Set operations on object attributes question

[TheSeeker]

Hi,

I have run into something I would like to do, but am not sure how to
code it up. I would like to perform 'set-like' operations (union,
intersection, etc) on a set of objects, but have the set operations
based on an attribute of the object, rather than the whole object.

For instance, say I have (pseudo-code):

LoTuples1 = [(1,1,0),(1,2,1),(1,3,3)]
Set1=set(LoTuples1)
LoTuples2 = [(2,1,3),(2,2,4),(2,3,2)]
Set2=set(LoTuples2)

What I would like to be able to do is:

Set3 = Set1union(Set2)
Set3.intersection(Set2, <use object[2]>)

to return:
set([(2,1,3), (1,3,3)])

How can one do this operation?

Thanks,
Duane