I am not particularly sure that the backend algorithm can reliably support this, but it would be nice if the BloomFilter implementation could support the operations of the set primitive. Specifically:

>>> a = BloomFilter(10)
>>> a.add(1)
>>> b = BloomFilter(10)
>>> b.add(2)
>>> c = a | b
>>> 1 in c
True
>>> 2 in c
True
>>> 3 in c
False

I find sets to be incredibly useful in general coding practice, as it allows one to work naturally on data. Having a proxy to a set of data, without having to load said data, would increase the size of problems that one could easily work on, especially when one is more interested in set membership than the actual members.

This is just a feature suggestion (not even a request really), so no hurry. If I have time, I might even write a patch. I am unfamiliar with the hash function generation and the nuances there, so thats why I haven't jumped right into it.