of global_exception_handler, import JSONResponse from fastapi.responses and return a JSONResponse with status_code=500 and content {"detail": "Internal Server Error"}.

FastAPI

from fastapi import FastAPI

app = FastAPI()

async def global_exception_handler(request, call_next):
    try:
        response = await call_next(request)
        return response
    except Exception as e:
        # Return JSONResponse with status 500 and error message
        # Your code here

Need a hint?

Use return JSONResponse(status_code=500, content={"detail": "Internal Server Error"}) inside the except block.

4

Register the middleware with the FastAPI app

Register the global_exception_handler function as middleware on the app by using the decorator @app.middleware("http") placed immediately above the function definition.

FastAPI

from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

# Register the middleware decorator here
# Your code here
async def global_exception_handler(request, call_next):
    try:
        response = await call_next(request)
        return response
    except Exception as e:
        return JSONResponse(status_code=500, content={"detail": "Internal Server Error"})

Need a hint?

Use @app.middleware("http") decorator above the function.