・Supports both validation (check if a value is valid) and adaptation (convert a valid input to an appropriate output).
・Succinct: validation schemas can be specified in a declarative and extensible mini "language"; no need to define verbose schema classes upfront. A regular Python API is also available if the compact syntax is not your cup of tea.
・Batteries included: validators for most common types are included out of the box.
・Extensible: New custom validators and adaptors can be easily defined and registered.
・Informative, customizable error messages: Validation errors include the reason and location of the error.
・Well tested: Extensive test suite with 100% coverage.
・Agnostic: not tied to any particular framework or application domain (e.g. Web form validation).
・Production ready: Used for validating every access to the Podio API.
・Licence: MIT.
こちらは最初からエラーメッセージのカスタマイズを謳ってるな