Cerberus provides type checking and other base functionality out of
the box and is designed to be non-blocking and easily extensible,
allowing for custom validation. It has no dependencies and is
thoroughly tested under Python 2.6, Python 2.7, Python 3.3, Python
3.4, Python 3.5, Python 3.6, PyPy and PyPy3.

WWW: https://github.com/pyeve/cerberus