Union python dictioniaries with function on collision

Question

In Python, is there a way to merge dictionaries and do something on collision? I'm looking for an idiom equivalent to the unionWith function in Haskell: http://hackage.haskell.org/packages/archive/containers/0.5.0.0/doc/html/Data-Map-Lazy.html#v:unionWith

>>> unionWith(lambda x,y: x + y, {'a' : [42], 'b' : [12], c : [4]}, {'a' : [3], 'b' : [2], 'd' : [0]})
{'a' : [42,3], 'b' : [12,2], 'c' : [4], 'd': [0]}

Implementation based on @monkut's solution: https://github.com/cheecheeo/useful/commit/109885a27288ef53a3de2fa2b3a6e50075c5aecf#L1R18

Answer 1

def union_with(merge_func, x, y):
    result = dict(x)
    for key in y:
        if key in result:
              result[key] = merge_func(result[key], y[key])
        else:
              result[key] = y[key]
    return result

---

>>> union_with(lambda x,y: x + y, {'a' : [42], 'b' : [12], 'c' : [4]}, {'a' : [3], 'b' : [2], 'd' : [0]})
{'a': [42, 3], 'c': [4], 'b': [12, 2], 'd': [0]}

Answer 2

my 5 cents

 dict = {'a' : [42], 'b' : [12], 'c' : [4]}, {'a' : [3], 'b' : [2], 'd' : [0]}
 keyset = set([k for d in dict for k in d.keys()])
 union = {}
 for d in dict:
     for k in d.keys():
         if k in union:
             union[k].append(d[k])
         else:
             union[k]=[s[k]]