How to have a set of sets in Python?

Use frozenset,

>>> set([frozenset([1,2]), frozenset([2,3])])
set([frozenset([1, 2]), frozenset([2, 3])])

To represent a set of sets, the inner sets must be frozenset objects for the reason that the elements of a set must be hashable (all of Python’s immutable built-in objects are hashable). frozenset is immutable and set is mutable.

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