Finding average value in list of dictionaries based on another unique value

I have a list of dictionaries that have an “index” and a “weight” value. I want to average the dictionaries based on any unique index. So, with the below example, how can I find the average weight for any given index (e.g. 0, 1, 250, etc.)? There will be 8 total elements for each index.

values = [
{'index': 0, 'weight': 0.5},
{'index': 1, 'weight': 0.5},
{'index': 0, 'weight': 0.5},
{'index': 1, 'weight': 0.5},
{'index': 0, 'weight': 0.0},
{'index': 1, 'weight': 1.0},
{'index': 0, 'weight': 0.0},
{'index': 1, 'weight': 1.0},
{'index': 0, 'weight': 0.0},
{'index': 1, 'weight': 1.0},
{'index': 0, 'weight': 1.0},
{'index': 1, 'weight': 0.0},
{'index': 0, 'weight': 1.0},
{'index': 1, 'weight': 0.0},
{'index': 0, 'weight': 1.0},
{'index': 1, 'weight': 0.0}
]

I know I can get the average weight for the whole list using the following code, but I’m not sure how to do this per unique index:

print(sum(v['weight'] for v in values ) / len(values))

Answer

You need to group weights by index. defaultdict from the built-in collections module is useful here.

from collections import defaultdict
total = defaultdict(int)
cnts = defaultdict(int)
for d in values:
    # add weights
    total[d['index']] += d['weight']
    # count indexes
    cnts[d['index']] += 1
# find the mean
[{'index': k, 'mean weight': total[k]/cnts[k]} for k in total]
# [{'index': 0, 'mean weight': 0.5}, {'index': 1, 'mean weight': 0.5}]