merging lists, dedup and keeping order, in older pythons

[ODIS]

Sets don't keep order and python doesn't include built-in ordered sets. But you can use ordered dictionary from the collections package which is also in Python 2:

from collections import OrderedDict

list1 = ["one", "two"]
list2 = ["three", "four"]

merge = OrderedDict()

for a_list in [list1, list2]:
    for item in a_list:
        merge[item] = None

print(merge.keys())

Sets stay unordered in Python 3.7 :)