Type error Unhashable type:set

Question

The below code has an error in function U=set(p.enum()) which a type error of unhashable type : 'set' actually if you can see the class method enum am returning 'L' which is list of sets and the U in function should be a set so can you please help me to resolve the issue or How can I convert list of sets to set of sets?

class pattern(object):

        def __init__(self,node,sets,cnt):
            self.node=node
            self.sets=sets
            self.cnt=cnt

        def enum(self):
            L=[]
            if self.cnt==1:
                L = self.node
            else:
                for i in self.sets:
                    L=[]
                    for j in self.node:
                        if i!=j:
                            L.append(set([i])|set([j]))

            return L #List of sets              

    V=set([1,2,3,4])
    U=set()
    cnt=1
    for j in V:
        p=pattern(V,(U|set([j])),cnt)
        U=set(p.enum()) #type error Unhashable type:'set'   
        print U
            cnt+=1 

Answer 1

This error is raised because a set can only contain immutable types. Or sets are mutable. However there is the frozenset type :

In [4]: a, b = {1,2,3}, {2,3,4}

In [5]: set([a,b])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-6ca6d80d679c> in <module>()
----> 1 set([a,b])

TypeError: unhashable type: 'set'

In [6]: a, b = frozenset({1,2,3}), frozenset({2,3,4})

In [7]: set([a,b])
Out[7]: {frozenset({1, 2, 3}), frozenset({2, 3, 4})}

Answer 2

The individual items that you put into a set can't be mutable, because if they changed, the effective hash would change and thus the ability to check for inclusion would break down.

Instead, you need to put immutable objects into a set - e.g. frozensets.

If you change the return statement from your enum method to...

return [frozenset(i) for i in L]

...then it should work.