defaultdict

defaultdict

A dictionary subclass that returns a default value for missing keys instead of raising a KeyError. See associative array — Wikipedia.

Usage

from collections import defaultdict
d = defaultdict(factory)

The factory is any callable — list, int, lambda: None, etc. When a missing key is accessed, factory() is called to create the default value.

Common patterns

Factory	Default value	Useful for
list	[]	Grouping values by key
int	0	Counting occurrences
lambda: None	None	Optional lookups

Run

python defaultdict.py

Demonstrates missing key handling, grouping words by first letter, and counting word frequency.