After this lesson, you will be able to:

• Create lists and recognize their ordered, mutable nature.
• Retrieve elements with zero-based and negative indexes.
• Replace an existing element by its index.
• Use len() to count elements and understand valid index boundaries.

Python lists hold ordered values in square brackets. If you know arrays from JavaScript or Java, the idea is familiar—but Python lists can grow and can contain different kinds of values. We will focus on simple lists of related values here.

services = ["api", "worker", "database"]

An index starts at 0, so services[0] is the first value. Negative indexes count from the end: services[-1] is the last value, regardless of list length.

Lists are mutable: assigning to an existing position changes that element. The assignment must use a valid index. For a list with three elements, the positive indexes are 0, 1, and 2; 3 is out of range.

Use len(services) to find the number of elements. The last positive index of a non-empty list is len(services) - 1. An empty list has length zero and no first or last element.

1. Create a list and access an item

A list literal preserves the order of its elements.

queue_names = ["urgent", "normal", "low"]
print(queue_names[0])
print(queue_names[2])

Output:

urgent
low

The first position is 0, not 1.

2. Read from the end

Negative indexing avoids assuming a fixed list length.

releases = ["v1.2", "v1.3", "v1.4"]
print(releases[-1])
print(releases[-2])

Output:

v1.4
v1.3

The -1 index is the last item; -2 is the one before it.

3. Update an existing position

Changing a list element does not require creating a new list.

channels = ["dev", "test", "prod"]
channels[1] = "staging"
print(channels)

Output:

['dev', 'staging', 'prod']

Only the item at index 1 changes.

4. Use list length safely

len() counts values, not the last index.

active_regions = ["eu", "us"]
count = len(active_regions)
print(f"Regions: {count}")
print(f"Last positive index: {count - 1}")

Output:

Regions: 2
Last positive index: 1

1. Starting at index 1: Python lists are zero-based.
2. Using items[len(items)]: that position is beyond the last element; use -1 or len(items) - 1 for a non-empty list.
3. Assuming every list has a last item: indexing an empty list raises IndexError.
4. Confusing assignment with insertion: items[1] = value replaces an existing item; it does not insert a new one.