dictionary add values for the same keys

Question

I have a list of dictionary:

[{'name':'Jay', 'value':'1'},{'name':'roc', 'value':'9'},{'name':'Jay', 'value':'7'},{'name':'roc', 'value':'2'}]

I want it to be:

[{'name':'Jay', 'value':'8'},{'name':'roc', 'value':'11'}]

I tried looping through but I am not able to find an example where I can do this. Any hint or idea will be appreciated.

Answer 1

You can use a defaultdict:

lst = [{'name':'Jay', 'value':'1'},{'name':'roc', 'value':'9'},{'name':'Jay', 'value':'7'},{'name':'roc', 'value':'2'}]

1) sum values for each name:

from collections import defaultdict    
result = defaultdict(int)

for d in lst:
    result[d['name']] += int(d['value'])

2) convert the name-value pair to a dictionary within a list:

[{'name': name, 'value': value} for name, value in result.items()]
# [{'name': 'roc', 'value': 11}, {'name': 'Jay', 'value': 8}]

Or if you want the value as str type as commented by @Kevin:

[{'name': name, 'value': str(value)} for name, value in result.items()]
​# [{'name': 'roc', 'value': '11'}, {'name': 'Jay', 'value': '8'}]

Answer 2

This is a good use case for itertools.groupby.

from itertools import groupby
from operator import itemgetter

orig = [{'name':'Jay', 'value':'1'},
        {'name':'roc', 'value':'9'},
        {'name':'Jay', 'value':'7'},
        {'name':'roc', 'value':'2'}]

get_name = itemgetter('name')
result = [{'name': name, 'value': str(sum(int(d['value']) for d in dicts))}
           for name, dicts in groupby(sorted(orig, key=get_name), key=get_name)]

Breaking it down:

1. get_name is a function that given a dictionary, returns the value of its "name" key. I.e., get_name = lambda x: x['name'].

2. sorted returns the list of dictionaries sorted by the value of the "name" key.

3. groupby returns an iterator of (name, dicts) where dicts is a list (ok, generator) of the dicts that share name as the value of the "name" key. (Grouping only occurs for consecutive items with the same key value, hence the need to sort the list in the previous step.)

4. The result is a list of new dictionaries using the given name and the sum of all the related "value" elements.