Sum values of similar keys inside two nested dictionary in python

Question

I have nested dictionary like this:

data = {
    "2010":{
            'A':2,
            'B':3,
            'C':5,
            'D':-18,
        },
    "2011":{
            'A':1,
            'B':2,
            'C':3,
            'D':1,
        },
    "2012":{
            'A':1,
            'B':2,
            'C':4,
            'D':2
        }
    }

In my case, i need to sum all values based on its similar keys in every year, from 2010 till 2012.. So the result i expected should be like this:

data = {'A':4,'B':7, 'C':12, 'D':-15}

Answer 1

You can use collections.Counter() (works only for positive values!):

In [17]: from collections import Counter
In [18]: sum((Counter(d) for d in data.values()), Counter())
Out[18]: Counter({'C': 12, 'B': 7, 'A': 4, 'D': 3})

Note that based on python documentation Counter is designed only for use cases with positive values:

The multiset methods are designed only for use cases with positive values. The inputs may be negative or zero, but only outputs with positive values are created. There are no type restrictions, but the value type needs to support addition, subtraction, and comparison. The elements() method requires integer counts. It ignores zero and negative counts.

So if you want to get a comprehensive result you can do the summation manually. The collections.defaultdict() is a good way for getting around this problem:

In [28]: from collections import defaultdict

In [29]: d = defaultdict(int)

In [30]: for sub in data.values():
   ....:     for i, j in sub.items():
   ....:         d[i] += j
   ....:         

In [31]: d
Out[31]: defaultdict(<class 'int'>, {'D': -15, 'A': 4, 'C': 12, 'B': 7})

Answer 2

Try this,

reduce(lambda x, y: dict((k, v + y[k]) for k, v in x.iteritems()), data.values())

Result

{'A': 4, 'B': 7, 'C': 12, 'D': -15}