Python One Line Conditional Assignment

Problem: How to perform one-line if conditional assignments in Python?

Example: Say, you start with the following code.

x = 2
boo = True

You want to set the value of x to 42 if boo is True, and do nothing otherwise.

Let’s dive into the different ways to accomplish this in Python. We start with an overview:

Exercise: Run the code. Are all outputs the same?

Next, you’ll dive into each of those methods and boost your one-liner superpower!

Method 1: Ternary Operator

The most basic ternary operator x if c else y returns expression x if the Boolean expression c evaluates to True. Otherwise, if the expression c evaluates to False, the ternary operator returns the alternative expression y.

<OnTrue> if <Condition> else <OnFalse>
OperandDescription
<OnTrue>The return expression of the operator in case the condition evaluates to True
<Condition>The condition that determines whether to return the <On True> or the <On False> branch.
<OnFalse>The return expression of the operator in case the condition evaluates to False
Operands of the Ternary Operator

Let’s go back to our example problem! You want to set the value of x to 42 if boo is True, and do nothing otherwise. Here’s how to do this in a single line:

x = 42 if boo else x

While using the ternary operator works, you may wonder whether it’s possible to avoid the ...else x part for clarity of the code? In the next method, you’ll learn how!

If you need to improve your understanding of the ternary operator, watch the following video:

You can also read the related article:

Method 2: Single-Line If Statement

Like in the previous method, you want to set the value of x to 42 if boo is True, and do nothing otherwise. But you don’t want to have a redundant else branch. How to do this in Python?

The solution to skip the else part of the ternary operator is surprisingly simple—use a standard if statement without else branch and write it into a single line of code:

if boo: x = 42

To learn more about what you can pack into a single line, watch my tutorial video “If-Then-Else in One Line Python”:

Method 3: Ternary Tuple Syntax Hack

Python Ternary Tuple Syntax

A shorthand form of the ternary operator is the following tuple syntax.

Syntax: You can use the tuple syntax (x, y)[c] consisting of a tuple (x, y) and a condition c enclosed in a square bracket. Here’s a more intuitive way to represent this tuple syntax.

(<OnFalse>, <OnTrue>)[<Condition>]
OperandDescription
<OnTrue>The return expression of the operator in case the condition evaluates to True
<Condition>The condition that determines whether to return the <On True> or the <On False> branch.
<OnFalse>The return expression of the operator in case the condition evaluates to False
Tuple Syntax of the Ternary Operator

In fact, the order of the <OnFalse> and <OnTrue> operands is just flipped when compared to the basic ternary operator. First, you have the branch that’s returned if the condition does NOT hold. Second, you run the branch that’s returned if the condition holds.

x = (x, 42)[boo]

Clever! The condition boo holds so the return value passed into the x variable is the <OnTrue> branch 42.

Don’t worry if this confuses you—you’re not alone. You can clarify the tuple syntax once and for all by studying my detailed blog article.

Related Article: Python Ternary — Tuple Syntax Hack

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