Basic one: Aggregating from a dictionary

[buran]

it can probably be done with NumPy too
but there is defaultdict from collections module

from collections import defaultdict
import json
spam = defaultdict(list)
files = {'fle1.txt': 'John', 'file2.txt': 'James', 'file3.txt': 'John'}
for fname, owner in files.items():
    spam[owner].append(fname)
print(spam)

# dump as json
eggs = json.dumps(spam)
print(eggs)

Output:
defaultdict(<class 'list'>, {'John': ['fle1.txt', 'file3.txt'], 'James': ['file2.txt']})
{"John": ["fle1.txt", "file3.txt"], "James": ["file2.txt"]}