How Can I Efficiently Merge Python Dictionaries and Sum Values for Common Keys?

Merging Dictionaries in Python: Adding Values for Overlapping Keys

When working with multiple dictionaries, it's often necessary to combine them in a meaningful way. One common scenario involves adding the values of keys that appear in both dictionaries while keeping keys that only exist in one.

Problem:

Consider the following dictionaries:

Dict A: {'a': 1, 'b': 2, 'c': 3}
Dict B: {'b': 3, 'c': 4, 'd': 5}

The goal is to merge these dictionaries such that the result is:

{'a': 1, 'b': 5, 'c': 7, 'd': 5}

Solution Using Collections.Counter:

A Pythonic and efficient way to achieve this is by using the collections.Counter class. It's a subclass of dict that provides a convenient way to do element-based counting, which lends itself well to the task of merging dictionaries.

from collections import Counter

A = Counter({'a':1, 'b':2, 'c':3})
B = Counter({'b':3, 'c':4, 'd':5})

To merge the dictionaries, we simply add the Counter objects:

merged_dict = A + B

The result is a new Counter object, which automatically adds values for overlapping keys:

merged_dict.most_common()
# Output: [('c', 7), ('b', 5), ('d', 5), ('a', 1)]

Converting it to a regular dictionary is trivial:

merged_dict = dict(merged_dict)

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