The 3 Step Process on Writing List Comprehensions Easily

At some point in your Python journey, you’ve come across for loops. While this is an excellent tool to be able to iterate over a list of numbers, strings, or whatever, we can write for loops in a much different way through list comprehensions.

Python list comprehensions provide a more concise way to create lists compared to the traditional for loop approach. They allow you to write cleaner, more readable code that can often be understood at a glance.

List comprehensions let you generate lists in just one line of code. They follow a simple structure that combines iteration, conditionals, and expression evaluation in a compact format.

Writing Python List Comprehensions: The 3 Step Process

So how do we go from a traditional for loop to using list comprehension syntax?

The way I learned it was by writing the loop out originally by using the append function. Then, I took the name of the variable and assigned it to an empty list on a new line:

Following this, I took the variable name that I am appending to my list (val, in this case) and placed it inside of the list comprehension:

Then I took my entire for loop statement and copied/pasted it in after the variable, val:

Writing List Comprehensions: including if and else

You can also include if and else in your list comprehension, but it gets slightly tricky depending what you’re using.

Suppose you’re only including if - your list comprehension will have the if statement in the behind the for loop syntax:

my_list = [val for val in range(10) if val % 2 == 0]

However, if we’re including if and else, this will be inserted ahead of the for loop syntax:

my_list = [
     val if val % 2 != 0 else val / 2 
     for val in range(10)
]

» I split this code up into multiple lines for readability

This block of code will append the value if it’s an odd number (if val % 2 != 0), otherwise it’ll divide it by 2. The for loop equivalent may look something like:

my_list = []
for val in range(10):
    if val % 2 != 0:
         my_list.append(val)
    else:
         my_list.append(val / 2)

Nested list comprehensions

You can also get a bit crafty with nested list comprehensions - I’ll save this for the next newsletter, but if we wanted to flatten a 2D list:

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

# Using nested loops
flattened = []
for row in matrix:
    for val in row:
        flattened.append(value)

# Using list comprehension
flattened = [val for row in matrix for val in row]

Related articles

• Python List Comprehension (with examples)

• Advanced Python: Lists-of-Lists and List Comprehensions

• Choosing the Best Iteration Technique for Optimized Python Code

• Python Dictionary Comprehension: A Simple Explanation