Python Lists Explained: Ordering, Indexing, Negative Indexing and len()

Lists Are Ordered

A list maintains the order of its objects.

Consider:

Output:

Python does not automatically arrange these numbers in numerical order.

The order in which we place the objects is retained.

So:

is different from:

Key Point: A Python list preserves the order in which its objects are placed. It does not automatically sort the objects.

Accessing Objects in a List

A very important property of a Python list is that we can access individual objects using their index.

Consider:

The positions are:

Therefore:

means the first object.

means the second object.

means the third object.

Key Point: A Python list starts its indexing from 0. Therefore, index 0 refers to the first object, index 1 refers to the second object, and so on.

Example: Accessing Individual Objects

Output:

We can also access the last two objects:

Output:

Negative Indexing

Python also allows us to access objects from the end of a list using negative indexes.

For example:

The indexes can be viewed as:

Therefore:

Output:

And

Output:

Quick Reference:
-1 → Last object    -2 → Second-last object    -3 → Third-last object

Finding the Number of Objects in a List

Python provides the len() function to find the number of objects in a list.

Example:

Output:

Therefore, the list contains five objects.

Another example:

Output:

Remember: If a list contains 5 objects, then len(list) returns 5.