print({k: (x.get(k, 0) + y.get(k, 0)) for k in set(x)})
print({k: (x.get(k, 0) + y.get(k, 0)) for k in set(x) & set(y)})
print({k: (x.get(k, 0) + y.get(k, 0)) for k in set(x) | set(y)})
Merge and sum of two dictionaries