python one line if statement variable assignment

How to Write the Python if Statement in one Line

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Have you ever heard of writing a Python if statement in a single line? Here, we explore multiple ways to do exactly that, including using conditional expressions in Python.

The if statement is one of the most fundamental statements in Python. In this article, we learn how to write the Python if in one line.

The if is a key piece in writing Python code. It allows developers to control the flow and logic of their code based on information received at runtime. However, many Python developers do not know they may reduce the length and complexity of their if statements by writing them in a single line.

For this article, we assume you’re somewhat familiar with Python conditions and comparisons. If not, don’t worry! Our Python Basics Course will get you up to speed in no time. This course is included in the Python Basics Track , a full-fledged Python learning track designed for complete beginners.

We start with a recap on how Python if statements work. Then, we explore some examples of how to write if statements in a single line. Let’s get started!

How the if Statement Works in Python

Let’s start with the basics. An if statement in Python is used to determine whether a condition is True or False . This information can then be used to perform specific actions in the code, essentially controlling its logic during execution.

The structure of the basic if statement is as follows:

The <expression> is the code that evaluates to either True or False . If this code evaluates to True, then the code below (represented by <perform_action> ) executes.

Python uses whitespaces to indicate which lines are controlled by the if statement. The if statement controls all indented lines below it. Typically, the indentation is set to four spaces (read this post if you’re having trouble with the indentation ).

As a simple example, the code below prints a message if and only if the current weather is sunny:

The if statement in Python has two optional components: the elif statement, which executes only if the preceding if/elif statements are False ; and the else statement, which executes only if all of the preceding if/elif statements are False. While we may have as many elif statements as we want, we may only have a single else statement at the very end of the code block.

Here’s the basic structure:

Here’s how our previous example looks after adding elif and else statements. Change the value of the weather variable to see a different message printed:

How to Write a Python if in one Line

Writing an if statement in Python (along with the optional elif and else statements) uses a lot of whitespaces. Some people may find it confusing or tiresome to follow each statement and its corresponding indented lines.

To overcome this, there is a trick many Python developers often overlook: write an if statement in a single line !

Though not the standard, Python does allow us to write an if statement and its associated action in the same line. Here’s the basic structure:

As you can see, not much has changed. We simply need to “pull” the indented line <perform_action> up to the right of the colon character ( : ). It’s that simple!

Let’s check it with a real example. The code below works as it did previously despite the if statement being in a single line. Test it out and see for yourself:

Writing a Python if Statement With Multiple Actions in one Line

That’s all well and good, but what if my if statement has multiple actions under its control? When using the standard indentation, we separate different actions in multiple indented lines as the structure below shows:

Can we do this in a single line? The surprising answer is yes! We use semicolons to separate each action in the same line as if placed in different lines.

Here’s how the structure looks:

And an example of this functionality:

Have you noticed how each call to the print() function appears in its own line? This indicates we have successfully executed multiple actions from a single line. Nice!

By the way, interested in learning more about the print() function? We have an article on the ins and outs of the print() function .

Writing a Full Python if/elif/else Block Using Single Lines

You may have seen this coming, but we can even write elif and else statements each in a single line. To do so, we use the same syntax as writing an if statement in a single line.

Here’s the general structure:

Looks simple, right? Depending on the content of your expressions and actions, you may find this structure easier to read and understand compared to the indented blocks.

Here’s our previous example of a full if/elif/else block, rewritten as single lines:

Using Python Conditional Expressions to Write an if/else Block in one Line

There’s still a final trick to writing a Python if in one line. Conditional expressions in Python (also known as Python ternary operators) can run an if/else block in a single line.

A conditional expression is even more compact! Remember it took at least two lines to write a block containing both if and else statements in our last example.

In contrast, here’s how a conditional expression is structured:

The syntax is somewhat harder to follow at first, but the basic idea is that <expression> is a test. If the test evaluates to True , then <value_if_true> is the result. Otherwise, the expression results in <value_if_false> .

As you can see, conditional expressions always evaluate to a single value in the end. They are not complete replacements for an if/elif/else block. In fact, we cannot have elif statements in them at all. However, they’re most helpful when determining a single value depending on a single condition.

Take a look at the code below, which determines the value of is_baby depending on whether or not the age is below five:

This is the exact use case for a conditional expression! Here’s how we rewrite this if/else block in a single line:

Much simpler!

Go Even Further With Python!

We hope you now know many ways to write a Python if in one line. We’ve reached the end of the article, but don’t stop practicing now!

If you do not know where to go next, read this post on how to get beyond the basics in Python . If you’d rather get technical, we have a post on the best code editors and IDEs for Python . Remember to keep improving!

How to use python if else in one line with examples

December 31, 2023

Python Ternary Operator , Python

How do I write a simple python if else in one line? What are ternary operator in Python? Can we use one liner for complex if and else statements?

In this tutorial I will share different examples to help you understand and learn about usage of ternary operator in one liner if and else condition with Python. Conditional expressions (sometimes called a “ ternary operator ”) have the lowest priority of all Python operations. Programmers coming to Python from C, C++, or Perl sometimes miss the so-called ternary operator ?:. It’s most often used for avoiding a few lines of code and a temporary variable for simple decisions.

I will not go into details of generic ternary operator as this is used across Python for loops and control flow statements. Here we will concentrate on learning python if else in one line using ternary operator

Python if else in one line

The general syntax of single if and else statement in Python is:

Now if we wish to write this in one line using ternary operator, the syntax would be:

In this syntax, first of all the else condition is evaluated.

If condition returns True then value_when_true is returned
If condition returns False then value_when_false is returned

Similarly if you had a variable assigned in the general if else block based on the condition

The same can be written in single line:

Here as well, first of all the condition is evaluated.

if condition returns True then true-expr is assigned to value object
if condition returns False then false-expr is assigned to value object

For simple cases like this, I find it very nice to be able to express that logic in one line instead of four. Remember, as a coder, you spend much more time reading code than writing it, so Python's conciseness is invaluable.

Some important points to remember:

You can use a ternary expression in Python, but only for expressions , not for statements
You cannot use Python if..elif..else block in one line.
The name " ternary " means there are just 3 parts to the operator: condition , then , and else .
Although there are hacks to modify if..elif..else block into if..else block and then use it in single line but that can be complex depending upon conditions and should be avoided
With if-else blocks , only one of the expressions will be executed.
While it may be tempting to always use ternary expressions to condense your code, realise that you may sacrifice readability if the condition as well as the true and false expressions are very complex.

Python Script Example

This is a simple script where we use comparison operator in our if condition

First collect user input in the form of integer and store this value into b
If b is greater than or equal to 0 then return " positive " which will be True condition
If b returns False i.e. above condition was not success then return " negative "
The final returned value i.e. either " positive " or " negative " is stored in object a
Lastly print the value of value a

The multi-line form of this code would be:

Python if..elif..else in one line

Now as I told this earlier, it is not possible to use if..elif..else block in one line using ternary expressions. Although we can hack our way into this but make sure the maximum allowed length of a line in Python is 79 as per PEP-8 Guidelines

We have this if..elif..else block where we return expression based on the condition check:

We can write this if..elif..else block in one-line using this syntax:

In this syntax,

First of all condition2 is evaluated, if return True then expr2 is returned
If condition2 returns False then condition1 is evaluated, if return True then expr1 is returned
If condition1 also returns False then else is executed and expr is returned

As you see, it was easier if we read this in multi-line if..elif..else block while the same becomes hard to understand for beginners.

We can add multiple if else block in this syntax, but we must also adhere to PEP-8 guidelines

Python Script Example-1

In this sample script we collect an integer value from end user and store it in " b ". The order of execution would be:

If the value of b is less than 0 then " neg " is returned
If the value of b is greater than 0 then " pos " is returned.
If both the condition return False , then " zero " is returned

The multi-line form of the code would be:

Output(when if condition is True )

Output(when if condition is False and elif condition is True )

Output(when both if and elif condition are False )

Python script Example-2

We will add some more else blocks in this sample script, the order of the check would be in below sequence :

Collect user input for value b which will be converted to integer type
If value of b is equal to 100 then return " equal to 100 ", If this returns False then next if else condition would be executed
If value of b is equal to 50 then return " equal to 50 ", If this returns False then next if else condition would be executed
If value of b is equal to 40 then return " equal to 40 ", If this returns False then next if else condition would be executed
If value of b is greater than 100 then return " greater than 100 ", If this returns False then next go to else block
Lastly if all the condition return False then return " less than hundred "

The multi-line form of this example would be:

Python nested if..else in one line

We can also use ternary expression to define nested if..else block on one line with Python.

If you have a multi-line code using nested if else block , something like this:

The one line syntax to use this nested if else block in Python would be:

Here, we have added nested if..elif..else inside the else block using ternary expression. The sequence of the check in the following order

If condition1 returns True then expr1 is returned, if it returns False then next condition is checked
If condition-m returns True then expr-m is returned, if it returns False then else block with nested if..elif..else is checked
If condition3 returns True then expr3 is returned, if it returns False then next condition inside the nested block is returned
If condition-n returns True then expr-n is returned, if it returns False then expr5 is returned from the else condition

In this example I am using nested if else inside the else block of our one liner. The order of execution will be in the provided sequence:

First of all collect integer value of b from the end user
If the value of b is equal to 100 then the if condition returns True and " equal to 100 " is returned
If the value of b is equal to 50 then the elif condition returns True and " equal to 50 " is returned
If both if and elif condition returns False then the else block is executed where we have nested if and else condition
Inside the else block , if b is greater than 100 then it returns " greater than 100 " and if it returns False then " less than 100 " is returned

In this tutorial we learned about usage of ternary operator in if else statement to be able to use it in one line. Although Python does not allow if..elif..else statement in one line but we can still break it into if else and then use it in single line form. Similarly we can also use nested if with ternary operator in single line. I shared multiple examples to help you understand the concept of ternary operator with if and else statement of Python programming language

Lastly I hope this tutorial guide on python if else one line was helpful. So, let me know your suggestions and feedback using the comment section.

Deepak Prasad

He is the founder of GoLinuxCloud and brings over a decade of expertise in Linux, Python, Go, Laravel, DevOps, Kubernetes, Git, Shell scripting, OpenShift, AWS, Networking, and Security. With extensive experience, he excels in various domains, from development to DevOps, Networking, and Security, ensuring robust and efficient solutions for diverse projects. You can connect with him on his LinkedIn profile.

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Mastering Python’s One-Line If Statements – A concise guide to conditional expressions

Introduction.

Conditional statements play a crucial role in programming languages like Python as they allow the execution of specific code based on certain conditions. Python provides several ways to write conditional statements, including the use of one-line if statements. In this blog post, we will explore the concept of Python’s one-line if statements, their syntax, advantages, practical applications, tips for writing effective conditional expressions, and more.

python one line if statement variable assignment

Understanding Python’s Conditional Expressions

Python’s one-line if statements, also known as conditional expressions, provide a concise way to write conditional statements in a single line of code. Let’s delve into the syntax and structure of these statements.

Syntax of one-line if statements

The basic structure of a one-line if statement in Python follows the pattern: result_if_true if condition else result_if_false This means that if the given condition evaluates to True, the expression before the “if” keyword is executed; otherwise, the expression after the “else” keyword is executed. Using a single line to write conditional statements allows for more compact and readable code, making it easier to understand and maintain.

The ternary operator in Python

Python’s one-line if statements are often referred to as the “ternary operator” due to their similar behavior to the ternary operator found in other programming languages. The ternary operator is used to assign a value to a variable based on a specific condition. The syntax for the ternary operator in Python is: variable = value_if_true if condition else value_if_false This allows you to perform a conditional assignment in a single line without the need for separate if-else blocks.

Advantages of One-Line If Statements

There are several advantages to using one-line if statements in Python. Let’s explore two significant benefits:

Reduced code length and increased readability

One-line if statements condense multiple lines of code into a single line, leading to shorter and more concise code. This reduction in code length improves readability by eliminating unnecessary clutter and reducing the chances of introducing errors. For example, consider the following traditional if-else block: if x > 5: result = "greater than 5" else: result = "less than or equal to 5" With a one-line if statement, the same logic can be expressed as: result = "greater than 5" if x > 5 else "less than or equal to 5" The one-line version provides the same functionality with fewer lines of code, making it easier to understand the conditional logic at a glance.

Conciseness in expressing conditions

One-line if statements allow you to express conditions in a straightforward and concise manner. This can result in code that is easier to understand and maintain, especially when dealing with simple conditional expressions. For instance, consider the following code using a traditional if-else statement: if condition: result = "Condition is True" else: result = "Condition is False" With a one-line if statement, the same condition can be expressed as: result = "Condition is True" if condition else "Condition is False" By writing the condition on a single line, the code becomes more compact, and the intention is clearer and more concise.

Using One-Line If Statements in Practice

To gain a better understanding of how to use one-line if statements, let’s explore some practical examples.

Simple examples for conditional expressions

One-line if statements are commonly used to evaluate expressions based on a condition or assign values conditionally. For example, let’s say we have a variable called “age,” and we want to check if the person is of legal drinking age: drink = "Yes" if age >= 21 else "No" In this case, if the “age” variable is greater than or equal to 21, the value assigned to “drink” will be “Yes”; otherwise, it will be “No.” Similarly, one-line if statements can be used to assign a value based on other conditions, such as checking if a number is positive or negative: message = "Positive" if num > 0 else "Negative" If “num” is greater than 0, the value assigned to “message” will be “Positive”; otherwise, it will be “Negative.”

Chaining multiple conditions in a single line

One-line if statements also facilitate chaining multiple conditions together in a single line. This can be achieved by nesting multiple conditional expressions or using logical operators. For example, let’s say we have three variables, “x,” “y,” and “z,” and we want to assign different values based on their relative sizes: result = "x is greater" if x > y > z else "y is greater" if y > x > z else "z is greater" In this case, the condition is evaluated sequentially from left to right. If “x” is greater than “y” and “y” is greater than “z,” the value assigned to “result” will be “x is greater.” If “y” is greater than “x” and “x” is greater than “z,” the value will be “y is greater.” Otherwise, if none of these conditions are met, the value will be “z is greater.” Using logical operators such as “and” and “or” can also help simplify complex conditional expressions.

Tips for Writing Effective One-Line If Statements

To write effective one-line if statements, consider the following tips:

Proper formatting and readability

Although one-line if statements are meant to condense code, it is essential to maintain proper formatting and readability. Use spaces to separate the different components of the conditional expression to enhance clarity. For example: result = "Yes" if condition1 and condition2 else "No" By appropriately formatting the conditional expression, it becomes easier to identify the conditions and their corresponding results.

Avoiding complex expressions

While one-line if statements are useful for simple conditional expressions, avoid using them for complex conditions involving multiple nested conditions or complex logic. In such cases, it is often more readable and maintainable to opt for traditional if-else blocks.

Considering potential errors when using conditional expressions

When using one-line if statements, be mindful of potential errors that can arise due to incorrect or missing syntax. Ensure correct placement of the “if,” “else,” and the “:” symbol, as well as proper alignment of expressions.

Python’s one-line if statements provide a concise and efficient way to express conditional logic. By condensing multiple lines of code into a single line, one-line if statements reduce code length and improve readability. They are particularly useful for simple conditional expressions, allowing you to assign values conditionally or evaluate expressions based on specific conditions easily. In this blog post, we have explored the syntax and structure of one-line if statements, discussed their advantages, provided examples of their practical application, and shared tips for writing effective conditional expressions. By incorporating one-line if statements into your Python code, you can enhance readability and maintainability, ultimately resulting in more efficient and elegant code. For further practice and implementation, try applying one-line if statements to different scenarios in your Python projects. The more familiar you become with their usage, the more you can leverage their benefits and write cleaner, more concise code.

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Python Conditional Assignment

When you want to assign a value to a variable based on some condition, like if the condition is true then assign a value to the variable, else assign some other value to the variable, then you can use the conditional assignment operator.

In this tutorial, we will look at different ways to assign values to a variable based on some condition.

1. Using Ternary Operator

The ternary operator is very special operator in Python, it is used to assign a value to a variable based on some condition.

It goes like this:

Here, the value of variable will be value_if_true if the condition is true, else it will be value_if_false .

Let's see a code snippet to understand it better.

You can see we have conditionally assigned a value to variable c based on the condition a > b .

2. Using if-else statement

if-else statements are the core part of any programming language, they are used to execute a block of code based on some condition.

Using an if-else statement, we can assign a value to a variable based on the condition we provide.

Here is an example of replacing the above code snippet with the if-else statement.

3. Using Logical Short Circuit Evaluation

Logical short circuit evaluation is another way using which you can assign a value to a variable conditionally.

The format of logical short circuit evaluation is:

It looks similar to ternary operator, but it is not. Here the condition and value_if_true performs logical AND operation, if both are true then the value of variable will be value_if_true , or else it will be value_if_false .

Let's see an example:

But if we make condition True but value_if_true False (or 0 or None), then the value of variable will be value_if_false .

So, you can see that the value of c is 20 even though the condition a < b is True .

So, you should be careful while using logical short circuit evaluation.

While working with lists , we often need to check if a list is empty or not, and if it is empty then we need to assign some default value to it.

Let's see how we can do it using conditional assignment.

Here, we have assigned a default value to my_list if it is empty.

Assign a value to a variable conditionally based on the presence of an element in a list.

Now you know 3 different ways to assign a value to a variable conditionally. Any of these methods can be used to assign a value when there is a condition.

The cleanest and fastest way to conditional value assignment is the ternary operator .

if-else statement is recommended to use when you have to execute a block of code based on some condition.

Happy coding! 😊

One-Lining It in Python: Mastering Conditional Expressions and Concise If Blocks

Conditional Expressions (Ternary Operator):

This method uses a compact format with three parts:

Here's how it works:

condition : This is a Boolean expression that evaluates to True or False.
expression_if_true : This expression is evaluated and returned if the condition is True.
expression_if_false : This expression is evaluated and returned if the condition is False.

One-Line If Blocks:

While not technically one line, you can achieve a similar concise look by placing the if statement and its action on one line and using indentation for the else block:

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Python If-Else Statement in One Line - Ternary Operator Explained

Single-line conditionals in python here’s when to and when not to use them.

Python isn’t the fastest programming language out there, but boy is it readable and efficient to write. Everyone knows what conditional statements are, but did you know you can write if statements in one line of Python code? As it turns out you can, and you’ll learn all about it today.

After reading, you’ll know everything about Python’s If Else statements in one line. You’ll understand when to use them, and when it’s best to avoid them and stick to conventional conditional statements.

Don’t feel like reading? Watch my video instead:

Want to get hired as a data scientist? Running a data science blog might help:

Can Blogging About Data Science Really Get You Hired as a Data Scientist?

What’s Wrong With the Normal If Statement?

Absolutely nothing. Splitting conditional statements into multiple lines of code has been a convention for ages. Most programming languages require the usage of curly brackets, and hence the single line if statements are not an option. Other languages allow writing only simple conditionals in a single line.

And then there’s Python. Before diving into If Else statements in one line, let’s first make a short recap on regular conditionals.

For example, you can check if a condition is true with the following syntax:

The variable age is less than 18 in this case, so Go home. is printed to the console. You can spice things up by adding an else condition that gets evaluated if the first condition is False :

This time age is greater than 18, so Welcome! gets printed to the console. Finally, you can add one or multiple elif conditions. These are used to capture the in-between cases. For example, you can print something entirely different if age is between 16 (included) and 18 (excluded):

The variable age is 17, which means the condition under elif is True , hence Not sure... is printed to the console.

Pretty basic stuff, so we naturally don’t want to spend so many lines of code writing it. As it turns out, you can use the ternary operator in Python to evaluate conditions in a single line.

Ternary Operator in Python

A ternary operator exists in some programming languages, and it allows you to shorten a simple If-Else block. It takes in 3 or more operands:

Value if true - A value that’s returned if the condition evaluates to True.
Condition - A boolean condition that has to be satisfied to return value if true.
Value if false - A value that’s returned if the condition evaluates to False. In code, it would look like this:

You can even write else-if logic in Python’s ternary operator. In that case, the syntax changes slightly:

I have to admit - it looks a bit abstract when written like this. You’ll see plenty of practical examples starting from the next section.

One-Line If Statement (Without Else)

A single-line if statement just means you’re deleting the new line and indentation. You’re still writing the same code, with the only twist being that it takes one line instead of two.

Note: One-line if statement is only possible if there’s a single line of code following the condition. In any other case, wrap the code that will be executed inside a function.

Here’s how to transform our two-line if statement to a single-line conditional:

As before, age is less than 18 so Go home. gets printed.

What if you want to print three lines instead of one? As said before, the best practice is to wrap the code inside a function:

One-line if statements in Python are pretty boring. The real time and space saving benefit happens when you add an else condition.

You’ll benefit the most from one-line if statements if you add one or multiple else conditions.

One-Line If-Else Statement

Now we can fully leverage the power of Python’s ternary operator. The code snippet below stores Go home. to a new variable outcome if the age is less than 18 or Welcome! otherwise:

As you would guess, Welcome! is printed to the console as age is set to 19. If you want to print multiple lines or handle more complex logic, wrap everything you want to be executed into a function - just as before.

You now have a clear picture of how the ternary operator works on a simple one-line if-else statement. We can add complexity by adding more conditions to the operator.

One-Line If-Elif-Else Statement

Always be careful when writing multiple conditions in a single line of code. The logic will still work if the line is 500 characters long, but it’s near impossible to read and maintain it.

You should be fine with two conditions in one line, as the code is still easy to read. The following example prints Go home. if age is below 16, Not Sure... if age is between 16 (included) and 18 (excluded), and Welcome otherwise:

You’ll see Not sure... printed to the console, since age is set to 17. What previously took us six lines of code now only takes one. Neat improvement, and the code is still easy to read and maintain.

What else can you do with one-line if statements? Well, a lot. We’ll explore single-line conditionals for list operations next.

Example: One-Line Conditionals for List Operations

Applying some logic to a list involves applying the logic to every list item, and hence iterating over the entire list. Before even thinking about a real-world example, let’s see how you can write a conditional statement for every list item in a single line of code.

How to Write IF and FOR in One Line

You’ll need to make two changes to the ternary operator:

Surround the entire line of code with brackets [] Append the list iteration code (for element in array) after the final else Here’s how the generic syntax looks like:

It’s not that hard, but let’s drive the point home with an example. The following code snippet prints + if the current number of a range is greater than 5 and - otherwise. The numbers range from 1 to 10 (included):

Image 1 - If and For in a single line in Python (image by author)

Let’s now go over an additional real-world example.

Example: Did Student Pass the Exam?

To start, we’ll declare a list of students. Each student is a Python dictionary object with two keys: name and test score:

We want to print that the student has passed the exam if the score is 50 points or above. If the score was below 50 points, we want to print that the student has failed the exam.

In traditional Python syntax, we would manually iterate over each student in the list and check if the score is greater than 50:

Image 2 - List iteration with traditional Python syntax (image by author)

The code works, but we need 5 lines to make a simple check and store the results. You can use your newly-acquired knowledge to reduce the amount of code to a single line:

Image 3 - One-line conditional and a loop with Python (image by author)

The results are identical, but we have a much shorter and neater code. It’s just on the boundary of being unreadable, which is often a tradeoff with ternary operators and single-line loops. You often can’t have both readable code and short Python scripts.

Just because you can write a conditional in one line, it doesn’t mean you should. Readability is a priority. Let’s see in which cases you’re better off with traditional if statements.

Be Careful With One-Line Conditionals

Just because code takes less vertical space doesn’t mean it’s easier to read. Now you’ll see the perfect example of that claim.

The below snippet checks a condition for every possible grade (1-5) with a final else condition capturing invalid input. The conditions take 12 lines of code to write, but the entire snippet is extremely readable:

As expected, you’ll see Grade = 1 printed to the console, but that’s not what we’re interested in. We want to translate the above snippet into a one-line if-else statement with the ternary operator.

It’s possible - but the end result is messy and unreadable:

This is an example of an extreme case where you have multiple conditions you have to evaluate. It’s better to stick with the traditional if statements, even though they take more vertical space.

Take home point: A ternary operator with more than two conditions is just a nightmare to write and debug.

And there you have it - everything you need to know about one-line if-else statements in Python. You’ve learned all there is about the ternary operator, and how to write conditionals starting with a single if to five conditions in between.

Remember to keep your code simple. The code that’s easier to read and maintain is a better-written code at the end of the day. Just because you can cram everything into a single line, doesn’t mean you should. You’ll regret it as soon as you need to make some changes.

An even cleaner way to write long conditionals is by using structural pattern matching - a new feature introduced in Python 3.10. It brings the beloved switch statement to Python for extra readability and speed of development.

What do you guys think of one-line if-else statements in Python? Do you use them regularly or have you switched to structural pattern matching? Let me know in the comment section below.

If-Then-Else in One Line Python

Quick answer : How to put a simple if-then-else statement in one line of Python code?

To put an if-then-else statement in one line, use Python’s ternary operator x if c else y . This returns the result of expression x if the Boolean condition c evaluates to True . Otherwise, the ternary operator returns the alternative expression y .

While you read through the article to boost your one-liner power, you can listen to my detailed video explanation:

Here’s another minimal example where you return 21 if the condition 3>2 evaluates to True , otherwise, you return 42 :

The output 42 is stored in the variable var .

Introduction and Overview

Python is so powerful, that you can even compress whole algorithms in a single line of code .

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So the natural question arises: can you write conditional if-then-else statements in a single line of code?

This article explores this mission-critical question in all detail.

Can you write the if-then-else statement in a single line of code?

Yes, you can write most if statements in a single line of Python using any of the following methods:

Write the if statement without else branch as a Python one-liner: if 42 in range(100): print("42") .
If you want to set a variable, use the ternary operator : x = "Alice" if "Jon" in "My name is Jonas" else "Bob" .
If you want to conditionally execute a function, still use the ternary operator : print("42") if 42 in range(100) else print("21") .

In the previous paragraph, you’ve unwillingly learned about the ternary operator in Python.

The ternary operator is something you’ll see in most advanced code bases so make sure to understand it thoroughly by reading the following section of this article.

I drew this picture to show visually how the ternary operator works:

Let’s dive into the three different ways to write the if-then-else statement as a Python one-liner .

Related articles : Python One Line Ternary

How to Write the If-Then-Else Statement as a Python One-Liner?

Let’s have a look at all the ways how you can write the if-then-else statement in one line.

The trivial answer is to just write it in one line—but only if you don’t have an else branch:

Case 1: You Don’t Have an Else Branch

Consider the following code snippet where you check for the number 42 whether it falls in a range of numbers:

This code snippet will indeed print the output because the integer 42 falls into the range of numbers from 0 to 99.

But how can we write this if statement in a single line of code?

Just use a single line like this:

The two statements are identical so this is the way to do it—if you can write the conditional body in a single line of code.

However, if you try to become too fancy and you use a nested if body, it won’t work:

Python cannot handle this anymore: the interpreter throws an “invalid syntax” error because the statement became ambiguous.

But don’t worry, there’s a workaround: the ternary operator .

Case 2: You Have an Else Branch And You Want to Conditionally Assign a Value

In case you have an else branch and you want to conditionally assign a value to a variable, the ternary operator is your friend.

Say, you want to write the following if-then-else statement in a single line of code:

As the string "Jon" appears in the string "My name is Jonas" , the variable x will take value "Alice" .

Can we write it in a single line? Sure—by using the ternary operator .

The ternary operator is very intuitive. Just read it from left to right and you’ll understand its meaning.

We assign the value "Alice" to the variable x in case the following condition holds: "Jon" in "My name is Jonas" . Otherwise, we assign the string "Bob" to the variable x .

Ternary Operator Syntax : The three operands are written as x if c else y which reads as “return x if c else return y “. Let’s write this more intuitively as:

Case 3: What If You Don’t Want to Assign Any Value But You Have an Else Branch?

Well, there’s a quick and dirty hack: just ignore the return value of the ternary operator.

Say, we want to compress the following if-then-else statement in a single line of code:

The problem is that we don’t have a return value. So can we still use the ternary operator?

As it turns out, we can. Let’s write this if-then-else statement in a single line:

We use the ternary operator. The return value of the print() function is simply None . But we don’t really care about the return value, so we don’t store it in any variable.

We only care about executing the print function in case the if condition is met.

How to Write an If-Elif-Else Statement in a Single Line of Python?

In the previous paragraphs, you’ve learned that we can write the if-else statement in a single line of code.

But can you one-linerize an elif expression with multiple conditions?

Of course, you can!

(Heuristic: If you’re in doubt about whether you can do XYZ in a single line of Python, just assume that you can. See here .)

Say, you want to write the following if-then-else condition in a single line of code:

The elif branch wins! We print the output "yes" to the shell.

But how to do it in a single line of code? Just use the ternary operator with an elif statement won’t work (it’ll throw a syntax error).

The answer is simple: nest two ternary operators like so:

If the value x is larger than 42 , we print "no" to the shell.

Otherwise, we execute the remainder of the code (which is a ternary operator by itself). If the value x is equal to 42 , we print "yes" , otherwise "maybe" .

So by nesting multiple ternary operators, we can greatly increase our Python one-liner power!

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How to Write If Without Else in One Line?

We’ve already seen an example above: we simply write the if statement in one line without using the ternary operator: if 42 in range(100): print("42") .

Python is perfectly able to understand a simple if statement without an else branch in a single line of code.

How to Write Elif in One Line?

We cannot directly write the elif branch in one line of Python code. But we can nest two ternary operators instead:

Python If-Else One-Liner: What Does It Return?

The ternary operator always returns the result of the conditional evaluation. Again, the code snippet 100 if x>42 else 42 returns the integer value 42.

If you only execute functions within the ternary operator, it’ll return the None value.

Where to Go From Here

Knowing small Python one-liner tricks such as the ternary operator is vital for your success in the Python language. Every expert coder knows them by heart—after all, this is what makes them very productive.

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Python Enhancement Proposals

Python »
PEP Index »

PEP 572 – Assignment Expressions

The importance of real code, exceptional cases, scope of the target, relative precedence of :=, change to evaluation order, differences between assignment expressions and assignment statements, specification changes during implementation, _pydecimal.py, datetime.py, sysconfig.py, simplifying list comprehensions, capturing condition values, changing the scope rules for comprehensions, alternative spellings, special-casing conditional statements, special-casing comprehensions, lowering operator precedence, allowing commas to the right, always requiring parentheses, why not just turn existing assignment into an expression, with assignment expressions, why bother with assignment statements, why not use a sublocal scope and prevent namespace pollution, style guide recommendations, acknowledgements, a numeric example, appendix b: rough code translations for comprehensions, appendix c: no changes to scope semantics.

This is a proposal for creating a way to assign to variables within an expression using the notation NAME := expr .

As part of this change, there is also an update to dictionary comprehension evaluation order to ensure key expressions are executed before value expressions (allowing the key to be bound to a name and then re-used as part of calculating the corresponding value).

During discussion of this PEP, the operator became informally known as “the walrus operator”. The construct’s formal name is “Assignment Expressions” (as per the PEP title), but they may also be referred to as “Named Expressions” (e.g. the CPython reference implementation uses that name internally).

Naming the result of an expression is an important part of programming, allowing a descriptive name to be used in place of a longer expression, and permitting reuse. Currently, this feature is available only in statement form, making it unavailable in list comprehensions and other expression contexts.

Additionally, naming sub-parts of a large expression can assist an interactive debugger, providing useful display hooks and partial results. Without a way to capture sub-expressions inline, this would require refactoring of the original code; with assignment expressions, this merely requires the insertion of a few name := markers. Removing the need to refactor reduces the likelihood that the code be inadvertently changed as part of debugging (a common cause of Heisenbugs), and is easier to dictate to another programmer.

During the development of this PEP many people (supporters and critics both) have had a tendency to focus on toy examples on the one hand, and on overly complex examples on the other.

The danger of toy examples is twofold: they are often too abstract to make anyone go “ooh, that’s compelling”, and they are easily refuted with “I would never write it that way anyway”.

The danger of overly complex examples is that they provide a convenient strawman for critics of the proposal to shoot down (“that’s obfuscated”).

Yet there is some use for both extremely simple and extremely complex examples: they are helpful to clarify the intended semantics. Therefore, there will be some of each below.

However, in order to be compelling , examples should be rooted in real code, i.e. code that was written without any thought of this PEP, as part of a useful application, however large or small. Tim Peters has been extremely helpful by going over his own personal code repository and picking examples of code he had written that (in his view) would have been clearer if rewritten with (sparing) use of assignment expressions. His conclusion: the current proposal would have allowed a modest but clear improvement in quite a few bits of code.

Another use of real code is to observe indirectly how much value programmers place on compactness. Guido van Rossum searched through a Dropbox code base and discovered some evidence that programmers value writing fewer lines over shorter lines.

Case in point: Guido found several examples where a programmer repeated a subexpression, slowing down the program, in order to save one line of code, e.g. instead of writing:

they would write:

Another example illustrates that programmers sometimes do more work to save an extra level of indentation:

This code tries to match pattern2 even if pattern1 has a match (in which case the match on pattern2 is never used). The more efficient rewrite would have been:

Syntax and semantics

In most contexts where arbitrary Python expressions can be used, a named expression can appear. This is of the form NAME := expr where expr is any valid Python expression other than an unparenthesized tuple, and NAME is an identifier.

The value of such a named expression is the same as the incorporated expression, with the additional side-effect that the target is assigned that value:

There are a few places where assignment expressions are not allowed, in order to avoid ambiguities or user confusion:

This rule is included to simplify the choice for the user between an assignment statement and an assignment expression – there is no syntactic position where both are valid.

Again, this rule is included to avoid two visually similar ways of saying the same thing.

This rule is included to disallow excessively confusing code, and because parsing keyword arguments is complex enough already.

This rule is included to discourage side effects in a position whose exact semantics are already confusing to many users (cf. the common style recommendation against mutable default values), and also to echo the similar prohibition in calls (the previous bullet).

The reasoning here is similar to the two previous cases; this ungrouped assortment of symbols and operators composed of : and = is hard to read correctly.

This allows lambda to always bind less tightly than := ; having a name binding at the top level inside a lambda function is unlikely to be of value, as there is no way to make use of it. In cases where the name will be used more than once, the expression is likely to need parenthesizing anyway, so this prohibition will rarely affect code.

This shows that what looks like an assignment operator in an f-string is not always an assignment operator. The f-string parser uses : to indicate formatting options. To preserve backwards compatibility, assignment operator usage inside of f-strings must be parenthesized. As noted above, this usage of the assignment operator is not recommended.

An assignment expression does not introduce a new scope. In most cases the scope in which the target will be bound is self-explanatory: it is the current scope. If this scope contains a nonlocal or global declaration for the target, the assignment expression honors that. A lambda (being an explicit, if anonymous, function definition) counts as a scope for this purpose.

There is one special case: an assignment expression occurring in a list, set or dict comprehension or in a generator expression (below collectively referred to as “comprehensions”) binds the target in the containing scope, honoring a nonlocal or global declaration for the target in that scope, if one exists. For the purpose of this rule the containing scope of a nested comprehension is the scope that contains the outermost comprehension. A lambda counts as a containing scope.

The motivation for this special case is twofold. First, it allows us to conveniently capture a “witness” for an any() expression, or a counterexample for all() , for example:

Second, it allows a compact way of updating mutable state from a comprehension, for example:

However, an assignment expression target name cannot be the same as a for -target name appearing in any comprehension containing the assignment expression. The latter names are local to the comprehension in which they appear, so it would be contradictory for a contained use of the same name to refer to the scope containing the outermost comprehension instead.

For example, [i := i+1 for i in range(5)] is invalid: the for i part establishes that i is local to the comprehension, but the i := part insists that i is not local to the comprehension. The same reason makes these examples invalid too:

While it’s technically possible to assign consistent semantics to these cases, it’s difficult to determine whether those semantics actually make sense in the absence of real use cases. Accordingly, the reference implementation [1] will ensure that such cases raise SyntaxError , rather than executing with implementation defined behaviour.

This restriction applies even if the assignment expression is never executed:

For the comprehension body (the part before the first “for” keyword) and the filter expression (the part after “if” and before any nested “for”), this restriction applies solely to target names that are also used as iteration variables in the comprehension. Lambda expressions appearing in these positions introduce a new explicit function scope, and hence may use assignment expressions with no additional restrictions.

Due to design constraints in the reference implementation (the symbol table analyser cannot easily detect when names are re-used between the leftmost comprehension iterable expression and the rest of the comprehension), named expressions are disallowed entirely as part of comprehension iterable expressions (the part after each “in”, and before any subsequent “if” or “for” keyword):

A further exception applies when an assignment expression occurs in a comprehension whose containing scope is a class scope. If the rules above were to result in the target being assigned in that class’s scope, the assignment expression is expressly invalid. This case also raises SyntaxError :

(The reason for the latter exception is the implicit function scope created for comprehensions – there is currently no runtime mechanism for a function to refer to a variable in the containing class scope, and we do not want to add such a mechanism. If this issue ever gets resolved this special case may be removed from the specification of assignment expressions. Note that the problem already exists for using a variable defined in the class scope from a comprehension.)

See Appendix B for some examples of how the rules for targets in comprehensions translate to equivalent code.

The := operator groups more tightly than a comma in all syntactic positions where it is legal, but less tightly than all other operators, including or , and , not , and conditional expressions ( A if C else B ). As follows from section “Exceptional cases” above, it is never allowed at the same level as = . In case a different grouping is desired, parentheses should be used.

The := operator may be used directly in a positional function call argument; however it is invalid directly in a keyword argument.

Some examples to clarify what’s technically valid or invalid:

Most of the “valid” examples above are not recommended, since human readers of Python source code who are quickly glancing at some code may miss the distinction. But simple cases are not objectionable:

This PEP recommends always putting spaces around := , similar to PEP 8 ’s recommendation for = when used for assignment, whereas the latter disallows spaces around = used for keyword arguments.)

In order to have precisely defined semantics, the proposal requires evaluation order to be well-defined. This is technically not a new requirement, as function calls may already have side effects. Python already has a rule that subexpressions are generally evaluated from left to right. However, assignment expressions make these side effects more visible, and we propose a single change to the current evaluation order:

In a dict comprehension {X: Y for ...} , Y is currently evaluated before X . We propose to change this so that X is evaluated before Y . (In a dict display like {X: Y} this is already the case, and also in dict((X, Y) for ...) which should clearly be equivalent to the dict comprehension.)

Most importantly, since := is an expression, it can be used in contexts where statements are illegal, including lambda functions and comprehensions.

Conversely, assignment expressions don’t support the advanced features found in assignment statements:

Multiple targets are not directly supported: x = y = z = 0 # Equivalent: (z := (y := (x := 0)))
Single assignment targets other than a single NAME are not supported: # No equivalent a [ i ] = x self . rest = []
Priority around commas is different: x = 1 , 2 # Sets x to (1, 2) ( x := 1 , 2 ) # Sets x to 1
Iterable packing and unpacking (both regular or extended forms) are not supported: # Equivalent needs extra parentheses loc = x , y # Use (loc := (x, y)) info = name , phone , * rest # Use (info := (name, phone, *rest)) # No equivalent px , py , pz = position name , phone , email , * other_info = contact
Inline type annotations are not supported: # Closest equivalent is "p: Optional[int]" as a separate declaration p : Optional [ int ] = None
Augmented assignment is not supported: total += tax # Equivalent: (total := total + tax)

The following changes have been made based on implementation experience and additional review after the PEP was first accepted and before Python 3.8 was released:

for consistency with other similar exceptions, and to avoid locking in an exception name that is not necessarily going to improve clarity for end users, the originally proposed TargetScopeError subclass of SyntaxError was dropped in favour of just raising SyntaxError directly. [3]
due to a limitation in CPython’s symbol table analysis process, the reference implementation raises SyntaxError for all uses of named expressions inside comprehension iterable expressions, rather than only raising them when the named expression target conflicts with one of the iteration variables in the comprehension. This could be revisited given sufficiently compelling examples, but the extra complexity needed to implement the more selective restriction doesn’t seem worthwhile for purely hypothetical use cases.

Examples from the Python standard library

env_base is only used on these lines, putting its assignment on the if moves it as the “header” of the block.

Current: env_base = os . environ . get ( "PYTHONUSERBASE" , None ) if env_base : return env_base
Improved: if env_base := os . environ . get ( "PYTHONUSERBASE" , None ): return env_base

Avoid nested if and remove one indentation level.

Current: if self . _is_special : ans = self . _check_nans ( context = context ) if ans : return ans
Improved: if self . _is_special and ( ans := self . _check_nans ( context = context )): return ans

Code looks more regular and avoid multiple nested if. (See Appendix A for the origin of this example.)

Current: reductor = dispatch_table . get ( cls ) if reductor : rv = reductor ( x ) else : reductor = getattr ( x , "__reduce_ex__" , None ) if reductor : rv = reductor ( 4 ) else : reductor = getattr ( x , "__reduce__" , None ) if reductor : rv = reductor () else : raise Error ( "un(deep)copyable object of type %s " % cls )
Improved: if reductor := dispatch_table . get ( cls ): rv = reductor ( x ) elif reductor := getattr ( x , "__reduce_ex__" , None ): rv = reductor ( 4 ) elif reductor := getattr ( x , "__reduce__" , None ): rv = reductor () else : raise Error ( "un(deep)copyable object of type %s " % cls )

tz is only used for s += tz , moving its assignment inside the if helps to show its scope.

Current: s = _format_time ( self . _hour , self . _minute , self . _second , self . _microsecond , timespec ) tz = self . _tzstr () if tz : s += tz return s
Improved: s = _format_time ( self . _hour , self . _minute , self . _second , self . _microsecond , timespec ) if tz := self . _tzstr (): s += tz return s

Calling fp.readline() in the while condition and calling .match() on the if lines make the code more compact without making it harder to understand.

Current: while True : line = fp . readline () if not line : break m = define_rx . match ( line ) if m : n , v = m . group ( 1 , 2 ) try : v = int ( v ) except ValueError : pass vars [ n ] = v else : m = undef_rx . match ( line ) if m : vars [ m . group ( 1 )] = 0
Improved: while line := fp . readline (): if m := define_rx . match ( line ): n , v = m . group ( 1 , 2 ) try : v = int ( v ) except ValueError : pass vars [ n ] = v elif m := undef_rx . match ( line ): vars [ m . group ( 1 )] = 0

A list comprehension can map and filter efficiently by capturing the condition:

Similarly, a subexpression can be reused within the main expression, by giving it a name on first use:

Note that in both cases the variable y is bound in the containing scope (i.e. at the same level as results or stuff ).

Assignment expressions can be used to good effect in the header of an if or while statement:

Particularly with the while loop, this can remove the need to have an infinite loop, an assignment, and a condition. It also creates a smooth parallel between a loop which simply uses a function call as its condition, and one which uses that as its condition but also uses the actual value.

An example from the low-level UNIX world:

Rejected alternative proposals

Proposals broadly similar to this one have come up frequently on python-ideas. Below are a number of alternative syntaxes, some of them specific to comprehensions, which have been rejected in favour of the one given above.

A previous version of this PEP proposed subtle changes to the scope rules for comprehensions, to make them more usable in class scope and to unify the scope of the “outermost iterable” and the rest of the comprehension. However, this part of the proposal would have caused backwards incompatibilities, and has been withdrawn so the PEP can focus on assignment expressions.

Broadly the same semantics as the current proposal, but spelled differently.

Since EXPR as NAME already has meaning in import , except and with statements (with different semantics), this would create unnecessary confusion or require special-casing (e.g. to forbid assignment within the headers of these statements).

(Note that with EXPR as VAR does not simply assign the value of EXPR to VAR – it calls EXPR.__enter__() and assigns the result of that to VAR .)

Additional reasons to prefer := over this spelling include:

In if f(x) as y the assignment target doesn’t jump out at you – it just reads like if f x blah blah and it is too similar visually to if f(x) and y .
import foo as bar
except Exc as var
with ctxmgr() as var

To the contrary, the assignment expression does not belong to the if or while that starts the line, and we intentionally allow assignment expressions in other contexts as well.

NAME = EXPR
if NAME := EXPR

reinforces the visual recognition of assignment expressions.

This syntax is inspired by languages such as R and Haskell, and some programmable calculators. (Note that a left-facing arrow y <- f(x) is not possible in Python, as it would be interpreted as less-than and unary minus.) This syntax has a slight advantage over ‘as’ in that it does not conflict with with , except and import , but otherwise is equivalent. But it is entirely unrelated to Python’s other use of -> (function return type annotations), and compared to := (which dates back to Algol-58) it has a much weaker tradition.

This has the advantage that leaked usage can be readily detected, removing some forms of syntactic ambiguity. However, this would be the only place in Python where a variable’s scope is encoded into its name, making refactoring harder.

Execution order is inverted (the indented body is performed first, followed by the “header”). This requires a new keyword, unless an existing keyword is repurposed (most likely with: ). See PEP 3150 for prior discussion on this subject (with the proposed keyword being given: ).

This syntax has fewer conflicts than as does (conflicting only with the raise Exc from Exc notation), but is otherwise comparable to it. Instead of paralleling with expr as target: (which can be useful but can also be confusing), this has no parallels, but is evocative.

One of the most popular use-cases is if and while statements. Instead of a more general solution, this proposal enhances the syntax of these two statements to add a means of capturing the compared value:

This works beautifully if and ONLY if the desired condition is based on the truthiness of the captured value. It is thus effective for specific use-cases (regex matches, socket reads that return '' when done), and completely useless in more complicated cases (e.g. where the condition is f(x) < 0 and you want to capture the value of f(x) ). It also has no benefit to list comprehensions.

Advantages: No syntactic ambiguities. Disadvantages: Answers only a fraction of possible use-cases, even in if / while statements.

Another common use-case is comprehensions (list/set/dict, and genexps). As above, proposals have been made for comprehension-specific solutions.

This brings the subexpression to a location in between the ‘for’ loop and the expression. It introduces an additional language keyword, which creates conflicts. Of the three, where reads the most cleanly, but also has the greatest potential for conflict (e.g. SQLAlchemy and numpy have where methods, as does tkinter.dnd.Icon in the standard library).

As above, but reusing the with keyword. Doesn’t read too badly, and needs no additional language keyword. Is restricted to comprehensions, though, and cannot as easily be transformed into “longhand” for-loop syntax. Has the C problem that an equals sign in an expression can now create a name binding, rather than performing a comparison. Would raise the question of why “with NAME = EXPR:” cannot be used as a statement on its own.

As per option 2, but using as rather than an equals sign. Aligns syntactically with other uses of as for name binding, but a simple transformation to for-loop longhand would create drastically different semantics; the meaning of with inside a comprehension would be completely different from the meaning as a stand-alone statement, while retaining identical syntax.

Regardless of the spelling chosen, this introduces a stark difference between comprehensions and the equivalent unrolled long-hand form of the loop. It is no longer possible to unwrap the loop into statement form without reworking any name bindings. The only keyword that can be repurposed to this task is with , thus giving it sneakily different semantics in a comprehension than in a statement; alternatively, a new keyword is needed, with all the costs therein.

There are two logical precedences for the := operator. Either it should bind as loosely as possible, as does statement-assignment; or it should bind more tightly than comparison operators. Placing its precedence between the comparison and arithmetic operators (to be precise: just lower than bitwise OR) allows most uses inside while and if conditions to be spelled without parentheses, as it is most likely that you wish to capture the value of something, then perform a comparison on it:

Once find() returns -1, the loop terminates. If := binds as loosely as = does, this would capture the result of the comparison (generally either True or False ), which is less useful.

While this behaviour would be convenient in many situations, it is also harder to explain than “the := operator behaves just like the assignment statement”, and as such, the precedence for := has been made as close as possible to that of = (with the exception that it binds tighter than comma).

Some critics have claimed that the assignment expressions should allow unparenthesized tuples on the right, so that these two would be equivalent:

(With the current version of the proposal, the latter would be equivalent to ((point := x), y) .)

However, adopting this stance would logically lead to the conclusion that when used in a function call, assignment expressions also bind less tight than comma, so we’d have the following confusing equivalence:

The less confusing option is to make := bind more tightly than comma.

It’s been proposed to just always require parentheses around an assignment expression. This would resolve many ambiguities, and indeed parentheses will frequently be needed to extract the desired subexpression. But in the following cases the extra parentheses feel redundant:

Frequently Raised Objections

C and its derivatives define the = operator as an expression, rather than a statement as is Python’s way. This allows assignments in more contexts, including contexts where comparisons are more common. The syntactic similarity between if (x == y) and if (x = y) belies their drastically different semantics. Thus this proposal uses := to clarify the distinction.

The two forms have different flexibilities. The := operator can be used inside a larger expression; the = statement can be augmented to += and its friends, can be chained, and can assign to attributes and subscripts.

Previous revisions of this proposal involved sublocal scope (restricted to a single statement), preventing name leakage and namespace pollution. While a definite advantage in a number of situations, this increases complexity in many others, and the costs are not justified by the benefits. In the interests of language simplicity, the name bindings created here are exactly equivalent to any other name bindings, including that usage at class or module scope will create externally-visible names. This is no different from for loops or other constructs, and can be solved the same way: del the name once it is no longer needed, or prefix it with an underscore.

(The author wishes to thank Guido van Rossum and Christoph Groth for their suggestions to move the proposal in this direction. [2] )

As expression assignments can sometimes be used equivalently to statement assignments, the question of which should be preferred will arise. For the benefit of style guides such as PEP 8 , two recommendations are suggested.

If either assignment statements or assignment expressions can be used, prefer statements; they are a clear declaration of intent.
If using assignment expressions would lead to ambiguity about execution order, restructure it to use statements instead.

The authors wish to thank Alyssa Coghlan and Steven D’Aprano for their considerable contributions to this proposal, and members of the core-mentorship mailing list for assistance with implementation.

Appendix A: Tim Peters’s findings

Here’s a brief essay Tim Peters wrote on the topic.

I dislike “busy” lines of code, and also dislike putting conceptually unrelated logic on a single line. So, for example, instead of:

instead. So I suspected I’d find few places I’d want to use assignment expressions. I didn’t even consider them for lines already stretching halfway across the screen. In other cases, “unrelated” ruled:

is a vast improvement over the briefer:

The original two statements are doing entirely different conceptual things, and slamming them together is conceptually insane.

In other cases, combining related logic made it harder to understand, such as rewriting:

as the briefer:

The while test there is too subtle, crucially relying on strict left-to-right evaluation in a non-short-circuiting or method-chaining context. My brain isn’t wired that way.

But cases like that were rare. Name binding is very frequent, and “sparse is better than dense” does not mean “almost empty is better than sparse”. For example, I have many functions that return None or 0 to communicate “I have nothing useful to return in this case, but since that’s expected often I’m not going to annoy you with an exception”. This is essentially the same as regular expression search functions returning None when there is no match. So there was lots of code of the form:

I find that clearer, and certainly a bit less typing and pattern-matching reading, as:

It’s also nice to trade away a small amount of horizontal whitespace to get another _line_ of surrounding code on screen. I didn’t give much weight to this at first, but it was so very frequent it added up, and I soon enough became annoyed that I couldn’t actually run the briefer code. That surprised me!

There are other cases where assignment expressions really shine. Rather than pick another from my code, Kirill Balunov gave a lovely example from the standard library’s copy() function in copy.py :

The ever-increasing indentation is semantically misleading: the logic is conceptually flat, “the first test that succeeds wins”:

Using easy assignment expressions allows the visual structure of the code to emphasize the conceptual flatness of the logic; ever-increasing indentation obscured it.

A smaller example from my code delighted me, both allowing to put inherently related logic in a single line, and allowing to remove an annoying “artificial” indentation level:

That if is about as long as I want my lines to get, but remains easy to follow.

So, in all, in most lines binding a name, I wouldn’t use assignment expressions, but because that construct is so very frequent, that leaves many places I would. In most of the latter, I found a small win that adds up due to how often it occurs, and in the rest I found a moderate to major win. I’d certainly use it more often than ternary if , but significantly less often than augmented assignment.

I have another example that quite impressed me at the time.

Where all variables are positive integers, and a is at least as large as the n’th root of x, this algorithm returns the floor of the n’th root of x (and roughly doubling the number of accurate bits per iteration):

It’s not obvious why that works, but is no more obvious in the “loop and a half” form. It’s hard to prove correctness without building on the right insight (the “arithmetic mean - geometric mean inequality”), and knowing some non-trivial things about how nested floor functions behave. That is, the challenges are in the math, not really in the coding.

If you do know all that, then the assignment-expression form is easily read as “while the current guess is too large, get a smaller guess”, where the “too large?” test and the new guess share an expensive sub-expression.

To my eyes, the original form is harder to understand:

This appendix attempts to clarify (though not specify) the rules when a target occurs in a comprehension or in a generator expression. For a number of illustrative examples we show the original code, containing a comprehension, and the translation, where the comprehension has been replaced by an equivalent generator function plus some scaffolding.

Since [x for ...] is equivalent to list(x for ...) these examples all use list comprehensions without loss of generality. And since these examples are meant to clarify edge cases of the rules, they aren’t trying to look like real code.

Note: comprehensions are already implemented via synthesizing nested generator functions like those in this appendix. The new part is adding appropriate declarations to establish the intended scope of assignment expression targets (the same scope they resolve to as if the assignment were performed in the block containing the outermost comprehension). For type inference purposes, these illustrative expansions do not imply that assignment expression targets are always Optional (but they do indicate the target binding scope).

Let’s start with a reminder of what code is generated for a generator expression without assignment expression.

Original code (EXPR usually references VAR): def f (): a = [ EXPR for VAR in ITERABLE ]
Translation (let’s not worry about name conflicts): def f (): def genexpr ( iterator ): for VAR in iterator : yield EXPR a = list ( genexpr ( iter ( ITERABLE )))

Let’s add a simple assignment expression.

Original code: def f (): a = [ TARGET := EXPR for VAR in ITERABLE ]
Translation: def f (): if False : TARGET = None # Dead code to ensure TARGET is a local variable def genexpr ( iterator ): nonlocal TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Let’s add a global TARGET declaration in f() .

Original code: def f (): global TARGET a = [ TARGET := EXPR for VAR in ITERABLE ]
Translation: def f (): global TARGET def genexpr ( iterator ): global TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Or instead let’s add a nonlocal TARGET declaration in f() .

Original code: def g (): TARGET = ... def f (): nonlocal TARGET a = [ TARGET := EXPR for VAR in ITERABLE ]
Translation: def g (): TARGET = ... def f (): nonlocal TARGET def genexpr ( iterator ): nonlocal TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Finally, let’s nest two comprehensions.

Original code: def f (): a = [[ TARGET := i for i in range ( 3 )] for j in range ( 2 )] # I.e., a = [[0, 1, 2], [0, 1, 2]] print ( TARGET ) # prints 2
Translation: def f (): if False : TARGET = None def outer_genexpr ( outer_iterator ): nonlocal TARGET def inner_generator ( inner_iterator ): nonlocal TARGET for i in inner_iterator : TARGET = i yield i for j in outer_iterator : yield list ( inner_generator ( range ( 3 ))) a = list ( outer_genexpr ( range ( 2 ))) print ( TARGET )

Because it has been a point of confusion, note that nothing about Python’s scoping semantics is changed. Function-local scopes continue to be resolved at compile time, and to have indefinite temporal extent at run time (“full closures”). Example:

This document has been placed in the public domain.

Source: https://github.com/python/peps/blob/main/peps/pep-0572.rst

Last modified: 2023-10-11 12:05:51 GMT

Python Numerical Methods

This notebook contains an excerpt from the Python Programming and Numerical Methods - A Guide for Engineers and Scientists , the content is also available at Berkeley Python Numerical Methods .

The copyright of the book belongs to Elsevier. We also have this interactive book online for a better learning experience. The code is released under the MIT license . If you find this content useful, please consider supporting the work on Elsevier or Amazon !

< 2.0 Variables and Basic Data Structures | Contents | 2.2 Data Structure - Strings >

Variables and Assignment ¶

When programming, it is useful to be able to store information in variables. A variable is a string of characters and numbers associated with a piece of information. The assignment operator , denoted by the “=” symbol, is the operator that is used to assign values to variables in Python. The line x=1 takes the known value, 1, and assigns that value to the variable with name “x”. After executing this line, this number will be stored into this variable. Until the value is changed or the variable deleted, the character x behaves like the value 1.

TRY IT! Assign the value 2 to the variable y. Multiply y by 3 to show that it behaves like the value 2.

A variable is more like a container to store the data in the computer’s memory, the name of the variable tells the computer where to find this value in the memory. For now, it is sufficient to know that the notebook has its own memory space to store all the variables in the notebook. As a result of the previous example, you will see the variable “x” and “y” in the memory. You can view a list of all the variables in the notebook using the magic command %whos .

TRY IT! List all the variables in this notebook

Note that the equal sign in programming is not the same as a truth statement in mathematics. In math, the statement x = 2 declares the universal truth within the given framework, x is 2 . In programming, the statement x=2 means a known value is being associated with a variable name, store 2 in x. Although it is perfectly valid to say 1 = x in mathematics, assignments in Python always go left : meaning the value to the right of the equal sign is assigned to the variable on the left of the equal sign. Therefore, 1=x will generate an error in Python. The assignment operator is always last in the order of operations relative to mathematical, logical, and comparison operators.

TRY IT! The mathematical statement x=x+1 has no solution for any value of x . In programming, if we initialize the value of x to be 1, then the statement makes perfect sense. It means, “Add x and 1, which is 2, then assign that value to the variable x”. Note that this operation overwrites the previous value stored in x .

There are some restrictions on the names variables can take. Variables can only contain alphanumeric characters (letters and numbers) as well as underscores. However, the first character of a variable name must be a letter or underscores. Spaces within a variable name are not permitted, and the variable names are case-sensitive (e.g., x and X will be considered different variables).

TIP! Unlike in pure mathematics, variables in programming almost always represent something tangible. It may be the distance between two points in space or the number of rabbits in a population. Therefore, as your code becomes increasingly complicated, it is very important that your variables carry a name that can easily be associated with what they represent. For example, the distance between two points in space is better represented by the variable dist than x , and the number of rabbits in a population is better represented by nRabbits than y .

Note that when a variable is assigned, it has no memory of how it was assigned. That is, if the value of a variable, y , is constructed from other variables, like x , reassigning the value of x will not change the value of y .

EXAMPLE: What value will y have after the following lines of code are executed?

WARNING! You can overwrite variables or functions that have been stored in Python. For example, the command help = 2 will store the value 2 in the variable with name help . After this assignment help will behave like the value 2 instead of the function help . Therefore, you should always be careful not to give your variables the same name as built-in functions or values.

TIP! Now that you know how to assign variables, it is important that you learn to never leave unassigned commands. An unassigned command is an operation that has a result, but that result is not assigned to a variable. For example, you should never use 2+2 . You should instead assign it to some variable x=2+2 . This allows you to “hold on” to the results of previous commands and will make your interaction with Python must less confusing.

You can clear a variable from the notebook using the del function. Typing del x will clear the variable x from the workspace. If you want to remove all the variables in the notebook, you can use the magic command %reset .

In mathematics, variables are usually associated with unknown numbers; in programming, variables are associated with a value of a certain type. There are many data types that can be assigned to variables. A data type is a classification of the type of information that is being stored in a variable. The basic data types that you will utilize throughout this book are boolean, int, float, string, list, tuple, dictionary, set. A formal description of these data types is given in the following sections.

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A variable is a segment of memory with a unique name used to hold data that will later be processed. Although each programming language has a different mechanism for declaring variables, the name and the data that will be assigned to each variable are always the same. They are capable of storing values of data types.

The assignment operator(=) assigns the value provided to its right to the variable name given to its left. Given is the basic syntax of variable declaration:

Assign Values to Multiple Variables in One Line

Given above is the mechanism for assigning just variables in Python but it is possible to assign multiple variables at the same time. Python assigns values from right to left. When assigning multiple variables in a single line, different variable names are provided to the left of the assignment operator separated by a comma. The same goes for their respective values except they should be to the right of the assignment operator.

While declaring variables in this fashion one must be careful with the order of the names and their corresponding value first variable name to the left of the assignment operator is assigned with the first value to its right and so on.

Variable assignment in a single line can also be done for different data types.

Not just simple variable assignment, assignment after performing some operation can also be done in the same way.

Assigning different operation results to multiple variable.

Here, we are storing different characters in a different variables.

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python
inline if statement while multiple variable assginment. 0. How to use the result of a function call as condition of the loop and in the body of the loop? 0. ... One line if assignment in python. 1. Python one-liner if else statement. 0. Assign, compare and use value in one line. 0. one line if statement. 3.
How to Write the Python if Statement in one Line
You may have seen this coming, but we can even write elif and else statements each in a single line. To do so, we use the same syntax as writing an if statement in a single line. Here's the general structure: if <expression_01>: <perform_action_01>. elif <expression_02>: <perform_action_02>.
How to use python if else in one line with examples
The general syntax of single if and else statement in Python is: bash. if condition: value_when_true else: value_when_false. Now if we wish to write this in one line using ternary operator, the syntax would be: bash. value_when_true if condition else value_when_false. In this syntax, first of all the else condition is evaluated.
Python One Line Conditional Assignment
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>. Operand.
Mastering Python's One-Line If Statements
The basic structure of a one-line if statement in Python follows the pattern: ... The syntax for the ternary operator in Python is: variable = value_if_true if condition else value_if_false This allows you to perform a conditional assignment in a single line without the need for separate if-else blocks.
Python Conditional Assignment (in 3 Ways)
Let's see a code snippet to understand it better. a = 10. b = 20 # assigning value to variable c based on condition. c = a if a > b else b. print(c) # output: 20. You can see we have conditionally assigned a value to variable c based on the condition a > b. 2. Using if-else statement.
Conditional Statements in Python
Python is one of a relatively small set of off-side rule languages. ... It is customary to write if <expr> on one line and <statement> indented on the following line like this: Python. if < expr >: < statement > ... A common use of the conditional expression is to select variable assignment. For example, suppose you want to find the larger of ...
if statement
Complexity: One-line statements can be less readable for complex conditions or actions. Use multi-line blocks for clarity. Nesting: You cannot nest ternary operators, but you can combine them with regular if statements. Variable Assignment: The ternary operator is mainly for expressions, not variable assignments. Use multi-line if blocks for ...
Python If-Else Statement in One Line
Before diving into If Else statements in one line, let's first make a short recap on regular conditionals. For example, you can check if a condition is true with the following syntax: age = 16 if age < 18: print('Go home.') The variable age is less than 18 in this case, so Go home. is printed to the console.
python
You can do it in one line as assignment expressions with the walrus (:=) operator (Python 3.8+): (wins := wins + 1) if num > num2 else (lose := lose + 1) but it doesn't make much sense to. You're modifying two different objects, and trying to cram both into one statement isn't especially logical.
One line if without else in Python
If your conditional involves an assignment, then you need to use the regular if statement.. Conclusion. This tutorial has shown you examples of writing a one line if without else statement in Python.. In practice, writing a one line if statement is discouraged as it means you're writing at least two statements in one line: the condition and the code to run when that condition is True.
If-Then-Else in One Line Python
Yes, you can write most if statements in a single line of Python using any of the following methods: Write the if statement without else branch as a Python one-liner: if 42 in range(100): print("42"). If you want to set a variable, use the ternary operator: x = "Alice" if "Jon" in "My name is Jonas" else "Bob".
Python's Assignment Operator: Write Robust Assignments
To create a new variable or to update the value of an existing one in Python, you'll use an assignment statement. This statement has the following three components: A left operand, which must be a variable. The assignment operator ( =) A right operand, which can be a concrete value, an object, or an expression.
Variable Assignment
Variable Assignment. Think of a variable as a name attached to a particular object. In Python, variables need not be declared or defined in advance, as is the case in many other programming languages. To create a variable, you just assign it a value and then start using it. Assignment is done with a single equals sign ( = ).
python
x = 2. elif i < 100: x = 1. else: x = 0. If you want to use the above-mentioned code in one line, you can use the following: x = 2 if i > 100 else 1 if i < 100 else 0. On doing so, x will be assigned 2 if i > 100, 1 if i < 100 and 0 if i = 100. answered Jan 10, 2019 at 3:24.
PEP 572
Unparenthesized assignment expressions are prohibited at the top level of the right hand side of an assignment statement. Example: y0 = y1 := f(x) # INVALID y0 = (y1 := f(x)) # Valid, though discouraged. Again, this rule is included to avoid two visually similar ways of saying the same thing.
How To Use Assignment Expressions in Python
Python 3.8, released in October 2019, adds assignment expressions to Python via the := syntax. The assignment expression syntax is also sometimes called "the walrus operator" because := vaguely resembles a walrus with tusks. Assignment expressions allow variable assignments to occur inside of larger expressions.
Variables and Assignment
Variables and Assignment¶. When programming, it is useful to be able to store information in variables. A variable is a string of characters and numbers associated with a piece of information. The assignment operator, denoted by the "=" symbol, is the operator that is used to assign values to variables in Python.The line x=1 takes the known value, 1, and assigns that value to the variable ...
python
One line if assignment in python. 0. Assigning variable values to if statement. 0. Assign, compare and use value in one line. 2. Assigning multiple values in an inline if/else statement. 0. How to do one line if condition assignment in Python. Hot Network Questions
Assigning multiple variables in one line in Python
Python assigns values from right to left. When assigning multiple variables in a single line, different variable names are provided to the left of the assignment operator separated by a comma. The same goes for their respective values except they should be to the right of the assignment operator. While declaring variables in this fashion one ...
Python Multiple Assignment Statements In One Line
All credit goes to @MarkDickinson, who answered this in a comment: Notice the + in (target_list "=")+, which means one or more copies.In foo = bar = 5, there are two (target_list "=") productions, and the expression_list part is just 5. All target_list productions (i.e. things that look like foo =) in an assignment statement get assigned, from left to right, to the expression_list on the right ...