How to create a dictionary whose elements are lists

Question

I want to create a dictionary whose values are lists. What is the best way to achieve this? Is there some way to express that I want a dictionary of empty lists, even if I don't know every key when I first create the list?

Answer 1

A simple way to do this is by using defaultdict.The defaultdict tool is a container in the collections class of Python. It's similar to the usual dictionary (dict) container, but the only difference is that a defaultdict will have a default value if that key has not been set yet. If you didn't use a defaultdict you'd have to check to see if that key exists, and if it doesn't, set it to what you want. You can refer to the following code to know more.

>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> a = ['1', '2']
>>> for i in a:
...   for j in range(int(i), int(i) + 2):
...     d[j].append(i)
...
>>> d
defaultdict(<type 'list'>, {1: ['1'], 2: ['1', '2'], 3: ['2']})
>>> d.items()
[(1, ['1']), (2, ['1', '2']), (3, ['2'])]