new_test.py

#!/usr/bin/env python

class Foo(object):
    def __new__(cls):
        new_foo = super(Foo, cls).__new__(cls)
        new_foo.x = None
        new_foo.y = None
        new_foo.z = None
        return new_foo

class Bar(Foo):
    def __init__(self, x):
        self.x = x

class Baz(Foo):
    def __init__(self, y):
        self.y = y

my_bar = Bar(1)