For just a list like this, my favourite neat little trick is just to use sum;
sum has an optional argument: sum(iterable [, start]), so you can do:
list_of_lists = [[1,2,3], [4,5,6], [7,8,9]]
print sum(list_of_lists, []) # [1,2,3,4,5,6,7,8,9]
this works because the + operator happens to be the concatenation operator for lists, and you've told it that the starting value is [] - an empty list.
but the documentaion for sum advises that you use itertools.chain instead, as it's much clearer.
