Pythonic way to create union of all values contained in multiple lists

Question

I have a list of lists:

lists = [[1,4,3,2,4], [4,5]] 

I want to flatten this list and remove all duplicates; or, in other words, apply a set union operation:

desired_result = [1, 2, 3, 4, 5] 

What's the easiest way to do this?

Answer 1

set.union does what you want:

>>> results_list = [[1,2,3], [1,2,4]] >>> results_union = set().union(*results_list) >>> print(results_union) set([1, 2, 3, 4]) 

You can also do this with more than two lists.

Answer 2

Since you seem to be using Python 2.5 (it would be nice to mention in your Q if you need an A for versions != 2.6, the current production one, by the way;-) and want a list rather than a set as the result, I recommend:

import itertools  ...  return list(set(itertools.chain(*result_list))) 

itertools is generally a great way to work with iterators (and so with many kinds of sequences or collections) and I heartily recommend you become familiar with it. itertools.chain, in particular, is documented here.