Combining 2 dictionaries with common key

Question

I have two dictionaries and need to combine the values of similar keys in them. Here's an example:

dict1 = {'key1':[value11,value12,value13] , 'key2':[value21,value22,value23]}
dict2 = {'key1':[value14,value15] , 'key2':[value24,value25]}

I used :

dict3 = {}
for key in (dict1.viewkeys() | dict2.keys()):
    if key in dict1: dict3.setdefault(key, []).append(dict1[key])
    if key in dict2: dict3.setdefault(key, []).append(dict2[key])

which gives me:

dict3 = {'key1':[[value11,value12,value13],[value14,value15]] , 'key2':[[value21,value22,value23],[value24,value25]]}

What I want is a simple one like:

Desired output :

 dict3 = {'key1':[value11,value12,value13,value14,value15] , 'key2':[value21,value22,value23,value24,value25]}

Answer 1

All you need to do is to modify append to extend which will then add the elements of the list rather than adding the list itself. See the list docs for more details on the difference between append and extend.

dict1 = {'key1':['value11','value12','value13'] , 'key2':['value21','value22','value23']}
dict2 = {'key1':['value14','value15'] , 'key2':['value24','value25']}

dict3 = {}
for key in set().union(dict1, dict2):
    if key in dict1: dict3.setdefault(key, []).extend(dict1[key])
    if key in dict2: dict3.setdefault(key, []).extend(dict2[key])

print(dict3)
# {'key2': ['value21', 'value22', 'value23', 'value24', 'value25'], 'key1': ['value11', 'value12', 'value13', 'value14', 'value15']}

Alternatively you could use a collections.defaultdict with the default set to list as shown below.

from collections import defaultdict
dict3 = defaultdict(list)

for key in set().union(dict1, dict2):
  for dic in [dict1, dict2]:
    if key in dic:
      dict3[key] += dic[key]

Answer 2

You can do it much simpler but if you want to use your code just change append to extend

dict1 = {'key1':['value11','value12','value13'] , 'key2':['value21','value22','value23']}
dict2 = {'key1':['value14','value15'] , 'key2':['value24','value25']}

dict3 = {}
for key in (dict1.viewkeys() | dict2.keys()):
    if key in dict1: dict3.setdefault(key, []).extend(dict1[key])
    if key in dict2: dict3.setdefault(key, []).extend(dict2[key])

print dict3

output:

{'key2': ['value21', 'value22', 'value23', 'value24', 'value25'], 'key1': ['value11', 'value12', 'value13', 'value14', 'value15']}

You can read this post about the difference ov append to extend

Answer 3

Here is a generic method on which you can pass as many dict as you want as parameter.

>>> def mix_dict(*args):
       res = {}
       for d in args:
           if not isinstance(d, dict):
               continue
           for k, v in d.iteritems():
               res.setdefault(k, [])
               if isinstance(v, list):
                   res[k].extend(v)
               else:
                   res[k].append(v)
       return res
>>> dict1 = {'key1':['value11','value12','value13'] , 'key2':['value21','value22','value23']}
>>> dict2 = {'key1':['value14','value15'] , 'key2':['value24','value25']}
>>> dict3 = mix_dict(dict1, dict2)
>>> print dict3
... {'key1': ['value11', 'value12', 'value13', 'value14', 'value15'],
     'key2': ['value21', 'value22', 'value23', 'value24', 'value25']}