Merge two dictionaries and keep the values for duplicate keys in Python

Question

Let\'s suppose that I have two dictionaries:

dic1 =  { \"first\":1, \"second\":4, \"third\":8} 
dic2 =  { \"first\":9, \"second\":5, \"fourth\":3}

Answer 1

You can use a defaultdict to hold lists, and then just append the values to them. This approach easily extends to an arbitrary number of dictionaries.

from collections import defaultdict

dd = defaultdict(list)

dics = [dic1, dic2]
for dic in dics:
    for key, val in dic.iteritems():  # .items() in Python 3.
        dd[key].append(val)

>>> dict(dd)
{'first': [1, 9], 'fourth': [3], 'second': [4, 5], 'third': [8]}

All of the keys with a single value are still held within a list, which is probably the best way to go. You could, however, change anything of length one into the actual value, e.g.

for key, val in dd.iteritems():  # .items() in Python 3.
    if len(val) == 1
        dd[key] = val[0]