Summing 2 level of nested dictionaries in Python

Question

I have 2 nested dictionaries variable that have the similar keys, each defining different values :

data1 = {
"2010":{
        'A':2,
        'B':3,
        'C':5
    },
"2011":{
        'A':1,
        'B':2,
        'C':3
    },
"2012":{
        'A':1,
        'B':2,
        'C':4
    }
}

data2 = {
"2010":{
        'A':4,
        'B':4,
        'C':5
    },
"2011":{
        'A':1,
        'B':1,
        'C':3
    },
"2012":{
        'A':3,
        'B':2,
        'C':4
    }
}

In my case, i need to sum both dictionaries values based on the same keys, so the answer will be like this:

data3 = {
"2010":{
        'A':6,
        'B':7,
        'C':10
    },
"2011":{
        'A':2,
        'B':3,
        'C':6
    },
"2012":{
        'A':4,
        'B':4,
        'C':8
    }
}

How can i do that?

Answer 1

Given the structure of the two dictionaries is the same, you can use dictionary comprehension for that:

data3 = {key:{key2:val1+data2[key][key2] for key2,val1 in subdic.items()} for key,subdic in data1.items()}

In the repl:

>>> {key:{key2:val1+data2[key][key2] for key2,val1 in subdic.items()} for key,subdic in data1.items()}
{'2010': {'B': 7, 'C': 10, 'A': 6}, '2012': {'B': 4, 'C': 8, 'A': 4}, '2011': {'B': 3, 'C': 6, 'A': 2}}

The comprehension works as follows: in the outerloop, we iterate over the key,subdic of data1. So in your case, key is a year and subdic is the dictionary (of data1) for that year.

Now for each of these years, we iterate over the items of the subdic and here key2 is 'A', 'B' and 'C'. val1 is the value that we find in data1 for these keys. We get the other value by querying data2[key][key2]. We sum these up and construct new dictionaries for that.