Short story about how metaclasses work

metaclasses.py

print('class Meta(type)')


class Meta(type):
    @classmethod
    def __prepare__(mcs, name, bases, **kwargs):
        print('  Meta.__prepare__(mcs=%s, name=%r, bases=%s, **%s)' % (
            mcs, name, bases, kwargs
        ))
        return super().__prepare__(mcs, name, bases)

    @staticmethod
    def __new__(mcs, name, bases, attrs, **kwargs):
        print('  Meta.__new__(mcs=%s, name=%r, bases=%s, attrs=[%s], **%s)' % (
            mcs, name, bases, ', '.join(attrs), kwargs
        ))
        return super().__new__(mcs, name, bases, attrs)

    def __init__(cls, name, bases, attrs, **kwargs):
        print('  Meta.__init__(cls=%s, name=%r, bases=%s, attrs=[%s], **%s)' % (
            cls, name, bases, ', '.join(attrs), kwargs
        ))
        return super().__init__(name, bases, attrs)

    def __call__(cls, *args, **kwargs):
        print('  Meta.__call__(cls=%s, args=%s, kwargs=%s)' % (
            cls, args, kwargs
        ))
        return super().__call__(*args, **kwargs)

    def __getattribute__(cls, name):
        print('  Meta.__getattribute__(cls=%s, name=%s)' % (cls, name))
        return super().__getattribute__(name)


print('class Class(metaclass=Meta)')


class Class(metaclass=Meta):
    @staticmethod
    def __new__(cls, *args, **kwargs):
        print('  Class.__new__(cls=%s, args=%s, kwargs=%s)' % (
            cls, args, kwargs
        ))
        return super().__new__(cls, *args, **kwargs)

    def __init__(self, *args, **kwargs):
        print('  Class.__init__(self=%s, args=%s, kwargs=%s)' % (
            self, args, kwargs
        ))
        return super().__init__(*args, **kwargs)

    def __getattribute__(self, name):
        print('  Class.__getattribute__(self=%s, name=%s)' % (self, name))
        return super().__getattribute__(name)

    def foo(self):
        print('Class.foo(%s)' % self)

    def bar(self, *args, **kwargs):
        print('Class.bar(%s, *%r, **%r)' % (self, args, kwargs))


print('class SubClass(Class)')


class SubClass(Class):
    def foo(self):
        print('SubClass.foo(%s)' % self)


print('instances')

a = SubClass()
a.bar(1, a=2)
a.foo()