join two lists of dictionaries on a single key

by 
from collections import defaultdict

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:
        d[elem['index']].update(elem)
l3 = d.values()

# l3 is now:

[{'b': 2, 'c': 4, 'index': 1},
 {'b': 3, 'c': 5, 'index': 2},
 {'green': 'eggs', 'index': 3}]

EDIT: Since l3 is not guaranteed to be sorted (.values() returns items in no specific order), you can do as @user560833 suggests:

from operator import itemgetter

...

l3 = sorted(d.values(), key=itemgetter("index"))

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