Skipping on memory in Python classes

[the_rev_dharma_roadkill]

Hi,
I'm a Python newbie, but I have some experience in other languages.

I need to create about 100,000 instances of one class.  Each instance
has two lists, one usually empty, the other containing exactly 200
elements which differ widely between the 100,000 instances, but about
half of the elements in these lists will be empty strings:

instance x (one of 100,000) contains:
list A, 200 elements but half are emptyString
list B, usually empty (None, not []), but can contain a few small
elements
variable X, a fairly short string.
I also set __init__, __cmp__, and an attribute access function.

Question:  How can a reduce the memory used to a minimum?

I have already set __slots__ = A,B,X
and this shaved about 10% off of the used memory, which is well worth
it.

Any other proven techniques out there?  Is there much point in
creating a new metaclass for my class?  How about replacing
emptyStrings with Nones?  Is there a fast (runtime) way of translating
between '' and None?

Cheers,
Doug