Lesson 17 — defaultdict

Grouping made easy

A normal dict raises KeyError when you touch a missing key. defaultdict(list) creates an empty list automatically the first time a key is used, so you can append straight away. defaultdict(int) does the same with 0, which is handy for counting.

from collections import defaultdict
by_len = defaultdict(list)
by_len[3].append("cat")

Why it matters: grouping records by a field (city, date, category) is very common, and this removes the "is the key there yet?" checks.