FastAPI leverages Python type hints to automatically generate comprehensive and interactive API documentation. Here's how it works:

* Type Hints in Function Signatures : When defining routes in FastAPI, you use Python type hints in the function signatures to declare the expected data types for request parameters, request bodies, and response objects.

* Pydantic Models for Request and Response Bodies : FastAPI encourages the use of Pydantic models, which are Python classes with type hints provided. These models define the structure and validation rules for request and response bodies.

* Automatic Validation and Documentation : FastAPI uses the provided type hints to automatically validate incoming requests and generate OpenAPI and JSON Schema documentation. This documentation includes details about the expected request parameters, request bodies, response objects, and their data types.

* Swagger UI and ReDoc Integration : FastAPI provides two interactive web interfaces, Swagger UI and ReDoc, that are automatically generated based on the OpenAPI documentation. These interfaces allow developers to explore and test the API interactively. Swagger UI is accessible at http://localhost:8000/docs, and ReDoc at http://localhost:8000/redoc.

* Validation and Documentation for Query Parameters : Query parameters are also automatically documented based on the provided type hints. The documentation includes information about the parameter name, its type, whether it's required or optional, default values, etc.