Demo of how the underlying Python object model works because I always forget and need to re-read it

python_object_model.py

class MyMetaClass(type):
    def __new__(cls, name, bases, attrs):
        """
        Step 1:
        Called when the module is loaded and class definitions are read.
        Prepares the new type MyObj.

        cls is MyMetaclass, name, bases, and attrs is the code
        in the class definition, aka name, bases, attrs
        """
        print(f'Messagee 1: Hi from meta new')
        assert name == 'MyObj'
        assert bases == ()
        assert attrs['cls_attr'] == 'my class attribute'
        my_obj_cls = super().__new__(cls, name, bases, attrs)
        return my_obj_cls

    def __call__(cls, *args, **kwargs):
        """
        Step 2: Called when a child class is instantiated. Consider this a metahook for object creation.

        args and kwargs are whatever data will eventually be passed to the instances init

        Args:
            *args:
            **kwargs:

        Returns:

        """
        assert cls is MyObj
        print(f'Message 3: hi from meta call {args}, {kwargs}')
        instance = super().__call__(*args, **kwargs)
        print('Message 6: Return the object instance from meta call')
        assert isinstance(instance, MyObj)
        return instance


class MyObj(metaclass=MyMetaClass):
    cls_attr = 'my class attribute'

    def __new__(cls, *args, **kwargs):
        """Creates a new instance of MyObj and
        then passes it init.

        Note *args and **kwargs is the same data sent to init
        """
        print('Message 4: hi from MyObj new')
        assert args == ('arg1', 'arg2')
        assert kwargs == {'kwarg1': 'val1', 'kwarg2': 'val2'}

        assert isinstance(cls, MyMetaClass)
        assert type(cls) is MyMetaClass
        new = super().__new__
        # Under the hood we're using object to create a new instance of cls
        assert new is object.__new__
        # Return an instance of cls so init is triggered
        instance = new(cls)
        return instance

    def __init__(self, *args, **kwargs):
        print('Message 5: hi from MyObj init')
        assert args == ('arg1', 'arg2')
        assert kwargs == {'kwarg1': 'val1', 'kwarg2': 'val2'}


print('Message 2: Prior to instantiating MyObj instance')
obj = MyObj('arg1', 'arg2', kwarg1='val1', kwarg2='val2')
assert isinstance(obj, MyObj)
print('Message 7: All one')