Python - Group similar value list to dictionary

Sometimes, while working with Python list, we can have a problem in which we need to group similar value lists indices to values into a dictionary. This can have a good application in domains in which we need grouped dictionary as output for pair of lists. Lets discuss certain ways in which this task can be performed.

Method #1 : Using dictionary comprehension
This is way in which this task can be performed. In this, we iterate one list along with other and keep constructing dictionary with similar key from one list and corresponding values from other. This is one liner solution.

# Python3 code to demonstrate  
# Group similar value list to dictionary 
# using dictionary comprehension 
  
# Initializing lists 
test_list1 = [4, 4, 4, 5, 5, 6, 6, 6, 6] 
test_list2 = ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't'] 
  
# printing original lists 
print("The original list 1 is : " + str(test_list1)) 
print("The original list 2 is : " + str(test_list2)) 
  
# Group similar value list to dictionary 
# using dictionary comprehension 
res = {key : [test_list2[idx]  
      for idx in range(len(test_list2)) if test_list1[idx]== key] 
      for key in set(test_list1)} 
  
# printing result  
print ("Mapped resultant dictionary : " + str(res)) 

Output :

The original list 1 is : [4, 4, 4, 5, 5, 6, 6, 6, 6]
The original list 2 is : [‘G’, ‘f’, ‘g’, ‘i’, ‘s’, ‘b’, ‘e’, ‘s’, ‘t’]
Mapped resultant dictionary : {4: [‘G’, ‘f’, ‘g’], 5: [‘i’, ‘s’], 6: [‘b’, ‘e’, ‘s’, ‘t’]}

Method #2 : Using defaultdict() + loop
This is yet another way in which this task can be performed. In this we avoid testing for keys presence in dictionary by creating a defaultdict().

# Python3 code to demonstrate  
# Group similar value list to dictionary 
# using defaultdict() + loop 
from collections import defaultdict 
  
# Initializing lists 
test_list1 = [4, 4, 4, 5, 5, 6, 6, 6, 6] 
test_list2 = ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't'] 
  
# printing original lists 
print("The original list 1 is : " + str(test_list1)) 
print("The original list 2 is : " + str(test_list2)) 
  
# Group similar value list to dictionary 
# using defaultdict() + loop 
res = defaultdict(set) 
for key, val in zip(test_list1, test_list2): 
    res[key].add(val) 
res = {key: list(val) for key, val in res.items()} 
  
# printing result  
print ("Mapped resultant dictionary : " + str(res)) 

Output :

The original list 1 is : [4, 4, 4, 5, 5, 6, 6, 6, 6]
The original list 2 is : [‘G’, ‘f’, ‘g’, ‘i’, ‘s’, ‘b’, ‘e’, ‘s’, ‘t’]
Mapped resultant dictionary : {4: [‘G’, ‘f’, ‘g’], 5: [‘i’, ‘s’], 6: [‘b’, ‘e’, ‘s’, ‘t’]}

My Personal Notes

Python – Group similar value list to dictionary

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