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

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.

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.

You can also follow this style

In [12]: a = ['Orange and Banana', 'Orange Banana']
In [13]: b = ['Grapes', 'Orange Banana']
In [14]: c = ['Foobanana', 'Orange and Banana']

In [20]: list(set(a) | set(b) | set(c))
Out[20]: ['Orange and Banana', 'Foobanana', 'Orange Banana', 'Grapes']

In [21]: list(set(a) & set(b) | set(c))
Out[21]: ['Orange and Banana', 'Foobanana', 'Orange Banana']