[FIXED] How to sum up dict values based on combined key values?

Issue

My question is similar to this(Python sum on keys for List of Dictionaries), but need to sum up the values based on two or more key-value elements. I have a list of dictionaries as following:

list_to_sum=
        [{'Name': 'A', 'City': 'W','amt':100},
         {'Name': 'B', 'City': 'A','amt':200},
         {'Name': 'A', 'City': 'W','amt':300},
         {'Name': 'C', 'City': 'X','amt':400},
         {'Name': 'C', 'City': 'X','amt':500},
         {'Name': 'A', 'City': 'W','amt':600}]
            

So based on a combination of Name and City key values, amt should be summed. Please let me know how to solve this.

Output: [{'Name': 'A', 'City': 'W','amt':900},
         {'Name': 'B', 'City': 'A','amt':200},
         {'Name': 'C', 'City': 'X','amt':900}]

Solution

You could create a collections.Counter.Then you can simply add the values as the appear using the tuple as (Name, City) as the key:

from collections import Counter

list_to_sum=[
    {'Name': 'A', 'City': 'W','amt':100},
    {'Name': 'B', 'City': 'A','amt':200},
    {'Name': 'A', 'City': 'W','amt':300},
    {'Name': 'C', 'City': 'X','amt':400},
    {'Name': 'C', 'City': 'X','amt':500},
    {'Name': 'A', 'City': 'W','amt':600}
]
    
totals = Counter()

for d in list_to_sum:
    totals[(d['Name'],d['City'])] += d['amt']

print(totals[('A','W')]) # 1000
print(totals[('B','A')]) # 200
print(totals[('C','X')]) # 900
      

This will produce a dictionary-like object Counter:

Counter({('A', 'W'): 1000, ('B', 'A'): 200, ('C', 'X'): 900})

With this you can convert the dict back to a list of dicts like:

sums_list = [{'Name':Name, 'City':City, 'amt':amt} for (Name, City), amt in totals.items()]

giving sums_list:

[{'Name': 'A', 'City': 'W', 'amt': 1000},
 {'Name': 'B', 'City': 'A', 'amt': 200},
 {'Name': 'C', 'City': 'X', 'amt': 900}]

Answered By - Mark