Detailed Explanation of FastAPI Parameters: Path Parameters, Query Parameters, and Request Body

FastAPI is a high-performance Python web framework that supports automatic document generation and parameter validation. Its core parameter types include path parameters, query parameters, and request bodies. **Path Parameters**: Defined as `{parameter_name}` in the URL path. They are declared with type hints in function parameters (e.g., `item_id: int`), and FastAPI automatically extracts and converts their types. Multiple parameters are supported (e.g., `/users/{user_id}/orders/{order_id}`). **Query Parameters**: Presented as `key=value` after the question mark in the URL. They are defined similarly to regular function parameters, supporting default values (e.g., `item_id: int = None`). FastAPI automatically parses list parameters (e.g., `tags=python&tags=fastapi` is converted to a list). **Request Bodies**: JSON data sent in POST requests, defined using Pydantic models for structure (e.g., an `Item` class with fields like `name` and `price`). FastAPI validates data types using Pydantic and supports nested models. **Applicable Scenarios**: Path parameters identify resources (e.g., IDs), query parameters filter and paginate data, and request bodies transmit complex data. FastAPI automatically identifies parameters in a specific order, returning a 422 validation error for type mismatches.