How can I create a Set of Sets in Python?

python collections nested set
Matt picture Matt · May 9, 2011 · Viewed 57.9k times · Source

I'm trying to make a set of sets in Python. I can't figure out how to do it.

Starting with the empty set xx:

xx = set([])
# Now we have some other set, for example
elements = set([2,3,4])
xx.add(elements)

but I get

TypeError: unhashable type: 'list'

or 

TypeError: unhashable type: 'set'

Is it possible to have a set of sets in Python?

I am dealing with a large collection of sets and I want to be able to not have to deal duplicate sets (a set B of sets A1, A2, ...., An would "cancel" two sets if Ai = Aj)

Answer

a3nm picture a3nm · May 9, 2011

Python's complaining because the inner set objects are mutable and thus not hashable. The solution is to use frozenset for the inner sets, to indicate that you have no intention of modifying them.

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