pythonlistset-difference

In python, why do I get an error message of " unhashable type:'list'" when doing list(set(listB)-set(listA))?

I have listA=[[0,1,2]], and listB=[[0,1,2],[0,1,3],[0,2,3]], and want to obtain elements that are in listB but not in listA, i.e., my desired output is listC=[[0,1,3],[0,2,3]].

I already got listC using list comprehension:

listA=[[0,1,2]]
listB=[[0,1,2],[0,1,3],[0,2,3]]
listC=[b for b in listB if b not in listA]

But if I want to do it in another way, i.e., using setdifference in the following code, I got the error message unhashable type:'list'.

 listA=[[0,1,2]]
listB=[[0,1,2],[0,1,3],[0,2,3]]
listC=list(set(listB)-set(listA))

Why does this error pop up? And how it could be fixed? Thanks!

Solution

  • sets require to hash the objects, and lists are not hashable (as they are mutable).

    Thus you need to convert to tuple and back to list:

    listA=[[0,1,2]]
listB=[[0,1,2],[0,1,3],[0,2,3]]
listC=list(map(list,set(map(tuple,listB))-set(map(tuple,listA))))
listC

    output: [[0, 1, 3], [0, 2, 3]]