Source: 📖 Python Cookbook ch1.7 p12

Using defaultdict to map keys to multiple values

Manually mapping a key to multiple values seems trivial, but it can make for clunky, error-prone code—you have to define the key, assign a list/set to it, and then append or add values to that container.

Using defaultdict dict simplifies this process and makes for terser code.

from collections import defaultdict

d = defaultdict(list)
d['a'].append(1)
d['a'].append(2)
d['b'].append(4)

Default dict will automatically create a new empty list (or any provided callable) for each new key that is indexed, so the programmer doesn't have to.

See also:

• defaultdict automatically sets a default value to missing keys