My Notes on Python Metaclasses

Today I read an interesting article on Python Metaclasses (see Advanced Python: Metaclasses). My notes are as follows:

1. What is a metaclasses?

Metaclasses are like classes for classes. They define how classes themselves should behave. In other words, they are responsible for defining the rules and behaviors that classes follow. The relation between objects, classes and metaclasses is shown in the following chart:

Image by Ilija Lazarevic from Advanced Python: Metaclasses

2. Can you give me an example on how to use Python metaclasses?

class MyMeta(type):

    def __call__(cls, *args, **kwargs):
        print(f'{cls.__name__} is called'
              f'with args={args}, kwargs={kwargs}')
        print('metaclass calls __new__')
        obj = cls.__new__(cls, *args, **kwargs)

        if isinstance(obj, cls):
            print('metaclass calls __init__')
            cls.__init__(obj, *args, **kwargs)

        return obj

class Parent(metaclass=MyMeta):

    def __new__(cls, name, age):
        print('new is called')
        return super().__new__(cls)

    def __init__(self, name, age):
        print('init is called')
        self.name = name
        self.age = age

parent = Parent('John', 35)
# Parent is called with args=('John', 35), kwargs={}
# metaclass calls __new__
# new is called
# metaclass calls __init__
# init is called

type(parent)
# Parent

str(parent)
# '<__main__.Parent object at 0x103d540a0>'