Python - Group a list of dicts by key value, count separate key value as dict? [duplicate]

Question

I'm trying to group up the name key values here as a key for a dict value, and count the source value as a key for said parent key, and have the count value with as its value.

data = [
{'name':'Gill', 'source':'foo'},
{'name':'Gill', 'source':'foo'},
{'name':'Gill', 'source':'foo'},
{'name':'Gill', 'source':'bar'},
{'name':'Gill', 'source':'bar'},
{'name':'Gill', 'source':'bar'},
{'name':'Gill', 'source':'bar'},
{'name':'Gill', 'source':'bar'},
{'name':'Dave', 'source':'foo'},
{'name':'Dave', 'source':'foo'},
{'name':'Dave', 'source':'foo'},
{'name':'Dave', 'source':'foo'},
{'name':'Dave', 'source':'egg'},
{'name':'Dave', 'source':'egg'},
{'name':'Dave', 'source':'egg'},
{'name':'Dave', 'source':'egg'},
{'name':'Dave', 'source':'egg'},
{'name':'Dave', 'source':'egg'},
{'name':'Dave', 'source':'egg'}
]

How do I achieve the below output?

{'Gill': {'foo':3, 'bar':5}, 'Dave': {'foo':4, 'egg':7}}

I think it may be possible with a 1 liner...

Answer 1

Use itertools.groupby to group by names, then collections.Counter to count the source categories belonging to each name:

from collections import Counter
from itertools import groupby

f = lambda x: x['name']
dct = {k: Counter(d['source'] for d in g) for k, g in groupby(data, f)}
print(dct)
# {'Gill': Counter({'bar': 5, 'foo': 3}), 'Dave': Counter({'egg': 7, 'foo': 4})}