How to Customize FastAPI\'s 422 Error Response to 401 for Missing Headers?

Customizing Error Response for Specific Routes in FastAPI

Problem Overview

In FastAPI, endpoints can require a specific Header. When this Header is absent from a client request, the server returns an error code of 422 Unprocessable Entity. However, in some scenarios, it may be desirable to customize this error response to 401 Unauthorized.

Solution Options

Option 1: Optional Header with Custom Error Handling

Use Header(None) to indicate that the Header is optional. Within the function body, check for None and raise a 401 Unauthorized error if the Header is not present.

Option 2: Override Exception Handler

Override the RequestValidationError exception handler. Check if the error relates to the custom Header and return a custom error response (e.g., 401 Unauthorized) accordingly.

Option 3: Sub-Application with Custom Exception Handling

Create a sub-application and mount it to the main app. Override the exception handler for RequestValidationError in the sub-application to handle errors for routes in that sub-application only.

Option 4: APIRouter with Custom APIRoute Class

Use a custom APIRoute class to handle exception handling. Within this class, define a custom route handler that catches RequestValidationError and returns a custom error response.

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