Python One Line For Loop With If

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This tutorial will teach you how to write one-line for loops in Python using the popular expert feature of list comprehension. After you’ve learned the basics of list comprehension, you’ll learn how to restrict list comprehensions so that you can write custom filters quickly and effectively.

Are you ready? Let’s roll up your sleeves and learn about list comprehension in Python!

List Comprehension Basics

The following section is based on my detailed article List Comprehension [Ultimate Guide]. Read the shorter version here or the longer version on the website—you decide!

This overview graphic shows how to use list comprehension statement to create Python lists programmatically:

List Comprehension

List comprehension is a compact way of creating lists. The simple formula is [expression + context].

  • Expression: What to do with each list element?
  • Context: What elements to select? The context consists of an arbitrary number of for and if statements.

The example [x for x in range(3)] creates the list [0, 1, 2].

A Simple Introduction to List Comprehension in Python

Have a look at the following interactive code snippet—can you figure out what’s printed to the shell? Go ahead and click “Run” to see what happens in the code:

Exercise: Run the code snippet and compare your guessed result with the actual one. Were you correct?

Now, that you know about the basics of list comprehension (expression + context!), let’s dive into a more advanced example where list comprehension is used for filtering by adding an if clause to the context part.

List Comprehension for Filtering (using If Clauses)

How to Filter a List in Python?

You can also modify the list comprehension statement by restricting the context with another if statement:

Problem: Say, we want to create a list of squared numbers—but you only consider even and ignore odd numbers.

Example: The multi-liner way would be the following.

squares = []

for i in range(10):
    if i%2==0:
# [0, 4, 16, 36, 64]

You create an empty list squares and successively add another square number starting from 0**2 and ending in 8**2—but only considering the even numbers 0, 2, 4, 6, 8. Thus, the result is the list [0, 4, 16, 36, 64].

Again, you can use list comprehension [i**2 for i in range(10) if i%2==0] with a restrictive if clause (in bold) in the context part to compress this in a single line of Python code:

print([i**2 for i in range(10) if i%2==0])
# [0, 4, 16, 36, 64]

This line accomplishes the same output with much less bits.

Related Article: Python One Line For Loop

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