List Comprehension

List comprehension is an elegant way to define and create lists based on existing lists in Python. It can be used to create a new list from an existing one by specifying instructions on how to transform its elements, and also allows you to filter out items from the original list.

Syntax#

The typical syntax for list comprehension in Python is as follows:

1new_list = [expression for item in iterable if condition]

Here is what each segment represents:

1. expression: Transforms the item before it is added to the new list.
2. iterable: Any object you can iterate over.
3. condition (Optional): An optional condition that must be met for an item to be included in the new list.

Example#

Let’s look at an example where we want to create a new list of squares for all numbers in an existing list.

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

Output:

[1, 4, 9, 16, 25]

Here, the expression is number**2, the iterable is numbers and there is no condition.

We can also apply a condition to filter items from the original list. For instance, we can create a list of squares for only the even numbers in our original list:

1numbers = [1, 2, 3, 4, 5]
2even_squares = [number**2 for number in numbers if number % 2 == 0]
3print(even_squares)

Output:

[4, 16]

Here, the condition is number % 2 == 0, which verifies if a number is even.