List Comprehensions

List comprehensions create lists concisely by applying an expression to each item in an iterable, with optional filtering based on a condition. They are often more readable and concise than traditional loops, making them a preferred method for list creation in Python.

A list comprehension includes an expression, a for clause, and optionally an if clause for filtering elements.

Syntax

[expression for item in iterable if condition]

• expression: The expression or transformation to apply to each item in the iterable.
• item: The current element from the iterable.
• iterable: The collection (such as a list, range, or string) to iterate over.
• condition (Optional): A filter that only includes elements satisfying the condition.

Example

Example 1: Basic List Comprehension

The following list comprehension generates a list of squares for the numbers 0 through 4:

squares = [x ** 2 for x in range(5)]
print(squares)

The output will be as follows :

[0, 1, 4, 9, 16]

In the above example, the expression x ** 2 is applied to each number in the range 0 to 4, generating the list of squared values.

Example 2: List Comprehension with a Condition

This example generates a list of even squares for the numbers 0 through 9:

even_squares = [x ** 2 for x in range(10) if x % 2 == 0]
print(even_squares)

The output will be as follows :

[0, 4, 16, 36, 64]

Here, the if x % 2 == 0 condition filters out odd numbers, and only the squares of even numbers are included in the result.

Example 3: List Comprehension with Multiple Loops

Multiple for loops can also be used in a list comprehension. The below example creates a list of pairs (tuples) from two lists :

pairs = [(x, y) for x in range(3) for y in range(3)]
print(pairs)

The output will be as follows :

[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

In this example, the comprehension generates all possible pairs (tuples) from the numbers 0, 1, and 2 by combining them.

Codebyte Example

Imagine we’re grading a class of students and want to create a list of students who passed. A list comprehension makes it easy to filter and label grades.

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