Lists

A list is an ordered collection of values under one name, where each item has a position that starts at 0.

Last updated: 30 Sep, 2026 · Python 3.14

One variable per ticket works for two tickets and fails for two hundred. A list holds any number of them.

lst = ['Mathematics', 'chemistry', 100, 200, 300, 204]
print(len(lst))

lst.append("Krish")
print(lst)
print(lst[3])
print(lst[6])
print(lst[1:])
print(lst[1:6])

6
['Mathematics', 'chemistry', 100, 200, 300, 204, 'Krish']
200
Krish
['chemistry', 100, 200, 300, 204, 'Krish']
['chemistry', 100, 200, 300, 204]

One list can hold text and numbers together. append put the new name at position 6, and the slice lst[1:6] stopped before position 6, so it left that name out. The video names its list lst; below, the list is named after what it holds, a list of tickets.

Syntax:

items = [first, second, third]
items[0]           # the first item
items.append(x)    # add x at the end

Creating a list and reading items

tickets = [
    "I was charged twice for one order",
    "My parcel has not arrived",
    "How do I change my password?",
]
print(len(tickets))
print(tickets[0])
print(tickets[-1])

3
I was charged twice for one order
How do I change my password?

Square brackets make a list, with commas between the items. Writing one item per line is only for reading; Python does not mind either way.

tickets[0] is the first item, because positions count from 0. tickets[-1] counts from the end, so it is always the last item however long the list gets.

Adding items and slicing

tickets.append("The parcel arrived but the box was crushed")
print(len(tickets))
print(tickets[:2])

4
['I was charged twice for one order', 'My parcel has not arrived']

append adds an item to the end and changes the list itself. tickets[:2] is a slice: a new list of the items from the start up to, but not including, position 2.

Asking for a position that is not there

print(tickets[4])

Traceback (most recent call last):
  File "main.py", line 1, in <module>
    print(tickets[4])
IndexError: list index out of range

Four items sit at positions 0 to 3. Asking for position 4 is an IndexError. When a list's length can vary, check len first, or loop over the list instead, which for loops covers.

append vs insert

Where lists show up in AI code

• A batch of texts to send to a model, one after another.
• The answers that come back, collected with append.
• The messages of a chat, which Lists of dictionaries builds.

Related

• Previous: if, elif and else
• Next: for loops
• Reference: More on Lists in the Python tutorial