Pythonic Way to Create Union of All Values Contained in Multiple Lists

Question

Hi all,

Doing some XML processing in python. (Edit: I'm forced to use Python 2.4 for this project, boo!) I want to know what is the most Pythonic way to do this (create union of all values in multiple lists):

def getUniqueAttributeValues(xml_attribute_nodes):
    # split attribute values by whitespace into lists
    result_lists=list(item.getContent().split() for item in xml_attribute_nodes)

    # find all unique values
    unique_results=[]
    for result_list in result_lists:
        for result in result_list:
            if result in unique_results:
                continue
            unique_results.append(result)

    return unique_results

Thanks,

-aj

Answer 1

def getUniqueAttributeValues(xml_attribute_nodes):
    return set(l 
       for item in xml_attribute_nodes
       for l in item.getContent().split())

If you want to have a list, just convert the set to a list before returning.

Answer 2

Unions are not supported by lists, which are ordered, but are supported by sets. Check out set.union.

Answer 3

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 4

I used the following to do intersections, which avoids the need for sets.

a, b= [[1,2,3], [1,2]]
s = filter( lambda x: x in b, a)

or,

s = [ x for x in b if x in a ]

Answer 5

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.