Python List and List Comprehension

What is a List in Python?

A list in Python is a mutable, ordered collection of items, allowing duplicate elements. Lists can hold any data type — strings, integers, floats, other lists, etc.

Syntax:

my_list = [item1, item2, item3, ...]

Example: Create and Print a List

fruits = ["apple", "banana", "cherry"]
print(fruits)

Output:

['apple', 'banana', 'cherry']

Python List Operations

Loop Through a List

for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

Python List Comprehension

What is List Comprehension?

List comprehension provides a shorter, more readable syntax to create new lists from existing iterables.

Syntax:

new_list = [expression for item in iterable if condition]

Example 1: Basic List Comprehension

numbers = [1, 2, 3, 4, 5]
squares = [n ** 2 for n in numbers]
print(squares)

Output:

[1, 4, 9, 16, 25]

Example 2: With Condition

even_numbers = [n for n in numbers if n % 2 == 0]
print(even_numbers)

Output:

[2, 4]

Example 3: Using range() and List Comprehension

multiples_of_3 = [n for n in range(1, 21) if n % 3 == 0]
print(multiples_of_3)

Output:

[3, 6, 9, 12, 15, 18]

Example 4: Nested List Comprehension

matrix = [[row * col for col in range(1, 4)] for row in range(1, 4)]
print(matrix)

Output:

[[1, 2, 3], [2, 4, 6], [3, 6, 9]]

Real-World Example: Filter Words by Length

words = ["python", "is", "awesome", "fun"]
long_words = [word for word in words if len(word) > 3]
print(long_words)

Output:

['python', 'awesome']

Python List Methods with Examples

1. append()

Adds an element to the end of the list.

fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits)

Output:

['apple', 'banana', 'cherry']

2. insert(index, element)

Inserts an element at a specific index.

fruits.insert(1, "mango")
print(fruits)

Output:

['apple', 'mango', 'banana', 'cherry']

3. extend(iterable)

Adds all elements from another iterable (list, tuple, etc.).

fruits.extend(["orange", "grape"])
print(fruits)

Output:

['apple', 'mango', 'banana', 'cherry', 'orange', 'grape']

4. remove(value)

Removes the first occurrence of a value.

fruits.remove("banana")
print(fruits)

Output:

['apple', 'mango', 'cherry', 'orange', 'grape']

5. pop(index=-1)

Removes and returns the item at the given index (default is last).

fruits.pop()
print(fruits)

Output:

['apple', 'mango', 'cherry', 'orange']

6. clear()

Removes all elements from the list.

fruits.clear()
print(fruits)

Output:

[]

7. index(value)

Returns the index of the first matching element.

numbers = [10, 20, 30, 40]
print(numbers.index(30))

Output:

2

8. count(value)

Counts the number of times a value occurs.

marks = [90, 85, 90, 75]
print(marks.count(90))

Output:

2

9. sort(reverse=False)

Sorts the list in ascending (default) or descending order.

nums = [3, 1, 4, 2]
nums.sort()
print(nums)

Output:

[1, 2, 3, 4]

Descending sort:

nums.sort(reverse=True)
print(nums)

Output:

[4, 3, 2, 1]

10. reverse()

Reverses the order of elements in the list.

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

Output:

[3, 2, 1]

11. copy()

Returns a shallow copy of the list.

original = [1, 2, 3]
copy_list = original.copy()
print(copy_list)

Output:

[1, 2, 3]

Bonus: len() and Membership Operators

Although not list methods, these are commonly used with lists:

len()

print(len([1, 2, 3]))

Output:

3

in / not in

print(2 in [1, 2, 3])
print(5 not in [1, 2, 3])

Output:

True
True

Summary Table