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d = {}
for name in names:
    key = len(name)
    if key not in d:
        d[key] = []
    d[key].append(name)


# slightly better
d = {}
for name in names:
    key = len(name)
    d.setdefault(key, []).append(name)


# way better
from collections import defaultdict
d = defaultdict(list)
for name in names:
    key = len(name)
    d[key].append(name)