Some brief example implementations of collections.defaultdict

defaultdict-examples.py

from collections import defaultdict

#------------------------
#   $COUNTING
#------------------------

names = ["tom", "dick", "harry", "harry", "dick", "tom", "tom", "dick", "harry"]

d = defaultdict(int)
for name in names:
    d[name] += 1

print(d)

#------------------------
#   $GROUPING-1
#------------------------

d = defaultdict(list)
for name in names:
    key = len(name)
    d[key].append(name)

print(d)

#------------------------
#   $GROUPING-2
#------------------------

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

print(d)