__slots_ and inheritance

[Blair Hall]

I'm having trouble understanding exactly how __slots__
is supposed to behave when one class is derived from
another. For example:

	# First case: works as I would expect
	class A(object):
     		__slots__ = ('x','y')
     		def __init__(self):
         		pass

	class B(A):
     		__slots__ = ('z',)
     		def __init__(self):
         		A.__init__(self)

	b = B()
	b.x = 1
	b.z = 3
	# b.c = 3 # fails

This script would fail at the last line as expected, because
there is no attribute 'c' allowed. However,


	# Second case: class B has no slots
	class A(object):
     		__slots__ = ('x','y')
     		def __init__(self):
         		pass

	class B(A):
     		def __init__(self):
         		A.__init__(self)

	b = B()
	b.x = 1
	b.c = 3 # passes

In this case the attribute 'c' can be created. Perhaps a __slots__
in the base class does not prevent attributes from being added
to the derived class (I guess this is good). But,

	# Third case: class A has no slots
	class A(object):
     		def __init__(self):
         		pass

	class B(A):
     		__slots__ = ('z',)
     		def __init__(self):
         		A.__init__(self)

	b = B()
	b.c = 3 # passes

in this case, although the class B has slots its base class
does not. This seems to allow instances of B to ignore their
own __slots__ requirements. This, to my mind, is a problem:
it seems to require knowledge of the __slots__ in all
base classes.

Can anyone steer me towards a better understanding of __slots__?