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.

Answer 2

This simple recursive procedure will merge one dictionary into another while overriding conflicting keys:

#!/usr/bin/env python2.7

def merge_dicts(dict1, dict2):
    """ Recursively merges dict2 into dict1 """
    if not isinstance(dict1, dict) or not isinstance(dict2, dict):
        return dict2
    for k in dict2:
        if k in dict1:
            dict1[k] = merge_dicts(dict1[k], dict2[k])
        else:
            dict1[k] = dict2[k]
    return dict1

print (merge_dicts({1:{"a":"A"}, 2:{"b":"B"}}, {2:{"c":"C"}, 3:{"d":"D"}}))
print (merge_dicts({1:{"a":"A"}, 2:{"b":"B"}}, {1:{"a":"A"}, 2:{"b":"C"}}))

Output:

{1: {'a': 'A'}, 2: {'c': 'C', 'b': 'B'}, 3: {'d': 'D'}}
{1: {'a': 'A'}, 2: {'b': 'C'}}

Answer 3

If you have an unknown level of dictionaries, then I would suggest a recursive function:

def combineDicts(dictionary1, dictionary2):
    output = {}
    for item, value in dictionary1.iteritems():
        if dictionary2.has_key(item):
            if isinstance(dictionary2[item], dict):
                output[item] = combineDicts(value, dictionary2.pop(item))
        else:
            output[item] = value
    for item, value in dictionary2.iteritems():
         output[item] = value
    return output

Answer 4

Easiest way i can think of is :

#!/usr/bin/python

from copy import deepcopy
def dict_merge(a, b):
    if not isinstance(b, dict):
        return b
    result = deepcopy(a)
    for k, v in b.iteritems():
        if k in result and isinstance(result[k], dict):
                result[k] = dict_merge(result[k], v)
        else:
            result[k] = deepcopy(v)
    return result

a = {1:{"a":'A'}, 2:{"b":'B'}}
b = {2:{"c":'C'}, 3:{"d":'D'}}

print dict_merge(a,b)

Output:

{1: {'a': 'A'}, 2: {'c': 'C', 'b': 'B'}, 3: {'d': 'D'}}

Answer 5

And just another slight variation:

Here is a pure python3 set based deep update function. It updates nested dictionaries by looping through one level at a time and calls itself to update each next level of dictionary values:

def deep_update(dict_original, dict_update):
    if isinstance(dict_original, dict) and isinstance(dict_update, dict):
        output=dict(dict_original)
        keys_original=set(dict_original.keys())
        keys_update=set(dict_update.keys())
        similar_keys=keys_original.intersection(keys_update)
        similar_dict={key:deep_update(dict_original[key], dict_update[key]) for key in similar_keys}
        new_keys=keys_update.difference(keys_original)
        new_dict={key:dict_update[key] for key in new_keys}
        output.update(similar_dict)
        output.update(new_dict)
        return output
    else:
        return dict_update

A simple example:

x={'a':{'b':{'c':1, 'd':1}}}
y={'a':{'b':{'d':2, 'e':2}}, 'f':2}

print(deep_update(x, y))
>>> {'a': {'b': {'c': 1, 'd': 2, 'e': 2}}, 'f': 2}