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how do i extract the values from the existing list-dictionary in a python to create a new dictionary as shown below ..any help will be appreciated

old_list_dict=[{'A':'FRUITS','B':'APPLE'} , {'A':'FRUITS','B':'BANANA'} ,{'A':'FRUITS','B':'PECAN'}]


new_dict={'FRUITS':'APPLE','FRUITS':'BANANA','FRUITS':'PECAN'}

Or 

new_dict ={'FRUITS':{'APPLE','BANANA','PECAN'}
shirop
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2 Answers2

1

Assuming all your dictionaries have exactly two items, you can use map to extract values in pairs for the dictionary constructor:

old_list_dict=[{'A':'APPLE','B':'FRUITS'} ,
               {'A':'Banana','B':'FRUITS'} ,
               {'A':'PECAN','B':'FRUITS'}]

new_dict = dict( map(dict.values,old_list_dict) )

print(new_dict)
                                                                          
{'APPLE': 'FRUITS', 'Banana': 'FRUITS', 'PECAN': 'FRUITS'}

Note that, if you have duplicate 'A' values in the list (ex: 'PECAN' multiple times, you will only get one of them in the new dictionary

For the second form, using update() will allow the values to be accumulated in a set for each key:

new_dict = dict()
new_dict.update( (k,new_dict.get(k,set())|{v})
                 for (v,k) in map(dict.values,old_list_dict)) 
print(new_dict)

{'FRUITS': {'Banana', 'PECAN', 'APPLE'}}

In this case the .update() method will receive tuples containing a key and a set() value to be added or updated in new_dict.

These tuples will be build from the pairing of values from the old_lit_dict list as (v,k). For example ('APPLE','FRUITS').

But, since we want the final values to be sets, accumulated for the 'FRUITS' key, the actual tuple produced for the update reads the existing key new_dict.get(k,set()) to get its set of values and adds the current value to it | {v} (the | is a union operator with the single value set {v}).

The ,set()) part of the get method ensures that an initial (empty) set is returned when the key is not yet present in the new_dict (e.g. the first time we encounter 'FRUITS')

The comprehension in this example is functionally equivalent to a more traditional for loop:

new_dict = dict()
for D in old_list_dict:
   v,k = D.values()           # extract value and key (e.g. APPLE,FRUITS)
   if k not in new_dict:      # assign empty set to key if it is new
       new_dict[k] = set()
   new_dict[k].add(v)         # add value to key's set
Alain T.
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  • thanks , how do i update the new_dictionary with new key/pair values derived from the for loop , is there any similar method like append() for updating dictionary ? i tried the update method its seems to be overwriting the previous dict entries – shirop Feb 28 '23 at 03:18
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    dictionaries cannot have duplicate keys so, if your other dictionary's values produces the same key as the first one, you will only get one item for them in the new dictionary. You would need to use lists for values of your new dictionary if you want to store multiple values for duplicate keys from the other ones. – Alain T. Feb 28 '23 at 03:29
  • It would be easier to help if you provide a more representative sample input and output in your question. – Alain T. Feb 28 '23 at 03:40
  • yes, updated the question accordingly thanks – shirop Feb 28 '23 at 03:42
  • thanks @Alain T for the help , appreciate the same , where should i start to understand the code flow - new_dict.update( (k,new_dict.get(k,set())|{v}) for (v,k) in map(dict.values,old_list_dict)) – shirop Feb 28 '23 at 04:01
  • Added explanations for the second form – Alain T. Feb 28 '23 at 04:09
-2

Try this

new_dictionary = {
    existing_dictionary["current"]: existing_dictionary["area"]
}
ProfDFrancis
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