sum list of dictionary values

Question

I have a list of dictionary in this form :

[
{'signal_8': 1, 'signal_1': 7, 'signal_10': 5, 'signal_5': 2, 'signal_2': 5, 'signal_6': 3, 'signal_4': 9, 'signal_3': 6, 'signal_9': 4, 'signal_7': 8}, 

{'signal_8': 1, 'signal_1': 7, 'signal_10': 5, 'signal_5': 2, 'signal_2': 5, 'signal_6': 3, 'signal_4': 9, 'signal_3': 6, 'signal_9': 4, 'signal_7': 8},

{'signal_8': 1, 'signal_1': 7, 'signal_10': 5, 'signal_5': 2, 'signal_2': 5, 'signal_6': 3, 'signal_4': 9, 'signal_3': 6, 'signal_9': 4, 'signal_7': 8},
]

and I want to sum the values in this by key for each element in the list like that :

    {
     'signal_8': 3,
     'signal_1': 21,
     'signal_10': 15,
     'signal_5': 6,
     'signal_2': 15,
     'signal_6': 9,
     'signal_4': 27,
     'signal_3': 18,
     'signal_9': 12,
     'signal_7': 24
    }

what I have tried is the following :

    result = {}
    sm = 0
    for elm in original_list:
        for k,v in elm.items():
            sm += v
            result[k] = sm
    print(result)

but it still doesn't work.

Answer 1

Similar to daveruinseverything's answer, I'd solve this with a Counter, but make use of its update method.

Let signals be your list of dicts.

>>> from collections import Counter
>>> c = Counter()
>>> for d in signals:
...     c.update(d)
... 
>>> c
Counter({'signal_4': 27, 'signal_7': 24, 'signal_1': 21, 'signal_3': 18, 'signal_10': 15, 'signal_2': 15, 'signal_9': 12, 'signal_6': 9, 'signal_5': 6, 'signal_8': 3})

For Op's sake, can you briefly describe what's happening here?

A Counter works similar to a dict, but its update method adds values to the values of pre-existing keys instead of overriding them.

Answer 2

The problem with your code is that you are summing sm and v no matter the key. Below you can find a reformatted version of your code that works. It simply adds the values from each element from the list to the result object:

from collections import defaultdict

result = defaultdict(int)

for elm in original_list:
    for k, v in elm.items():
        result[k] += v
print(result)

Or, with a one liner you can have:

result = {key: sum(e[key] for e in original_list) for key in original_list[0].keys()}