Python Comprehensions

What Is A Comprehension?

A comprehension is a compact way to build a collection from another iterable. It combines a loop, an expression, and sometimes a condition into one readable line.

numbers = [1, 2, 3, 4]
squares = [number * number for number in numbers]

print(squares)

Output:

[1, 4, 9, 16]

This is equivalent to creating an empty list and appending inside a loop, but shorter.

List Comprehensions

The basic pattern is:

[expression for item in iterable]

The expression decides what goes into the new list.

names = ["asha", "riya", "kabir"]
capitalized = [name.title() for name in names]

Filtering Values

Add an if condition at the end to keep only some values.

numbers = [1, 2, 3, 4, 5, 6]
evens = [number for number in numbers if number % 2 == 0]

This produces [2, 4, 6].

Set And Dictionary Comprehensions

Set comprehensions use braces and keep unique results.

words = ["hi", "hello", "hi"]
lengths = {len(word) for word in words}

Dictionary comprehensions create key-value pairs.

scores = {"Asha": 90, "Riya": 84}
passed = {name: score >= 35 for name, score in scores.items()}

Conditional Expressions

An if at the end filters items. A conditional expression before the for chooses between two output values.

labels = ["even" if number % 2 == 0 else "odd" for number in numbers]

This expression produces one label for every number.

When To Avoid Comprehensions

Comprehensions are great when the transformation is simple. If the logic needs several steps, nested conditions, or side effects, a normal loop is usually clearer.

Readable code matters more than short code.

Common Mistakes

A common mistake is placing the filter condition in the wrong location.

[number if number > 0 for number in numbers]

That is invalid because the conditional expression is incomplete. Use a filter at the end:

[number for number in numbers if number > 0]

Another mistake is using comprehensions only for side effects such as printing. Use a normal loop for that.

Quick Summary

• A comprehension builds a collection from an iterable.
• List comprehensions use square brackets.
• Set and dictionary comprehensions use braces.
• An if at the end filters items.
• A conditional expression before for chooses output values.
• Prefer normal loops when logic becomes hard to read.

Practice Quiz

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