Merging from Python 3.5

With the following code you can merge two dictionary and get also the sum of the value of the common keys.

a = {'a': 1, 'b': 2, 'c': 3}
b = {'b': 3, 'c': 4, 'd': 5}
# this sums the the values of common keys
for k in b:
    if k in a:
        b[k] = b[k] + a[k]
# Merge the two dict with sums of common keys
c = {**a, **b}
print(c)

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

Reusable code

In this code, instead, you can do the same thing, but with a function that is callable and so the code is nicer, more readeable and you can use it more times withou having to rewrite the ‘engine’ of the process.

a = {'a': 1, 'b': 2, 'c': 3}
b = {'b': 3, 'c': 4, 'd': 5}


def mergsum(a, b):
    for k in b:
        if k in a:
            b[k] = b[k] + a[k]
    c = {**a, **b}
    return c


print(mergsum(a, b))

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