Lists | AI Pathshala

Definition

How do you keep the marks of fifty students in one place?

A list is an ordered, changeable collection of items, written in square brackets, such as [72, 85, 64]. Items are accessed by index from 0, can be of any type, and can be added, removed, changed and sorted.

In simple words

A list is a row of numbered boxes in one variable. marks[0] is the first box. marks.append(90) adds a box at the end. len(marks) counts the boxes. sum(marks) / len(marks) is the average. Unlike a string, you can change any box.

Why it matters

Datasets are collections: rows of data, lists of words, batches of images. Lists are Python's basic collection, and NumPy arrays and pandas columns, used throughout AI, behave in similar ways.

How it works

Operation	Example	Result for marks = [72, 85, 64]
Access	marks[1]	85
Change	marks[2] = 70	[72, 85, 70]
Add to end	marks.append(90)	[72, 85, 64, 90]
Remove a value	marks.remove(85)	[72, 64]
Sort	sorted(marks)	[64, 72, 85]
Length, total, largest	len, sum, max	3, 221, 85

In code

A marks list with average, topper and a sorted copy.

marks_list.py

names = ["Aarav", "Meera", "Kabir", "Sana"]
marks = [72, 91, 64, 85]

print("Average:", sum(marks) / len(marks))
best = max(marks)
print("Topper:", names[marks.index(best)], "with", best)

marks.append(78)
names.append("Ishaan")
print("After adding Ishaan:", marks)
print("Sorted copy:", sorted(marks))
print("Original order kept:", marks)
print("Highest three:", sorted(marks, reverse=True)[:3])

Output

Average: 78.0
Topper: Meera with 91
After adding Ishaan: [72, 91, 64, 85, 78]
Sorted copy: [64, 72, 78, 85, 91]
Original order kept: [72, 91, 64, 85, 78]
Highest three: [91, 85, 78]

Try it yourself

Predict the list

Start each time from nums = [3, 1, 2]. What does each operation give?

nums.append(5); nums
sorted(nums)
nums[0] = 9; nums
len(nums)
nums[-1]

Common mistakes

Writing x = x.sort(), which sets x to None.
Using an index that is too big: IndexError.
Copying with b = a and being surprised that changing b changes a. Use a.copy().

Check yourself

1. For a = [5, 6, 7], what is a[1]?

2. Which adds an item to the end?

3. What does sorted([3, 1, 2]) return?

Summary

Lists are ordered, changeable collections in square brackets.
append, remove, sort, len, sum and max cover most needs.
sorted() returns a new list; .sort() changes the original.