How to merge dictionaries of dictionaries?

Question

I need to merge multiple dictionaries, here\'s what I have for instance:

dict1 = {1:{\"a\":{A}}, 2:{\"b\":{B}}}

dict2 = {2:{\"c\":{C}}, 3:{\"d\":{D}}
         


        

Answer 1

I had two dictionaries (a and b) which could each contain any number of nested dictionaries. I wanted to recursively merge them, with b taking precedence over a.

Considering the nested dictionaries as trees, what I wanted was:

• To update a so that every path to every leaf in b would be represented in a
• To overwrite subtrees of a if a leaf is found in the corresponding path in b
  • Maintain the invariant that all b leaf nodes remain leafs.

The existing answers were a little complicated for my taste and left some details on the shelf. I hacked together the following, which passes unit tests for my data set.

  def merge_map(a, b):
    if not isinstance(a, dict) or not isinstance(b, dict):
      return b

    for key in b.keys():
      a[key] = merge_map(a[key], b[key]) if key in a else b[key]
    return a

Example (formatted for clarity):

 a = {
    1 : {'a': 'red', 
         'b': {'blue': 'fish', 'yellow': 'bear' },
         'c': { 'orange': 'dog'},
    },
    2 : {'d': 'green'},
    3: 'e'
  }

  b = {
    1 : {'b': 'white'},
    2 : {'d': 'black'},
    3: 'e'
  }


  >>> merge_map(a, b)
  {1: {'a': 'red', 
       'b': 'white',
       'c': {'orange': 'dog'},},
   2: {'d': 'black'},
   3: 'e'}

The paths in b that needed to be maintained were:

• 1 -> 'b' -> 'white'
• 2 -> 'd' -> 'black'
• 3 -> 'e'.

a had the unique and non-conflicting paths of:

• 1 -> 'a' -> 'red'
• 1 -> 'c' -> 'orange' -> 'dog'

so they are still represented in the merged map.