What is a List Comprehension in Python?

Scenario 1: Let's say you have been given a list of numbers and you need to double each element in the list and store it in a new list. How would you do it?

list_numbers = [1,2,3,4]
list_double=[ ]
for element in list_numbers:
    list_double.append(2*element)
print(list_double)
---> [2, 4, 6, 8]

Is there a better way of writing this? In a more Pythonic way? Enter List Comprehension!!

list_double_with_comprehension = [2*element for element in list_numbers]
print(list_double_with_comprehension)
---> [2, 4, 6, 8]

Using List comprehension, a 3-line code gets converted to a 1-line code. In general, the format for a list comprehension requires at-least 2 pieces of information.

1. The expression which needs to be computed
2. Iterating through the iterable

When we created list_double_with_comprehension, (2*element) is the expression which is computed and "for element in list_numbers" is the way we iterate through the original list called "list_numbers".

Scenario 2: Let's assume we have been given a list of numbers and we want to create a new list which contains only even numbers. First, let's create a list of numbers

list_nos = [1,2,5,8,9,11,12,14,18,21]

Now let's create an empty list called list_even_nos. This is how we would usually write code

list_even_nos=[ ]
for element in list_nos:
    if element%2==0:
        list_even_nos.append(element)
print(list_even_nos)
--> [2, 8, 12, 14, 18]

A list comprehension way of writing would be as shown below

list_even_nos_comprehension = [element for element in list_nos if element%2==0]
print(list_even_nos_comprehension)
---> [2, 8, 12, 14, 18]

We mentioned above that we need at-least 2 pieces of information to write a list comprehension. Technically, there is a 3rd piece of information required if we are trying to check for a condition. In this case, we want to check if any element in the list_nos is even. Hence,we use if element%2==0. To reiterate, the format for a list comprehension requires at-least 2 pieces of information with the 3rd piece being optional.

1. The expression which needs to be computed
2. Iterating through the iterable
3. Condition to be checked (If required)

This covers broadly the ideas around list comprehension. More on this topic in the next post