l1 = [{'index': 1, 'b': 2}, {'index': 2, 'b': 3}, {'index': 3, 'green': 'eggs'}
    ]
l2 = [{'index': 1, 'c': 4}, {'index': 2, 'c': 5}]
d = defaultdict(dict)
for l in (l1, l2):
    for elem in l:
        pass
join two lists of dictionaries on a single key