How does collections.defaultdict work?

Question

I\'ve read the examples in python docs, but still can\'t figure out what this method means. Can somebody help? Here are two examples from the python docs

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Answer 1

The behavior of defaultdict can be easily mimicked using dict.setdefault instead of d[key] in every call.

In other words, the code:

from collections import defaultdict

d = defaultdict(list)

print(d['key'])                        # empty list []
d['key'].append(1)                     # adding constant 1 to the list
print(d['key'])                        # list containing the constant [1]

is equivalent to:

d = dict()

print(d.setdefault('key', list()))     # empty list []
d.setdefault('key', list()).append(1)  # adding constant 1 to the list
print(d.setdefault('key', list()))     # list containing the constant [1]

The only difference is that, using defaultdict, the list constructor is called only once, and using dict.setdefault the list constructor is called more often (but the code may be rewriten to avoid this, if really needed).

Some may argue there is a performance consideration, but this topic is a minefield. This post shows there isn't a big performance gain in using defaultdict, for example.

IMO, defaultdict is a collection that adds more confusion than benefits to the code. Useless for me, but others may think different.