How to aggregate data from a python list to a dictionary?

Question

I have a list:

A = ['a-1', 'b-1', 'c-2', 'c-1', 'a-2']

What is the pythonic way (I don't want to use lots of nested for loops) of aggregating the data in a dictionary (or any other data structure) to get a result like:

{
'a': ['1', '2'],
'b': ['1'],
'c': ['1', '2'],
}

Answer 1

IIUC, here's one way:

from collections import defaultdict

result = defaultdict(list)
for i in A:
    a, b = i.split('-')
    result[a].append(b)

OUTPUT:

defaultdict(list, {'a': ['1', '2'], 'b': ['1'], 'c': ['2', '1']})

NOTE: you can also use setdefault :

result = {}
for i in A:
    a, b = i.split('-')
    result.setdefault(a, []).append(b)