Modern Python Standard Library Cookbook

How to do it...

Proceed with the following steps for this recipe:

1. As we already know, defaultdict will create a default value by calling the provided callable for every missing key. We can provide the list constructor as a callable:

>>> from collections import defaultdict
>>> rd = defaultdict(list)

2. So, we insert keys into our multimap by using rd[k].append(v) instead of the usual rd[k] = v:

>>> for name, num in [('ichi', 1), ('one', 1), ('uno', 1), ('un', 1)]:
...   rd[num].append(name)
...
>>> rd
defaultdict(<class 'list'>, {1: ['ichi', 'one', 'uno', 'un']})