List Indexing and Lookup

We can also access elements of lists individually. The bracket lookup expression, a new kind of expression, takes in a number for the index of an element to look up. For example:

>>> some_numbers = [46, 12, 3, 9]
>>> some_numbers[0]
46
>>> some_numbers[1]
12
>>> some_numbers[2]
3

What we might call in English the “first” element, 46, is at what we call index 0 in the list (using index 0 got us back 46). This is called 0-based indexing, and is a common method for indexing in programming languages. It’s worth highlighting that len function still reports that this list has 4 elements (it doesn’t say 3, counting from 0).

Trying to access an element at an index that’s too large raises an error:

>>> some_numbers[4]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range

Trying to access an element at a negative index has a very interesting behavior. When you access the element at index -1, it gives the last element in the list. So we can say that negative indices starting from -1 can be used to access the list starting from the end (like we used positive indices starting from 0 to access the list starting from the beginning of the list).

>>> some_numbers[-1]
9
>>> some_numbers[-2]
3
>>> some_numbers[-5]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range

NOTE: Indexing in lists is very similar to indexing in strings that we learned previously!