Python sum list of dicts by key with nested dicts

I would do this by performing a recursive merge on a recursively defined collections.defaultdict object.

from collections import defaultdict

def merge(d, new_d):
    for k, v in new_d.items():
        if isinstance(v, dict):
            merge(d[k], v)
        else: 
            d[k] = d.setdefault(k, 0) + v

# https://stackoverflow.com/a/19189356/4909087    
nested = lambda: defaultdict(nested)
d = nested()

for subd in data:
    merge(d, subd)

Using default_to_regular to convert it back, we have:

default_to_regular(d)
# {
#     "fruits": {
#         "apples": 17,
#         "oranges": 27,
#         "grapes": 44
#     },
#     "vegetables": {
#         "carrots": 21,
#         "beans": 12,
#         "peas": 9
#     },
#     "grains": 9,
#     "meats": 6
# }