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

When you say a = [...], a is effectively a pointer to a PyObject containing an array of pointers to PyObjects.

When you ask for a[2], the interpreter first follows the pointer to the list's PyObject, then adds 2 to the address of the array inside it, then returns that pointer. The same happens if you ask for a[0] or a[9999].

Basically, all Python objects are accessed by reference instead of by value, even integer literals like 2. There are just some tricks in the pointer system to keep this all efficient. And pointers have a known size, so they can be stored conveniently in C-style arrays.