class A(object):
    pass

class B(object):
    __slots__ = ('foobar',)

    def __init__(self):
        self.x = 123

class C(A):
    __slots__ = ('foobar',)

    def __init__(self):
        self.x = 123

>>> A()
<__main__.A object at 0x10caf4d10>
>>> C()
<__main__.C object at 0x10caf38c0>
>>> B()
Traceback (most recent call last):
  File "slots.py", line 18, in <module>
    B()
  File "slots.py", line 8, in __init__
    self.x = 123
AttributeError: 'B' object has no attribute 'x'