Python List Comprehension
Create Lists

In Python, there's a short and neat way to create new lists. It’s called list comprehension. It looks a little different from what you've seen before, but don't worry — you'll understand it step by step.

What is List Comprehension?

List comprehension is a way to create a new list by doing something to each item in another list (or a range of numbers).

Instead of writing a loop and appending to a list, you can write it all in one line.

Normal Way vs List Comprehension

Let’s first see how we normally make a new list from numbers 1 to 5:

Using a for loop:

numbers = []
for i in range(1, 6):
    numbers.append(i)
print(numbers)

[1, 2, 3, 4, 5]

Now, using list comprehension:

numbers = [i for i in range(1, 6)]
print(numbers)

[1, 2, 3, 4, 5]

Explanation:

• i for i in range(1, 6) means: “for every number i from 1 to 5, take i and put it into the list.”
• The square brackets [ ] mean we’re making a list.

Example 2: Square Each Number

squares = [i*i for i in range(1, 6)]
print(squares)

[1, 4, 9, 16, 25]

Why? Because:

• When i = 1 → 1*1 = 1
• When i = 2 → 2*2 = 4
• ... and so on

Example 3: Double Only Even Numbers

doubles = [i*2 for i in range(1, 6) if i % 2 == 0]
print(doubles)

[4, 8]

Why?

• i % 2 == 0 checks if a number is even
• Only 2 and 4 are even between 1 to 5
• So we double only those: 2→4, 4→8

Example 4: Convert to Uppercase

words = ["apple", "banana", "cherry"]
uppercased = [word.upper() for word in words]
print(uppercased)

['APPLE', 'BANANA', 'CHERRY']

Why?

• word.upper() turns each word into uppercase.
• Each word from the list is transformed and put into a new list.

Example 5: Filter Words Starting with "b"

words = ["apple", "banana", "cherry", "blueberry"]
starts_with_b = [word for word in words if word.startswith("b")]
print(starts_with_b)

['banana', 'blueberry']

Why?

• word.startswith("b") checks if the word starts with the letter “b”.
• Only “banana” and “blueberry” match that rule.

Summary

List comprehension is just a shortcut to make lists. It’s clean and easy once you practice a few times.

Here's the basic pattern to remember:

[expression for item in collection if condition]

You can skip the if part if you don’t need to filter anything.