I often used collections.defaultdict to be able to append an element to d[key] without having to initialize it first to [] (benefit: you don't need to do: if key not in d: d[key] = []):
import collections, random
d = collections.defaultdict(list)
for i in range(100):
j = random.randint(0,20)
d[j].append(i) # if d[j] does not exist yet, initialize it to [], so we can use append directly
Now I realize we can simply use a normal dict and setdefault:
import random
d = {}
for i in range(100):
j = random.randint(0,20)
d.setdefault(j, []).append(i)
Question: when using a dict whose values are lists, is there a good reason to use a collections.defaultdict instead of the second method (using a simple dict and setdefault), or are they purely equivalent?
collections.defaultdict is generally more performant, it is optimised exactly for this task and C-implemented. However, you should use dict.setdefault if you want accessing an absent key in your resulting dictionary to result in a KeyError rather than inserting an empty list. This is the most important practical difference.