PEP 544 protocols, implementing via subclass, and checking.

person.py

"""
Example of "Explicitly declaring implementation"

PEP 544 says you can state that an implementation supports a protocol by
`subclassing the protocol <https://www.python.org/dev/peps/pep-0544/#explicitly-declaring-implementation>`_.

It also says: "If one omits Protocol in the base class list, this would
be a regular (non-protocol) class that must implement Sized."

In the implementation below, ``FrenchPerson`` says that it implements
``Person``. But it doesn't. And yet, ``mypy`` reports no errors. Not on 
the definition nor when passing an instance to ``print_person``, which 
asks for a ``Person``.

Are there any ``mypy`` checks for "must implement [the protocol]" as 
stated in the PEP?
"""

from typing import Protocol


class Person(Protocol):
    first_name: str

    def greeting(self, message: str) -> str:
        ...


class FrenchPerson(Person):
    prenom: str

    def __init__(self, name: str):
        self.prenom = name

    def salutation(self, message: str) -> str:
        return f"{message} {self.prenom}"


def print_person(person: Person) -> str:
    return person.greeting(message="Je m'appelle")


def main():
    fp = FrenchPerson(name='Henri')
    response = print_person(fp)
    print(response)


if __name__ == '__main__':
    main()