Combining Python Dictionaries

I had a small task where I had to take two existing dict and combine them by adding together the values of any elements that already exist. Here's how I did it.

def dict_combine(dict1, dict2, combiner):
      """takes two dictionaries and combines them
      such that the value is the result of:
      result[a]  = combiner(dict1[a],dict2[a])"""
      result = {}
      dict1_keys = set(dict1.keys())
      dict2_keys = set(dict2.keys())

      # keys uniuqe to dict1 are included unchanged
      for x in dict1_keys - dict2_keys:
            result[x] = dict1[x]

      # keys uniuqe to dict2 are included unchanged
      for x in dict2_keys - dict1_keys:
            result[x] = dict2[x]

      # keys in both dictionaries get combined then included
      for x in dict1_keys & dict2_keys:
            result[x] = combiner(dict1[x],dict2[x])

      return result

def test_dict_combine():
      dict1 = dict(a=1, b=2, c=3)
      def adder(a,b):
            return a + b 
      assert dict_combine(dict1,dict1,adder) == dict(a=2, b=4, c=6)
test_dict_combine()