Q.Example 9.2: Given the nested list list1 = [1,2,'a','c',[6,7,8],4,9], access the inner list with list1[4] and access an element of the nested list using two indices list1[i][j] (e.g., list1[4][1]).

What is a List? The Intuition

Imagine you're organising a small party. You have a guest list — a single piece of paper where you write down names, one after another, in a specific order. You can add a new friend at the end, cross someone out, or check who is third on the list. You might even have a mix of things: names, phone numbers, maybe a note like "bring cake".

A list in programming is exactly that: a single container that holds multiple items in a definite order. It's the most natural way to group related data when the order matters and you expect the collection to change — items get added, removed, or replaced.

The Precise Statement

A list is a built-in data type in Python (and many other languages) that stores a sequence of elements. In Python, a list is written as a comma-separated sequence of values enclosed in square brackets [ ].

guests = ["Aisha", "Ben", "Carlos", "Diana"]

Each element in a list can be of any type — numbers, strings, even other lists — and different elements in the same list can have different types.

mixed = [42, "hello", 3.14, True, [1, 2, 3]]

How Lists Work: The Core Mechanics

Indexing: Finding Your Spot

Every element in a list has an index — its position number. Python uses zero-based indexing: the first element is at index 0, the second at index 1, and so on.

guests = ["Aisha", "Ben", "Carlos", "Diana"]
print(guests[0])   # Aisha
print(guests[2])   # Carlos

You can also use negative indices to count from the end: guests[-1] is the last element ("Diana"), guests[-2] is the second-last ("Carlos").

Mutability: Changing the List

Unlike strings (which are immutable), you can directly change any element of a list:

guests[1] = "Beatrice"   # Replace "Ben" with "Beatrice"

You can add elements with .append() (adds to the end) or .insert() (adds at a specific position):

guests.append("Elena")          # ["Aisha", "Beatrice", "Carlos", "Diana", "Elena"]
guests.insert(0, "Zara")        # ["Zara", "Aisha", "Beatrice", "Carlos", "Diana", "Elena"]

Remove elements with .remove() (by value) or .pop() (by index):

guests.remove("Carlos")         # Removes the first occurrence of "Carlos"
last_one = guests.pop()         # Removes and returns the last element
second_one = guests.pop(1)      # Removes and returns the element at index 1

Slicing: Getting a Sub-List

You can extract a portion of a list using the slice notation [start:stop:step]:

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(numbers[2:6])    # [2, 3, 4, 5]   (index 2 up to, but not including, 6)
print(numbers[:4])     # [0, 1, 2, 3]   (from start up to index 4)
print(numbers[5:])     # [5, 6, 7, 8, 9] (from index 5 to end)
print(numbers[::2])    # [0, 2, 4, 6, 8] (every second element)

Why Lists Matter …