set of list of lists in python

Question

I am having a list of lists :

mat = [[1,2,3],[4,5,6],[1,2,3],[7,8,9],[4,5,6]]

and I want to convert into a set i.e. remove the repeating lists and creating a new list out of it which will only contain the unique lists.

In above case the required answer will be

[[1,2,3],[4,5,6],[7,8,9]]

But when I do set(mat), it gives me error

TypeError: unhashable type: 'list'

Can you please solve my problem. Thanks in advance!

Answer 1

Since the lists are mutable, they cannot be hashed. The best bet is to convert them to a tuple and form a set, like this

>>> mat = [[1,2,3],[4,5,6],[1,2,3],[7,8,9],[4,5,6]]
>>> set(tuple(row) for row in mat)
set([(4, 5, 6), (7, 8, 9), (1, 2, 3)])

We iterate through the mat, one list at a time, convert that to a tuple (which is immutable, so sets are cool with them) and the generator is sent to the set function.

If you want the result as list of lists, you can extend the same, by converting the result of set function call, to lists, like this

>>> [list(item) for item in set(tuple(row) for row in mat)]
[[4, 5, 6], [7, 8, 9], [1, 2, 3]]