len.py

from abc import ABCMeta, abstractmethod


class Len(metaclass=ABCMeta):
    @abstractmethod
    def len(self):
        """Return length of object."""
        raise NotImplementedError()

    def checktype(method):
        def decorated_method(*args, **kwargs):
            val = method(*args, **kwargs)
            if type(val) is not int:
                raise TypeError('len method should return int')
            else:
                return val
        return decorated_method


class Person(Len):
    @Len.checktype
    def len(self):
        return 1


try:
    person = Person()
    print(person.len())
except Exception as err:
    print(err)


class Dog(Len):
    @Len.checktype
    def len(self):
        return 'Bow, bow!'


try:
    dog = Dog()
    print(dog.len())  # TypeError
except TypeError as err:
    print(TypeError, err)


class Cat(Len):
    pass


try:
    cat = Cat()  # NotImplementedError
    print(cat.len())
except TypeError as err:
    print(TypeError, err)

http://nihaoshijie.hatenadiary.jp/entry/2018/02/24/122308