Nested List Comprehension in Python with If-Else Statements

Welcome back, Python enthusiasts! Today, we’re diving into a particularly nifty aspect of Python programming: nested list comprehension with if-else. This concept might sound a bit intimidating at first, especially if you’re just getting your feet wet in the world of Python. But worry not! By the end of this post, you’ll see just how powerful and user-friendly this tool can be for your coding projects.

What Is Nested List Comprehension?

Before we jump into the ‘if-else’ part, let’s quickly revisit what nested list comprehension is. Simply put, it’s a way to create lists within lists in a concise and efficient manner. It’s like crafting a matrix or a multi-dimensional array with just a single line of code!

Basic Structure

The basic structure of a nested list comprehension with if-else is as follows:

[<expression> if <condition> else <other_expression> for item in iterable]

The Power of ‘If-Else’ in List Comprehension

Adding if-else conditions to your list comprehensions opens up a world of possibilities. It allows you to introduce logic into your list creations, making your code not just shorter, but also much smarter.

Let’s See It in Action

Example 1: Creating a Chessboard

Let’s start with something fun – creating an 8×8 chessboard pattern. We’ll use ‘B’ for black squares and ‘W’ for white squares.

chessboard = [['B' if (i+j) % 2 == 0 else 'W' for j in range(8)] for i in range(8)]

Here, we’re using a nested list comprehension. The if-else condition (i+j) % 2 == 0 checks if the sum of the row and column indices is even. If it is, we place a ‘B’ (representing a black square); otherwise, a ‘W’ (a white square).

Example 2: Transforming a Matrix

Imagine you have a matrix, and you want to transform it so that all positive numbers are replaced with 1, and all non-positive numbers with 0.

original_matrix = [[-1, 2, -3], [4, -5, 6]]
transformed_matrix = [[1 if num > 0 else 0 for num in row] for row in original_matrix]

In this example, we’re iterating over each number in each row. The if-else condition checks if the number is greater than zero. Depending on this check, we replace it with 1 or 0.

Breaking Down the Process

  1. Outer Loop First: The outer loop runs first. In our chessboard example, it’s for i in range(8).
  2. Inner Loop Next: For each iteration of the outer loop, the inner loop completes its full run (for j in range(8) in the chessboard example).
  3. Condition Check: The if-else condition is evaluated for each item generated by the inner loop.

Why Use Nested List Comprehension with If-Else?

The beauty of using nested list comprehension with if-else in Python lies in its brevity and readability. It can turn complex loops and conditional logic into a single, readable line of code. This not only makes your code cleaner but often also enhances its performance.


Nested list comprehension with if-else in Python is a powerful tool that, when used correctly, can significantly simplify your code and make it more efficient. As with any programming concept, the best way to get comfortable with it is to practice. Try creating different patterns, transform matrices, or even attempt to solve some basic algorithms using this technique. Happy coding, and may your Python journey be as enjoyable as it is enlightening!

Read more about how to initialize nested list in python.