Python merging dictionary of dictionaries into one dictionary by summing the value

Question

I want to merge all the dictionaries in a dictionary, while ignoring the main dictionary keys, and summing the value of the other dictionaries by value.

Input:

{'first':{'a': 5}, 'second':{'a': 10}, 'third':{'b': 5, 'c': 1}}

Output:

{'a': 15, 'b': 5, 'c': 1}

I did:

def merge_dicts(large_dictionary):
    result = {}
    for name, dictionary in large_dictionary.items():
        for key, value in dictionary.items():
            if key not in result:
                result[key] = value
            else:
                result[key] += value
    return result

Which works, but I don't think it's such a good way (or less "pythonic").

By the way, I don't like the title I wrote. If anybody thinks of a better wording please edit.

The only bad thing you do is going over `large_dictionary.items()` instead of `large_dictionary.values()`. — Stefan Pochmann, Nov 03 '17 at 18:05

score 6 · Accepted Answer · answered Nov 03 '17 at 18:06

6

You can sum counters, which are a dict subclass:

>>> from collections import Counter
>>> sum(map(Counter, d.values()), Counter())
Counter({'a': 15, 'b': 5, 'c': 1})

answered Nov 03 '17 at 18:06

wim

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daemon24 · Answer 2 · 2017-11-03T18:44:37.203

2

This will work

from collections import defaultdict
values = defaultdict(int)
def combine(d, values):
    for k, v in d.items():
        values[k] += v

for v in a.values():
    combine(v, values)

print(dict(values))

edited Nov 03 '17 at 18:44

answered Nov 03 '17 at 18:11

daemon24

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I hadn't put list in defaultdict, i had put defaultdict(lambda : 0), defaultdict(int) and defaultdict(lambda : 0) result to same. But int will be more pythonic, i also thought. – daemon24 Nov 03 '17 at 19:27
@wim Oh. Hmm. I thought it would always return the same list. But it doesn't. Gotta think this through when I have more time. Thanks for the correction. – Stefan Pochmann Nov 03 '17 at 19:35
@wim Yeah ok I'm an idiot... wouldn't have expected `def f(): return []` to always return the same one. Something about `defaultdict(lambda: [])` somehow threw me off. I guess because it looks too similar to parameter default notation. – Stefan Pochmann Nov 03 '17 at 19:47

score 0 · Answer 3 · edited Nov 06 '17 at 19:52

0

Almost similar, but it's just short and I like it a little better.

def merge_dicts(large_dictionary):
    result = {}
    for d in large_dictionary.values():
        for key, value in d.items():
            result[key] = result.get(key, 0) + value
    return result

edited Nov 06 '17 at 19:52

wpercy

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answered Nov 03 '17 at 18:08

harshil9968

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Python merging dictionary of dictionaries into one dictionary by summing the value

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