deep merge two nested dictionaries, while applying a function to each item

deep_fn_merge.py

import collections

def deep_fn_merge(fn, left, right):
  for k, v in right.iteritems():
    if k in left and isinstance(left[k], dict) and isinstance(right[k], collections.Mapping):
      deep_fn_merge(fn, left[k], right[k])
    else:
      left[k] = fn(left, right, k)

def sum(left, right, key):
  return left.get(key, 0) + right.get(key, 0)

d1 = { 'a': 1, 'b': 2, 'c': 3, 'd': { 'e': 10, 'f': 4 } }
d2 = { 'a': 0, 'b': 6, 'd': { 'e': 10, 'g': 3 } }

deep_fn_merge(sum, d1, d2)

print d1