How can I create a Set of Sets in Python?

Question

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 1

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.

xx = set([])
# Nested sets must be frozen
elements = frozenset([2,3,4])
xx.add(elements)

Answer 2

So I had the exact same problem. I wanted to make a data structure that works as a set of sets. The problem is that the sets must contain immutable objects. So, what you can do is simply make it as a set of tuples. That worked fine for me!

A = set()
A.add( (2,3,4) )##adds the element
A.add( (2,3,4) )##does not add the same element
A.add( (2,3,5) )##adds the element, because it is different!

Answer 3

People already mentioned that you can do this with a frozenset(), so I will just add a code how to achieve this:

For example you want to create a set of sets from the following list of lists:

t = [[], [1, 2], [5], [1, 2, 5], [1, 2, 3, 4], [1, 2, 3, 6]]

you can create your set in the following way:

t1 = set(frozenset(i) for i in t)

Answer 4

Use frozenset inside.