Many Pythonistas, this author included, reacted in shock and disbelief to the introduction of type hints in Python 3.5. We knew that duck typing was essential to idiomatic Python, but type hints did not support it until Python 3.8, with PEP 544 – Protocols: Structural subtyping (static duck typing) and the typing.Protocol special class.

Static duck typing, first popularized in the Go programming language, allows programmers to leverage the essential idea that “if it quacks like a duck then it is a duck”, when quacking behavior is required. In other words, it does not matter whether a class has the proper “ancestry” or DNA, as long as it implements the interface to play a role. With interfaces defined as typing.Protocol, the type checker can verify that an object’s class implements the required methods.