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

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

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

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"})

Hint

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

Practice

1. What is the main purpose of global exception middleware in a FastAPI application?

easy

A. To automatically generate API documentation

B. To speed up the API response time by caching results

C. To manage database connections efficiently

D. To catch and handle errors for the entire application in one place

Solution

1. Step 1: Understand middleware role

   Middleware runs for every request and can intercept errors globally.

2. Step 2: Identify purpose of global exception middleware

   It catches errors from any part of the app and handles them centrally.

3. Final Answer:

   To catch and handle errors for the entire application in one place -> Option D

4. Quick Check:

   Global error handling = catch all errors [OK]

Hint: Global middleware handles all errors in one place [OK]

Common Mistakes:

• Confusing middleware with caching or documentation
• Thinking it manages database connections
• Assuming it only handles specific routes

2. Which of the following is the correct way to add a global exception middleware in FastAPI?

easy

A. app.use_middleware(ExceptionMiddleware, handler=custom_handler)

B. app.add_middleware(ExceptionMiddleware, handler=custom_handler)

C. app.middleware(ExceptionMiddleware, handler=custom_handler)

D. app.register_middleware(ExceptionMiddleware, handler=custom_handler)

Solution

1. Step 1: Recall FastAPI middleware syntax

   FastAPI uses add_middleware method to add middleware.

2. Step 2: Match correct method name

   Only add_middleware is valid; others are incorrect method names.

3. Final Answer:

   app.add_middleware(ExceptionMiddleware, handler=custom_handler) -> Option B

4. Quick Check:

   Use add_middleware() to add middleware [OK]

Hint: Use add_middleware() method to add middleware [OK]

Common Mistakes:

• Using incorrect method names like use_middleware or register_middleware
• Confusing middleware with route decorators
• Missing required parameters in add_middleware

3. Given this FastAPI middleware code snippet, what will be the response if a ValueError is raised inside a route?

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

app = FastAPI()

@app.middleware("http")
async def catch_exceptions_middleware(request: Request, call_next):
    try:
        response = await call_next(request)
        return response
    except ValueError as e:
        return JSONResponse(status_code=400, content={"error": str(e)})

@app.get("/test")
async def test_route():
    raise ValueError("Invalid input")

medium

A. {"detail": "ValueError"} with status 422

B. 500 Internal Server Error with default HTML page

C. {"error": "Invalid input"} with status 400

D. Empty response with status 200

Solution

1. Step 1: Analyze middleware error handling

   The middleware catches ValueError and returns JSONResponse with status 400 and error message.

2. Step 2: Check route behavior

   The route raises ValueError("Invalid input"), triggering the middleware's except block.

3. Final Answer:

   {"error": "Invalid input"} with status 400 -> Option C

4. Quick Check:

   Middleware catches ValueError and returns JSON error [OK]

Hint: Middleware catches ValueError and returns JSON with status 400 [OK]

Common Mistakes:

• Assuming default 500 error instead of custom JSON response
• Confusing status codes 422 and 400
• Ignoring middleware and expecting normal route error

4. Identify the error in this FastAPI global exception middleware code:

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

app = FastAPI()

@app.middleware("http")
async def exception_middleware(request: Request, call_next):
    try:
        response = call_next(request)
        return response
    except Exception as e:
        return JSONResponse(status_code=500, content={"error": "Server error"})

medium

A. Missing await before call_next(request)

B. Incorrect exception type used

C. JSONResponse should not be returned in middleware

D. Middleware should not catch exceptions

Solution

1. Step 1: Check async call to call_next

   call_next is an async function and must be awaited.

2. Step 2: Identify missing await

   Code calls call_next(request) without await, causing a coroutine object to be returned instead of response.

3. Final Answer:

   Missing await before call_next(request) -> Option A

4. Quick Check:

   Always await async call_next() in middleware [OK]

Hint: Always await call_next(request) in async middleware [OK]

Common Mistakes:

• Forgetting to await async call_next
• Thinking JSONResponse can't be returned in middleware
• Believing middleware shouldn't catch exceptions

5. You want to create a global exception middleware in FastAPI that logs all exceptions and returns a JSON error with status 500. Which code snippet correctly implements this behavior?

hard

A. @app.middleware('http')
async def global_exception(request, call_next):
try:
return await call_next(request)
except Exception as e:
print(f"Error: {e}")
return JSONResponse(status_code=500, content={"error": "Internal server error"})

B. app.add_middleware(ExceptionMiddleware, handler=lambda req, exc: JSONResponse({"error": str(exc)}, status_code=500))

C. @app.exception_handler(Exception)
async def global_exception_handler(request, exc):
return JSONResponse(status_code=500, content={"error": str(exc)})

D. app.add_exception_handler(Exception, lambda request, exc: JSONResponse({"error": "Error occurred"}, status_code=500))

Solution

1. Step 1: Understand middleware vs exception handler

   Middleware wraps all requests and can catch exceptions globally; exception handlers are per-exception but not middleware.

2. Step 2: Check for logging and JSON response in middleware

   @app.middleware('http') async def global_exception(request, call_next): try: return await call_next(request) except Exception as e: print(f"Error: {e}") return JSONResponse(status_code=500, content={"error": "Internal server error"}) uses @app.middleware('http') with try-except, logs error with print, and returns JSONResponse with status 500.

3. Step 3: Verify other options

   app.add_middleware(ExceptionMiddleware, handler=lambda req, exc: JSONResponse({"error": str(exc)}, status_code=500)) uses add_middleware incorrectly; C and D are exception handlers, not middleware.

4. Final Answer:

   @app.middleware('http') async def global_exception(request, call_next): try: return await call_next(request) except Exception as e: print(f"Error: {e}") return JSONResponse(status_code=500, content={"error": "Internal server error"}) -> Option A

5. Quick Check:

   Middleware with try-except and logging = @app.middleware('http') async def global_exception(request, call_next): try: return await call_next(request) except Exception as e: print(f"Error: {e}") return JSONResponse(status_code=500, content={"error": "Internal server error"}) [OK]

Hint: Use @app.middleware('http') with try-except and logging [OK]

Common Mistakes:

• Confusing middleware with exception handlers
• Using add_middleware incorrectly for exceptions
• Not logging exceptions inside middleware