List Methods — index(), count(), enumerate(), extend()

What you will learn

Search · Count · Iterate · Merge lists

This lesson teaches how to find data without changing it and combine multiple lists. Unlike File 02 — mutation methods, these tools are mainly used for searching, reporting, and iteration.

Tool	Role
index()	First position of a value
count()	How many times a value appears
enumerate()	Index + value while looping
extend()	Merge another list in place

Part of Collections lesson 3 in the module roadmap.

Learning roadmap

Memory formula: Find → Count → Loop → Merge

1. index() — find the first position

Syntax: listobj.index(value)

index() tells you where a value appears first.

lst = [10, "RS", 45.67, "Python", True]
print(lst.index("Python"))  # 3

If value does not exist

print(lst.index("Java"))
# ValueError: 'Java' is not in list

Situation	Result
Value exists	Returns first index
Value missing	ValueError

2. count() — count frequency

Syntax: listobj.count(value)

Instead of finding where, count() finds how many.

lst = [10, 20, 30, 10, 30, 20, 40, 10]
print(lst.count(10))   # 3
print(lst.count(100))  # 0

Character frequency

word = "MISSISSIPPI"
letters = list(word)
print(letters.count("I"))  # 4
print(letters.count("S"))  # 4

count() never throws an error. Missing values simply return 0.

index() vs count()

Finds the first position

Counts all occurrences

index()	count()
Only first match	Every match
Returns single integer	Returns total count
ValueError if missing	Returns 0 if missing

3. enumerate() — index + value together

Syntax: for index, value in enumerate(iterable):

Normally you only get the value:

for value in lst:
    print(value)

With enumerate() you get both:

for index, value in enumerate(lst):
    print(index, value)

lst = [10, 20, 30, 10]
for i, v in enumerate(lst):
    print(i, "→", v)

Output:

0 → 10
1 → 20
2 → 30
3 → 10

Finding ALL positions of a value

This is a very common interview question.

lst = [10, 20, 30, 10, 40, 10]
for i, v in enumerate(lst):
    if v == 10:
        print(i)
# 0, 3, 5

index()	enumerate()
Only first match	Every match
Returns single integer	Loop through all

Character frequency with enumerate()

Works on strings too — strings are iterable.

word = "MISSISSIPPI"
for i, ch in enumerate(word):
    if ch == "S":
        print(i, ch)
# 2 S, 3 S, 5 S, 6 S

enumerate() works on any iterable (list, tuple, string), but not integers.

enumerate(10)  # TypeError: 'int' object is not iterable

4. extend() — merge lists

Syntax: listobj1.extend(listobj2)

append() adds one object. extend() adds every element from another iterable.

Visual difference

[1,2].append([3,4]) → [1, 2, [3, 4]]

[1,2].extend([3,4]) → [1, 2, 3, 4]

lst1 = [10, 20, 30]
lst2 = ["A", "B"]
lst1.extend(lst2)
print(lst1)  # [10, 20, 30, 'A', 'B']
print(lst2)  # ['A', 'B'] — unchanged

Notice: lst1 changes; lst2 stays the same. extend() takes exactly one iterable argument.

extend() vs +

Mutates existing list

Creates another list

a = [1, 2]
b = [3, 4]
c = a + b
print(c)  # [1, 2, 3, 4]
# a is still [1, 2]

Feature	extend()	+
Creates new list?	No	Yes
Original changes?	Yes	No (unless you rebind)
Memory efficient?	Better for in-place	Slightly more memory

Chain three lists:

lst1 = [10, 20]
lst2 = ["A", "B"]
lst3 = [1.2, 2.3]
lst1.extend(lst2)
lst1.extend(lst3)
# Or: lst1 = lst1 + lst2 + lst3  # new list

Mini project — shopping cart merge

Imagine two family members add items separately.

cart_a = ["Apple", "Milk", "Bread"]
cart_b = ["Milk", "Eggs"]

cart_a.extend(cart_b)

print(cart_a.count("Milk"))
print(cart_a.index("Bread"))

Visual

Output:

2
2

You used extend + count + index in one practical example.

Interview revision sheet

Method	Purpose	Returns
index(x)	First position	Integer
count(x)	Total occurrences	Integer
enumerate()	Index + value pairs	Iterator
extend(lst)	Merge another iterable	None

Practice questions

Test yourself before File 04 — sort, reverse, copy, del:

1. Find the first index of "Python" in a mixed list.
2. Count how many times 20 appears.
3. Print all indices where value is 10 (use enumerate()).
4. Merge three lists using extend().
5. Merge three lists using the + operator.

Knowledge check

Q1: How do you find all indices of value 10?
Answer: Loop with enumerate(lst) and collect indices where v == 10.

Q2: lst1.extend(lst2, lst3) — valid?
Answer: No — TypeError. Call extend twice or use lst1 + lst2 + lst3.

Q3: index() vs count() when value is missing?
Answer: index raises ValueError; count returns 0.

Q4: Practice Q3 — why not use index() for every match?
Answer: index() only returns the first occurrence.