Unit 2 · List Methods

List Methods

Learn the complete set of built-in Python list methods to add, remove, search, sort, and rearrange elements — all performed directly on the original list.

Introduction

One of the biggest advantages of a Python list is that it is mutable — its contents can be changed after creation. To make working with this mutability easy, Python provides a rich collection of built-in list methods that let you add new items, remove unwanted ones, search for values, and reorder elements — all without writing manual loops.

This is different from string methods. Since strings are immutable, string methods always return a new string. List methods, on the other hand, usually modify the list in-place — directly changing the original list object rather than creating a copy.

Easy Definition

List methods are built-in functions attached to a list object that let you modify the list's content directly — adding, removing, searching, or reordering elements.

Real-Life Example

Adding a new item to a shopping cart → append()
Removing a completed task from a to-do list → remove()
Finding a student's rank in a marks list → index()
Arranging exam scores from highest to lowest → sort()

Examination Point of View

Difference between append() and extend() — very frequently asked.
Difference between remove() and pop().
Which methods return a value and which return None.
Difference between sort() (in-place) and sorted() (returns new list).
Tracing the final state of a list after a sequence of method calls.

List Methods — **Figure 2.17** — Modifying list contents dynamically using append, insert, pop, and sort.

Adding Elements
Removing Elements
Searching & Counting
Sorting & Reversing
sort() vs sorted()
Summary

Adding Elements

append(item): Adds a single item to the end of the list.
extend(iterable): Appends every item from another iterable (list, tuple, etc.) to the end.
insert(index, item): Inserts an item at a specific index, shifting later items right.

fruits = ["apple", "banana"]
fruits.append("orange")
print(fruits)          # ['apple', 'banana', 'orange']

fruits.insert(1, "mango")
print(fruits)          # ['apple', 'mango', 'banana', 'orange']

fruits.extend(["grapes", "pear"])
print(fruits)          # ['apple', 'mango', 'banana', 'orange', 'grapes', 'pear']

append() vs extend() — Very Important Distinction

append() adds its argument as a single element (even if that argument is itself a list). extend() unpacks the iterable and adds each element individually.

a = [1, 2]
a.append([3, 4])
print(a)     # [1, 2, [3, 4]]   ← nested list added as ONE item

b = [1, 2]
b.extend([3, 4])
print(b)     # [1, 2, 3, 4]     ← each item added separately

Removing Elements

pop(index): Removes and returns the item at the specified index. If index is not specified, it removes and returns the last item.
remove(item): Searches for and removes the first occurrence of the specified value (not an index).
clear(): Removes all items, leaving an empty list [].
del list[index]: A Python keyword (not a method) that also deletes an item by index.

nums = [10, 20, 30, 40]
last = nums.pop()
print(last)            # 40
print(nums)            # [10, 20, 30]

nums.remove(20)
print(nums)            # [10, 30]

pop() vs remove() — Frequently Confused

Feature	pop()	remove()
Works by	Index position	Value
Returns	The removed item	`None`
Error if invalid	`IndexError`	`ValueError`

Warning: remove() only deletes the first matching value. If the value doesn't exist at all, Python raises a ValueError.

Searching & Counting

index(item): Returns the index of the first occurrence of the item. Raises ValueError if the item is not found.
count(item): Returns the number of times the item appears in the list.

vals = [5, 10, 5, 15, 5]
print(vals.count(5))  # 3
print(vals.index(10)) # 1

Real-Life Examples

Situation	Method
Find a student's roll number position	`roll_list.index(105)`
Count how many times "Absent" appears	`status.count("Absent")`

Sorting & Reversing

sort(): Sorts the list elements in ascending order, in-place. Set reverse=True for descending order.
reverse(): Reverses the current order of elements in-place (not a sort — just flips the order).

scores = [85, 90, 75, 95]
scores.sort()
print(scores)          # [75, 85, 90, 95]

scores.sort(reverse=True)
print(scores)          # [95, 90, 85, 75]

Exam Point

reverse() does NOT sort the list — it simply flips the current order. Reversing an unsorted list like [3, 1, 2] gives [2, 1, 3], not [3, 2, 1].

sort() vs sorted() — Very Important Distinction

Students frequently confuse the sort() method with the sorted() built-in function. They behave very differently.

Feature	`list.sort()`	`sorted(list)`
Type	List method	Built-in function
Modifies original list?	Yes (in-place)	No — leaves it unchanged
Return value	`None`	A brand-new sorted list
Works on	Lists only	Any iterable (list, tuple, string...)

nums = [3, 1, 2]
result = nums.sort()
print(result)   # None  ← common trap in exams
print(nums)     # [1, 2, 3]

nums2 = [3, 1, 2]
new_list = sorted(nums2)
print(new_list) # [1, 2, 3]
print(nums2)    # [3, 1, 2]  ← original untouched

Common Mistakes

Writing my_list = my_list.sort() — this sets the list to None because sort() returns nothing.
Confusing append() (adds one item) with extend() (adds multiple items).
Calling remove(item) with a value that doesn't exist, causing a ValueError.
Assuming reverse() sorts the list — it only flips existing order.
Forgetting that most list methods return None, not the modified list.

Important Examination Points

List methods generally modify the list in-place and return None.
append() adds one element; extend() adds multiple elements individually.
pop() works by index and returns the removed value; remove() works by value and returns nothing.
sort() changes the original list; sorted() returns a new list.
index() raises an error if the value is absent; count() never raises an error (returns 0 instead).

Interview Questions

What is the difference between append() and extend()?
What is the difference between remove() and pop()?
What is the difference between sort() and sorted()?
What does pop() return if called without an argument?
What error does index() raise if the item is not found?

Practice MCQs

1. What is the output?

a = [1, 2]
a.append([3, 4])
print(a)

Answer: [1, 2, [3, 4]]

2. What is the output?

nums = [3, 1, 2]
print(nums.sort())

Answer: None

3. Which method removes an item by its value, not its index?

A) pop() B) del C) remove() D) clear()

Answer: C) remove()

4. What is the output?

vals = [5, 10, 5, 15, 5]
print(vals.count(5))

Answer: 3

Summary

Built-in list methods execute direct, in-place operations on a list: elements are added via append(), insert(), or extend(); deleted via pop() or remove(); searched via index() or count(); and reordered using sort() and reverse(). Unlike string methods, most list methods return None and change the original list directly rather than producing a copy.