Get unique values in List of Lists in python

Question

I want to create a list (or set) of all unique values appearing in a list of lists in python. I have something like this:

aList=[['a','b'], ['a', 'b','c'], ['a']]

and i would like the following:

unique_values=['a','b','c']

I know that for a list of strings you can just use set(aList), but I can't figure how to solve this in a list of lists, since set(aList) gets me the error message

unhashable type: 'list'

How can i solve it?

Solution 1

array = [['a','b'], ['a', 'b','c'], ['a']]
result = {x for l in array for x in l}

Solution 2

You can use itertools's chain to flatten your array and then call set on it:

from itertools import chain

array = [['a','b'], ['a', 'b','c'], ['a']]
print set(chain(*array))

If you are expecting a list object:

print list(set(chain(*array)))

Solution 3

array = [['a','b'], ['a', 'b','c'], ['a']]
unique_values = list(reduce(lambda i, j: set(i) | set(j), array))

Solution 4

You can use numpy.unique:

import numpy
import operator
print numpy.unique(reduce(operator.add, [['a','b'], ['a', 'b','c'], ['a']]))
# ['a' 'b' 'c']

Solution 5

The 2 top voted answers did not work for me, I'm not sure why (but I have integer lists). In the end I'm doing this:

unique_values = [list(x) for x in set(tuple(x) for x in aList)]

View more solutions

Comments

[mihasa]

that did the job, thank you very much! Isn't there a better way to do this kind of think without needing to iterate? (i'm a beginner but i thought that avoiding loops was the way to go when having a big len(array))

[mihasa]

i've just created my account, can't raise anything yet :( Your answer was very helpful, but ive accepted the first answer. when i have enough point (or idk what gives permition to raise answers) i'll do it for yours

[Tanveer Alam]

@mihasa No worries. I'm glad my answer was helpful.

[mihasa]

Just got it... raised!

[dlask]

I am afraid there is no simpler way. You have a precisely defined input structure, you have a precisely defined output structure, and it's necessary to make the conversion. Since the conversion involves all elements it's necessary to "visit" all of them. Well, there are many different ways to obtain the result (see other posts) but the iteration is always present internally.

[chepner]

There's also set(chain.from_iterable(array)).