Using the __call__ method of a metaclass instead of __new__?

Question

When discussing metaclasses, the docs state:

You can of course also override other class methods (or add new methods); for example defining a custom

Answer 1

One difference is that by defining a metaclass __call__ method you are demanding that it gets called before any of the class's or subclasses's __new__ methods get an opportunity to be called.

class MetaFoo(type):
    def __call__(cls,*args,**kwargs):
        print('MetaFoo: {c},{a},{k}'.format(c=cls,a=args,k=kwargs))

class Foo(object):
    __metaclass__=MetaFoo

class SubFoo(Foo):
    def __new__(self,*args,**kwargs):
        # This never gets called
        print('Foo.__new__: {a},{k}'.format(a=args,k=kwargs))

 sub=SubFoo()
 foo=Foo()

 # MetaFoo: , (),{}
 # MetaFoo: , (),{}

Notice that SubFoo.__new__ never gets called. In contrast, if you define Foo.__new__ without a metaclass, you allow subclasses to override Foo.__new__.

Of course, you could define MetaFoo.__call__ to call cls.__new__, but that's up to you. By refusing to do so, you can prevent subclasses from having their __new__ method called.

I don't see a compelling advantage to using a metaclass here. And since "Simple is better than complex", I'd recommend using __new__.