Pydantic

Pydantic is the most widely used data validation library for Python, using type annotations to define data schemas and automatically validate, serialize, and transform data. It is the validation backbone of FastAPI and is used extensively in AI applications for defining structured LLM outputs, API request/response models, configuration management, and data pipeline schemas. Pydantic v2, rewritten with a Rust core, offers significant performance improvements. The library's emphasis on type safety and developer experience has made it fundamental to the Python AI ecosystem.