python - Why is the time complexity of getting the length of a python list constant?

Question

This Python wiki says the time complexity of getting the length of a list is constant, or O(1):

lst = [4, 5, 6, 7]
len(lst)
# 4

However, I would expect it to be linear, or O(n), because all of the items in the list need to be counted. Is the length cached by the list? If not, how is the time complexity constant?

标签： pythonarraystime-complexity