Path parameters with typed Pydantic validation

Declare the type of a path parameter with standard Python type annotations, as in async def read_item(item_id: int); that single declaration gives editor support with error checks and completion, automatic request parsing (the function receives the integer 3, not the string "3"), and data validation — a request for /items/foo returns a clear HTTP error stating exactly where validation failed, and the same error appears for a float such as /items/4.2. The same declaration also produces automatic, interactive API documentation integrating Swagger UI, and because the generated schema follows the OpenAPI standard, FastAPI ships alternative documentation with ReDoc, plus many compatible tools including code generation for many languages. All data validation is performed under the hood by Pydantic, and the same declarations work with str, float, bool, and many other complex data types. Order matters: path operations are evaluated in order, so declare a fixed path like /users/me before /users/{user_id}. Scope: FastAPI. Reviewed October 4, 2026.