Question

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Pythonic Way to reverse nested dictionaries

I am having hard time trying to find a solution for this nested dictionary problem.
I found some itertools functions that could be used, but it is not clear to me as how to use it. any help on this front would be great

input = { "a" : { "x": 1, "y": 2 },
          "b" : { "x": 3, "z": 4 } }


output = {'y': {'a': 2},
          'x': {'a': 1, 'b': 3},
          'z': {'b': 4} }

Answer 1

A simplistic way using dict.setdefault:

i = { "a" : { "x": 1, "y": 2 },
      "b" : { "x": 3, "z": 4 } }

d = {}
for key, value in i.iteritems():    
    for ikey, ivalue in value.iteritems():
        d.setdefault(ikey,{})[key] = ivalue
print d

out:

{'x': {'a': 1, 'b': 3}, 'y': {'a': 2}, 'z': {'b': 4}}