How come list element lookup is O(1) in Python?

Question

Today in class, we learned that retrieving an element from a list is O(1) in Python. Why is this the case? Suppose I have a list of four items, for example:

Answer 1

Short answer: Python lists are arrays.

Long answer: The computer science term list usually means either a singly-linked list (as used in functional programming) or a doubly-linked list (as used in procedural programming). These data structures support O(1) insertion at either the head of the list (functionally) or at any position that does not need to be searched for (procedurally). A Python ``list'' has none of these characteristics. Instead it supports (amortized) O(1) appending at the end of the list (like a C++ std::vector or Java ArrayList). Python lists are really resizable arrays in CS terms.

The following comment from the Python documentation explains some of the performance characteristics of Python ``lists'':

It is also possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, lists are not efficient for this purpose. While appends and pops from the end of list are fast, doing inserts or pops from the beginning of a list is slow (because all of the other elements have to be shifted by one).