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

With a dictionary comprehension.

>>> def merge_func(x,y):
...    return x + y
...
>>>
>>> d1 = {'a' : [42], 'b' : [12], 'c' : [4]}
>>> d2 = {'a' : [3], 'b' : [2], 'd' : [0]}
>>> { key: merge_func(d1.get(key, []), d2.get(key, [])) for key in set( list(d1.keys()) + list(d2.keys()))}
{'a': [42, 3], 'c': [4], 'b': [12, 2], 'd': [0]}