Errors and exception handlers

Error handling in a web service is a translation problem. Deep inside, code discovers that a ticket does not exist, that the caller does not own it, or that the model API timed out. At the edge, a client needs a status code and a body it can parse. If every route does its own translation, the same condition becomes a 404 in one place, a 400 in another, and a 500 with a stack trace in a third. FastAPI's exception handlers let you do the translation once, for the whole app.

The pattern is short. Services raise their own exception classes and know nothing about HTTP. The app registers one handler per class (or one per base class), and every route inherits consistent behaviour without a single try block.