When you build an API, you cannot assume that every request will contain the data you expect. A user might send an empty username, an invalid email address, or a negative age.

You could write if checks for every field in every endpoint. But as your application grows, that approach becomes repetitive and easy to miss.

FastAPI works with Pydantic to handle this for you. You define the shape of the data and the rules it must follow. FastAPI then validates incoming requests before passing the data to your endpoint.

In this guide, we'll use Pydantic v2 and go through the validation features you'll commonly need in a FastAPI project.

1. What happens when a request arrives?

Let's say your API receives this JSON:

{ "username" : "" , "email" : "not-an-email" , "age" : -5 }

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These values are not valid for a typical user account.…