Global exception middleware in FastAPI - Step-by-Step Execution

Concept Flow - Global exception middleware

Request received

↓

Middleware intercepts request

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Try to process request

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Return [Catch exception

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Create error response

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Return error response

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Response sent

The middleware catches all exceptions during request processing and returns a custom error response instead of crashing.

Execution Sample

FastAPI

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

app = FastAPI()

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

This middleware catches any exception from downstream handlers and returns a JSON error response with status 500.

Execution Table

Step	Action	Try Block Result	Exception Caught	Response Returned
1	Request received by middleware	No result yet	No	No response yet
2	call_next(request) called	Handler runs normally	No	Handler's normal response
3	Middleware returns normal response	N/A	No	Normal response sent
4	Request received by middleware	No result yet	No	No response yet
5	call_next(request) called	Exception raised in handler	Yes	No response yet
6	Exception caught in except block	N/A	Yes	Prepare JSON error response
7	Middleware returns JSON error response	N/A	Yes	Error response sent

💡 Execution stops after middleware returns either normal or error response.

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
request	Incoming HTTP request object	Same request passed to handler	Same request passed to handler	Same request
response	None	Normal response from handler	None (exception raised)	Response returned (normal or error)
e (exception)	None	None	Exception instance caught	Exception handled and response created

Key Moments - 3 Insights

Why does the middleware use try-except around call_next(request)?

Because call_next runs the next handler which might raise an exception. The try-except catches it to prevent server crash and return a friendly error response (see execution_table rows 5-7).

What happens if no exception occurs in the handler?

The middleware simply returns the normal response from the handler without modification (see execution_table rows 2-3).

Can the middleware catch exceptions from any route handler?

Yes, because it wraps all requests globally before they reach any route (see concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table, what is the response returned at step 3?

A A JSON error response with status 500

B No response yet

C The normal response from the handler

D An exception object

💡 Hint

Check the 'Response Returned' column at step 3 in the execution_table.

At which step does the middleware catch an exception?

A Step 6

B Step 4

C Step 5

D Step 2

💡 Hint

Look for 'Exception caught in except block' in the 'Action' column of the execution_table.

If the handler never raises an exception, how many steps in the execution_table show exception handling?

A Two steps

B Zero steps

C One step

D Three steps

💡 Hint

Check the 'Exception Caught' column for 'Yes' entries when no exception occurs.

Concept Snapshot

Global exception middleware in FastAPI:
- Wraps all requests with @app.middleware("http")
- Uses try-except around call_next(request)
- Returns normal response if no error
- Returns JSON error response with status 500 if exception
- Prevents server crash and gives friendly error messages

Full Transcript

Global exception middleware in FastAPI intercepts every HTTP request before it reaches route handlers. It uses a try-except block around the call_next function that calls the next handler. If the handler runs without errors, the middleware returns the normal response. If an exception occurs, the middleware catches it and returns a JSON response with status code 500 and the error details. This prevents the server from crashing and provides a consistent error response. The execution table shows the step-by-step flow of request handling, exception catching, and response returning. Variables like request, response, and exception instance change state as the middleware processes the request. Key moments clarify why try-except is needed and how the middleware works globally. The quiz tests understanding of response flow and exception handling steps.