Python: Elegantly merge dictionaries with sum() of values [duplicate]

Answer 1

You could use itertools' groupby:

from itertools import groupby, chain

a=[("13.5",100)]
b=[("14.5",100), ("15.5", 100)]
c=[("15.5",100), ("16.5", 100)]
input = sorted(chain(a,b,c), key=lambda x: x[0])

output = {}
for k, g in groupby(input, key=lambda x: x[0]):
  output[k] = sum(x[1] for x in g)

print output

The use of groupby instead of two loops and a defaultdict will make your code clearer.