dictionary - merging several python dictionaries

Question

I have to merge list of python dictionary. For eg:

dicts[0] = {'a':1, 'b':2, 'c':3}
dicts[1] = {'a':1, 'd':2, 'c':'foo'}
dicts[2] = {'e':57,'c':3}

super_dict = {'a':[1], 'b':[2], 'c':[3,'foo'], 'd':[2], 'e':[57]}    

I wrote the following code:

super_dict = {}
for d in dicts:
    for k, v in d.items():
        if super_dict.get(k) is None:
            super_dict[k] = []
        if v not in super_dict.get(k):
            super_dict[k].append(v)

Can it be presented more elegantly / optimized?

Note I found another question on SO but its about merging exactly 2 dictionaries.

Answer 1

You can iterate over the dictionaries directly -- no need to use range. The setdefault method of dict looks up a key, and returns the value if found. If not found, it returns a default, and also assigns that default to the key.

super_dict = {}
for d in dicts:
    for k, v in d.iteritems():  # d.items() in Python 3+
        super_dict.setdefault(k, []).append(v)

Also, you might consider using a defaultdict. This just automates setdefault by calling a function to return a default value when a key isn't found.

import collections
super_dict = collections.defaultdict(list)
for d in dicts:
    for k, v in d.iteritems():  # d.items() in Python 3+
        super_dict[k].append(v)

Also, as Sven Marnach astutely observed, you seem to want no duplication of values in your lists. In that case, set gets you what you want:

import collections
super_dict = collections.defaultdict(set)
for d in dicts:
    for k, v in d.iteritems():  # d.items() in Python 3+
        super_dict[k].add(v)