Understanding defaultdict behavior with a list default

Question

I am trying to understand the following code:

from collections import defaultdict

dd_pair = defaultdict(lambda: [0, 0])
dd_pair[2][1] = 1                       # now dd_pair contains {2: [0,1]}

I understand defaultdict defaults to a list of two elements here. But how is the result ending up to be {2: [0,1]}

Answer 1

dd_pair[2] is returning a reference to a freshly created list with the value [0, 0] . Ignore the defaultdict itself, and see how it works with a list like that:

>>> mylist = [0, 0]
>>> mylist[1] = 1
>>> mylist
[0, 1]

The defaultdict is just dynamically creating a list like mylist each time you access a heretofore unaccessed key, storing it as the value of the requested key, and returning a reference to it. By adding [1] you do an index assignment to that list , and since it's the same list reference that was stored and returned, modifying one modifies the other.