assignment python if else

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! 😊

Conditional Statements in Python

Table of Contents

Introduction to the if Statement

Python: it’s all about the indentation, what do other languages do, which is better, the else and elif clauses, one-line if statements, conditional expressions (python’s ternary operator), the python pass statement.

Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Conditional Statements in Python (if/elif/else)

From the previous tutorials in this series, you now have quite a bit of Python code under your belt. Everything you have seen so far has consisted of sequential execution , in which statements are always performed one after the next, in exactly the order specified.

But the world is often more complicated than that. Frequently, a program needs to skip over some statements, execute a series of statements repetitively, or choose between alternate sets of statements to execute.

That is where control structures come in. A control structure directs the order of execution of the statements in a program (referred to as the program’s control flow ).

Here’s what you’ll learn in this tutorial: You’ll encounter your first Python control structure, the if statement.

In the real world, we commonly must evaluate information around us and then choose one course of action or another based on what we observe:

If the weather is nice, then I’ll mow the lawn. (It’s implied that if the weather isn’t nice, then I won’t mow the lawn.)

In a Python program, the if statement is how you perform this sort of decision-making. It allows for conditional execution of a statement or group of statements based on the value of an expression.

The outline of this tutorial is as follows:

• First, you’ll get a quick overview of the if statement in its simplest form.
• Next, using the if statement as a model, you’ll see why control structures require some mechanism for grouping statements together into compound statements or blocks . You’ll learn how this is done in Python.
• Lastly, you’ll tie it all together and learn how to write complex decision-making code.

Ready? Here we go!

Take the Quiz: Test your knowledge with our interactive “Python Conditional Statements” quiz. You’ll receive a score upon completion to help you track your learning progress:

Interactive Quiz

Test your understanding of Python conditional statements

We’ll start by looking at the most basic type of if statement. In its simplest form, it looks like this:

In the form shown above:

• <expr> is an expression evaluated in a Boolean context, as discussed in the section on Logical Operators in the Operators and Expressions in Python tutorial.
• <statement> is a valid Python statement, which must be indented. (You will see why very soon.)

If <expr> is true (evaluates to a value that is “truthy”), then <statement> is executed. If <expr> is false, then <statement> is skipped over and not executed.

Note that the colon ( : ) following <expr> is required. Some programming languages require <expr> to be enclosed in parentheses, but Python does not.

Here are several examples of this type of if statement:

Note: If you are trying these examples interactively in a REPL session, you’ll find that, when you hit Enter after typing in the print('yes') statement, nothing happens.

Because this is a multiline statement, you need to hit Enter a second time to tell the interpreter that you’re finished with it. This extra newline is not necessary in code executed from a script file.

Grouping Statements: Indentation and Blocks

So far, so good.

But let’s say you want to evaluate a condition and then do more than one thing if it is true:

If the weather is nice, then I will: Mow the lawn Weed the garden Take the dog for a walk (If the weather isn’t nice, then I won’t do any of these things.)

In all the examples shown above, each if <expr>: has been followed by only a single <statement> . There needs to be some way to say “If <expr> is true, do all of the following things.”

The usual approach taken by most programming languages is to define a syntactic device that groups multiple statements into one compound statement or block . A block is regarded syntactically as a single entity. When it is the target of an if statement, and <expr> is true, then all the statements in the block are executed. If <expr> is false, then none of them are.

Virtually all programming languages provide the capability to define blocks, but they don’t all provide it in the same way. Let’s see how Python does it.

Python follows a convention known as the off-side rule , a term coined by British computer scientist Peter J. Landin. (The term is taken from the offside law in association football.) Languages that adhere to the off-side rule define blocks by indentation. Python is one of a relatively small set of off-side rule languages .

Recall from the previous tutorial on Python program structure that indentation has special significance in a Python program. Now you know why: indentation is used to define compound statements or blocks. In a Python program, contiguous statements that are indented to the same level are considered to be part of the same block.

Thus, a compound if statement in Python looks like this:

Here, all the statements at the matching indentation level (lines 2 to 5) are considered part of the same block. The entire block is executed if <expr> is true, or skipped over if <expr> is false. Either way, execution proceeds with <following_statement> (line 6) afterward.

Notice that there is no token that denotes the end of the block. Rather, the end of the block is indicated by a line that is indented less than the lines of the block itself.

Note: In the Python documentation, a group of statements defined by indentation is often referred to as a suite . This tutorial series uses the terms block and suite interchangeably.

Consider this script file foo.py :

Running foo.py produces this output:

The four print() statements on lines 2 to 5 are indented to the same level as one another. They constitute the block that would be executed if the condition were true. But it is false, so all the statements in the block are skipped. After the end of the compound if statement has been reached (whether the statements in the block on lines 2 to 5 are executed or not), execution proceeds to the first statement having a lesser indentation level: the print() statement on line 6.

Blocks can be nested to arbitrary depth. Each indent defines a new block, and each outdent ends the preceding block. The resulting structure is straightforward, consistent, and intuitive.

Here is a more complicated script file called blocks.py :

The output generated when this script is run is shown below:

Note: In case you have been wondering, the off-side rule is the reason for the necessity of the extra newline when entering multiline statements in a REPL session. The interpreter otherwise has no way to know that the last statement of the block has been entered.

Perhaps you’re curious what the alternatives are. How are blocks defined in languages that don’t adhere to the off-side rule?

The tactic used by most programming languages is to designate special tokens that mark the start and end of a block. For example, in Perl blocks are defined with pairs of curly braces ( {} ) like this:

C/C++, Java , and a whole host of other languages use curly braces in this way.

Other languages, such as Algol and Pascal, use keywords begin and end to enclose blocks.

Better is in the eye of the beholder. On the whole, programmers tend to feel rather strongly about how they do things. Debate about the merits of the off-side rule can run pretty hot.

On the plus side:

• Python’s use of indentation is clean, concise, and consistent.
• In programming languages that do not use the off-side rule, indentation of code is completely independent of block definition and code function. It’s possible to write code that is indented in a manner that does not actually match how the code executes, thus creating a mistaken impression when a person just glances at it. This sort of mistake is virtually impossible to make in Python.
• Use of indentation to define blocks forces you to maintain code formatting standards you probably should be using anyway.

On the negative side:

• Many programmers don’t like to be forced to do things a certain way. They tend to have strong opinions about what looks good and what doesn’t, and they don’t like to be shoehorned into a specific choice.
• Some editors insert a mix of space and tab characters to the left of indented lines, which makes it difficult for the Python interpreter to determine indentation levels. On the other hand, it is frequently possible to configure editors not to do this. It generally isn’t considered desirable to have a mix of tabs and spaces in source code anyhow, no matter the language.

Like it or not, if you’re programming in Python, you’re stuck with the off-side rule. All control structures in Python use it, as you will see in several future tutorials.

For what it’s worth, many programmers who have been used to languages with more traditional means of block definition have initially recoiled at Python’s way but have gotten comfortable with it and have even grown to prefer it.

Now you know how to use an if statement to conditionally execute a single statement or a block of several statements. It’s time to find out what else you can do.

Sometimes, you want to evaluate a condition and take one path if it is true but specify an alternative path if it is not. This is accomplished with an else clause:

If <expr> is true, the first suite is executed, and the second is skipped. If <expr> is false, the first suite is skipped and the second is executed. Either way, execution then resumes after the second suite. Both suites are defined by indentation, as described above.

In this example, x is less than 50 , so the first suite (lines 4 to 5) are executed, and the second suite (lines 7 to 8) are skipped:

Here, on the other hand, x is greater than 50 , so the first suite is passed over, and the second suite executed:

There is also syntax for branching execution based on several alternatives. For this, use one or more elif (short for else if ) clauses. Python evaluates each <expr> in turn and executes the suite corresponding to the first that is true. If none of the expressions are true, and an else clause is specified, then its suite is executed:

An arbitrary number of elif clauses can be specified. The else clause is optional. If it is present, there can be only one, and it must be specified last:

At most, one of the code blocks specified will be executed. If an else clause isn’t included, and all the conditions are false, then none of the blocks will be executed.

Note: Using a lengthy if / elif / else series can be a little inelegant, especially when the actions are simple statements like print() . In many cases, there may be a more Pythonic way to accomplish the same thing.

Here’s one possible alternative to the example above using the dict.get() method:

Recall from the tutorial on Python dictionaries that the dict.get() method searches a dictionary for the specified key and returns the associated value if it is found, or the given default value if it isn’t.

An if statement with elif clauses uses short-circuit evaluation, analogous to what you saw with the and and or operators. Once one of the expressions is found to be true and its block is executed, none of the remaining expressions are tested. This is demonstrated below:

The second expression contains a division by zero, and the third references an undefined variable var . Either would raise an error, but neither is evaluated because the first condition specified is true.

It is customary to write if <expr> on one line and <statement> indented on the following line like this:

But it is permissible to write an entire if statement on one line. The following is functionally equivalent to the example above:

There can even be more than one <statement> on the same line, separated by semicolons:

But what does this mean? There are two possible interpretations:

If <expr> is true, execute <statement_1> .

Then, execute <statement_2> ... <statement_n> unconditionally, irrespective of whether <expr> is true or not.

If <expr> is true, execute all of <statement_1> ... <statement_n> . Otherwise, don’t execute any of them.

Python takes the latter interpretation. The semicolon separating the <statements> has higher precedence than the colon following <expr> —in computer lingo, the semicolon is said to bind more tightly than the colon. Thus, the <statements> are treated as a suite, and either all of them are executed, or none of them are:

Multiple statements may be specified on the same line as an elif or else clause as well:

While all of this works, and the interpreter allows it, it is generally discouraged on the grounds that it leads to poor readability, particularly for complex if statements. PEP 8 specifically recommends against it.

As usual, it is somewhat a matter of taste. Most people would find the following more visually appealing and easier to understand at first glance than the example above:

If an if statement is simple enough, though, putting it all on one line may be reasonable. Something like this probably wouldn’t raise anyone’s hackles too much:

Python supports one additional decision-making entity called a conditional expression. (It is also referred to as a conditional operator or ternary operator in various places in the Python documentation.) Conditional expressions were proposed for addition to the language in PEP 308 and green-lighted by Guido in 2005.

In its simplest form, the syntax of the conditional expression is as follows:

This is different from the if statement forms listed above because it is not a control structure that directs the flow of program execution. It acts more like an operator that defines an expression. In the above example, <conditional_expr> is evaluated first. If it is true, the expression evaluates to <expr1> . If it is false, the expression evaluates to <expr2> .

Notice the non-obvious order: the middle expression is evaluated first, and based on that result, one of the expressions on the ends is returned. Here are some examples that will hopefully help clarify:

Note: Python’s conditional expression is similar to the <conditional_expr> ? <expr1> : <expr2> syntax used by many other languages—C, Perl and Java to name a few. In fact, the ?: operator is commonly called the ternary operator in those languages, which is probably the reason Python’s conditional expression is sometimes referred to as the Python ternary operator.

You can see in PEP 308 that the <conditional_expr> ? <expr1> : <expr2> syntax was considered for Python but ultimately rejected in favor of the syntax shown above.

A common use of the conditional expression is to select variable assignment. For example, suppose you want to find the larger of two numbers. Of course, there is a built-in function, max() , that does just this (and more) that you could use. But suppose you want to write your own code from scratch.

You could use a standard if statement with an else clause:

But a conditional expression is shorter and arguably more readable as well:

Remember that the conditional expression behaves like an expression syntactically. It can be used as part of a longer expression. The conditional expression has lower precedence than virtually all the other operators, so parentheses are needed to group it by itself.

In the following example, the + operator binds more tightly than the conditional expression, so 1 + x and y + 2 are evaluated first, followed by the conditional expression. The parentheses in the second case are unnecessary and do not change the result:

If you want the conditional expression to be evaluated first, you need to surround it with grouping parentheses. In the next example, (x if x > y else y) is evaluated first. The result is y , which is 40 , so z is assigned 1 + 40 + 2 = 43 :

If you are using a conditional expression as part of a larger expression, it probably is a good idea to use grouping parentheses for clarification even if they are not needed.

Conditional expressions also use short-circuit evaluation like compound logical expressions. Portions of a conditional expression are not evaluated if they don’t need to be.

In the expression <expr1> if <conditional_expr> else <expr2> :

• If <conditional_expr> is true, <expr1> is returned and <expr2> is not evaluated.
• If <conditional_expr> is false, <expr2> is returned and <expr1> is not evaluated.

As before, you can verify this by using terms that would raise an error:

In both cases, the 1/0 terms are not evaluated, so no exception is raised.

Conditional expressions can also be chained together, as a sort of alternative if / elif / else structure, as shown here:

It’s not clear that this has any significant advantage over the corresponding if / elif / else statement, but it is syntactically correct Python.

Occasionally, you may find that you want to write what is called a code stub: a placeholder for where you will eventually put a block of code that you haven’t implemented yet.

In languages where token delimiters are used to define blocks, like the curly braces in Perl and C, empty delimiters can be used to define a code stub. For example, the following is legitimate Perl or C code:

Here, the empty curly braces define an empty block. Perl or C will evaluate the expression x , and then even if it is true, quietly do nothing.

Because Python uses indentation instead of delimiters, it is not possible to specify an empty block. If you introduce an if statement with if <expr>: , something has to come after it, either on the same line or indented on the following line.

Consider this script foo.py :

If you try to run foo.py , you’ll get this:

The Python pass statement solves this problem. It doesn’t change program behavior at all. It is used as a placeholder to keep the interpreter happy in any situation where a statement is syntactically required, but you don’t really want to do anything:

Now foo.py runs without error:

With the completion of this tutorial, you are beginning to write Python code that goes beyond simple sequential execution:

• You were introduced to the concept of control structures . These are compound statements that alter program control flow —the order of execution of program statements.
• You learned how to group individual statements together into a block or suite .
• You encountered your first control structure, the if statement, which makes it possible to conditionally execute a statement or block based on evaluation of program data.

All of these concepts are crucial to developing more complex Python code.

The next two tutorials will present two new control structures: the while statement and the for statement. These structures facilitate iteration , execution of a statement or block of statements repeatedly.

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In computer programming, the if statement is a conditional statement. It is used to execute a block of code only when a specific condition is met. For example,

Suppose we need to assign different grades to students based on their scores.

• If a student scores above 90 , assign grade A
• If a student scores above 75 , assign grade B
• If a student scores above 65 , assign grade C

These conditional tasks can be achieved using the if statement.

• Python if Statement

An if statement executes a block of code only when the specified condition is met.

Here, condition is a boolean expression, such as number > 5 , that evaluates to either True or False .

• If condition evaluates to True , the body of the if statement is executed.
• If condition evaluates to False , the body of the if statement will be skipped from execution.

Let's look at an example.

• Example: Python if Statement

Sample Output 1

If user enters 10 , the condition number > 0 evaluates to True . Therefore, the body of if is executed.

Sample Output 2

If user enters -2 , the condition number > 0 evaluates to False . Therefore, the body of if is skipped from execution.

Indentation in Python

Python uses indentation to define a block of code, such as the body of an if statement. For example,

Here, the body of if has two statements. We know this because two statements (immediately after if ) start with indentation.

We usually use four spaces for indentation in Python, although any number of spaces works as long as we are consistent.

You will get an error if you write the above code like this:

Here, we haven't used indentation after the if statement. In this case, Python thinks our if statement is empty, which results in an error.

An if statement can have an optional else clause. The else statement executes if the condition in the if statement evaluates to False .

Here, if the condition inside the if statement evaluates to

• True - the body of if executes, and the body of else is skipped.
• False - the body of else executes, and the body of if is skipped

• Example: Python if…else Statement

If user enters 10 , the condition number > 0 evalutes to True . Therefore, the body of if is executed and the body of else is skipped.

If user enters 0 , the condition number > 0 evalutes to False . Therefore, the body of if is skipped and the body of else is executed.

• Python if…elif…else Statement

The if...else statement is used to execute a block of code among two alternatives.

However, if we need to make a choice between more than two alternatives, we use the if...elif...else statement.

• Example: Python if…elif…else Statement

Here, the first condition, number > 0 , evaluates to False . In this scenario, the second condition is checked.

The second condition, number < 0 , evaluates to True . Therefore, the statements inside the elif block is executed.

In the above program, it is important to note that regardless the value of number variable, only one block of code will be executed.

• Python Nested if Statements

It is possible to include an if statement inside another if statement. For example,

Here's how this program works.

More on Python if…else Statement

In certain situations, the if statement can be simplified into a single line. For example,

This code can be compactly written as

This one-liner approach retains the same functionality but in a more concise format.

Python doesn't have a ternary operator. However, we can use if...else to work like a ternary operator in other languages. For example,

can be written as

If needed, we can use logical operators such as and and or to create complex conditions to work with an if statement.

Here, we used the logical operator and to add two conditions in the if statement.

We also used >= (comparison operator) to compare two values.

Logical and comparison operators are often used with if...else statements. Visit Python Operators to learn more.

Table of Contents

• Introduction

Before we wrap up, let’s put your knowledge of Python if else to the test! Can you solve the following challenge?

Write a function to check whether a student passed or failed his/her examination.

• Assume the pass marks to be 50 .
• Return Passed if the student scored more than 50. Otherwise, return Failed .

Video: Python if...else Statement

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How to Use Conditional Statements in Python – Examples of if, else, and elif

Conditional statements are an essential part of programming in Python. They allow you to make decisions based on the values of variables or the result of comparisons.

In this article, we'll explore how to use if, else, and elif statements in Python, along with some examples of how to use them in practice.

How to Use the if Statement in Python

The if statement allows you to execute a block of code if a certain condition is true. Here's the basic syntax:

The condition can be any expression that evaluates to a Boolean value (True or False). If the condition is True, the code block indented below the if statement will be executed. If the condition is False, the code block will be skipped.

Here's an example of how to use an if statement to check if a number is positive:

In this example, we use the > operator to compare the value of num to 0. If num is greater than 0, the code block indented below the if statement will be executed, and the message "The number is positive." will be printed.

How to Use the else Statement in Python

The else statement allows you to execute a different block of code if the if condition is False. Here's the basic syntax:

If the condition is True, the code block indented below the if statement will be executed, and the code block indented below the else statement will be skipped.

If the condition is False, the code block indented below the else statement will be executed, and the code block indented below the if statement will be skipped.

Here's an example of how to use an if-else statement to check if a number is positive or negative:

In this example, we use an if-else statement to check if num is greater than 0. If it is, the message "The number is positive." is printed. If it is not (that is, num is negative or zero), the message "The number is negative." is printed.

How to Use the elif Statement in Python

The elif statement allows you to check multiple conditions in sequence, and execute different code blocks depending on which condition is true. Here's the basic syntax:

The elif statement is short for "else if", and can be used multiple times to check additional conditions.

Here's an example of how to use an if-elif-else statement to check if a number is positive, negative, or zero:

Use Cases For Conditional Statements

Example 1: checking if a number is even or odd..

In this example, we use the modulus operator (%) to check if num is evenly divisible by 2.

If the remainder of num divided by 2 is 0, the condition num % 2 == 0 is True, and the code block indented below the if statement will be executed. It will print the message "The number is even."

If the remainder is not 0, the condition is False, and the code block indented below the else statement will be executed, printing the message "The number is odd."

Example 2: Assigning a letter grade based on a numerical score

In this example, we use an if-elif-else statement to assign a letter grade based on a numerical score.

The if statement checks if the score is greater than or equal to 90. If it is, the grade is set to "A". If not, the first elif statement checks if the score is greater than or equal to 80. If it is, the grade is set to "B". If not, the second elif statement checks if the score is greater than or equal to 70, and so on. If none of the conditions are met, the else statement assigns the grade "F".

Example 3: Checking if a year is a leap year

In this example, we use nested if statements to check if a year is a leap year. A year is a leap year if it is divisible by 4, except for years that are divisible by 100 but not divisible by 400.

The outer if statement checks if year is divisible by 4. If it is, the inner if statement checks if it is also divisible by 100. If it is, the innermost if statement checks if it is divisible by 400. If it is, the code block indented below that statement will be executed, printing the message "is a leap year."

If it is not, the code block indented below the else statement inside the inner if statement will be executed, printing the message "is not a leap year.".

If the year is not divisible by 4, the code block indented below the else statement of the outer if statement will be executed, printing the message "is not a leap year."

Example 4: Checking if a string contains a certain character

In this example, we use the in operator to check if the character char is present in the string string. If it is, the condition char in string is True, and the code block indented below the if statement will be executed, printing the message "The string contains the character" followed by the character itself.

If char is not present in string, the condition is False, and the code block indented below the else statement will be executed, printing the message "The string does not contain the character" followed by the character itself.

Conditional statements (if, else, and elif) are fundamental programming constructs that allow you to control the flow of your program based on conditions that you specify. They provide a way to make decisions in your program and execute different code based on those decisions.

In this article, we have seen several examples of how to use these statements in Python, including checking if a number is even or odd, assigning a letter grade based on a numerical score, checking if a year is a leap year, and checking if a string contains a certain character.

By mastering these statements, you can create more powerful and versatile programs that can handle a wider range of tasks and scenarios.

It is important to keep in mind that proper indentation is crucial when using conditional statements in Python, as it determines which code block is executed based on the condition.

With practice, you will become proficient in using these statements to create more complex and effective Python programs.

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How to use python if else in one line with examples

December 31, 2023

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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Python One Line Conditional Assignment

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

Example : Say, you start with the following code.

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 .

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:

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:

• Python One Line Ternary

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 :

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

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.

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.

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

Python One-Liners Book: Master the Single Line First!

Python programmers will improve their computer science skills with these useful one-liners.

Python One-Liners will teach you how to read and write “one-liners”: concise statements of useful functionality packed into a single line of code. You’ll learn how to systematically unpack and understand any line of Python code, and write eloquent, powerfully compressed Python like an expert.

The book’s five chapters cover (1) tips and tricks, (2) regular expressions, (3) machine learning, (4) core data science topics, and (5) useful algorithms.

Detailed explanations of one-liners introduce key computer science concepts and boost your coding and analytical skills . You’ll learn about advanced Python features such as list comprehension , slicing , lambda functions , regular expressions , map and reduce functions, and slice assignments .

You’ll also learn how to:

• Leverage data structures to solve real-world problems , like using Boolean indexing to find cities with above-average pollution
• Use NumPy basics such as array , shape , axis , type , broadcasting , advanced indexing , slicing , sorting , searching , aggregating , and statistics
• Calculate basic statistics of multidimensional data arrays and the K-Means algorithms for unsupervised learning
• Create more advanced regular expressions using grouping and named groups , negative lookaheads , escaped characters , whitespaces, character sets (and negative characters sets ), and greedy/nongreedy operators
• Understand a wide range of computer science topics , including anagrams , palindromes , supersets , permutations , factorials , prime numbers , Fibonacci numbers, obfuscation , searching , and algorithmic sorting

By the end of the book, you’ll know how to write Python at its most refined , and create concise, beautiful pieces of “Python art” in merely a single line.

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Python - if, elif, else Conditions

By default, statements in the script are executed sequentially from the first to the last. If the processing logic requires so, the sequential flow can be altered in two ways:

Python uses the if keyword to implement decision control. Python's syntax for executing a block conditionally is as below:

Any Boolean expression evaluating to True or False appears after the if keyword. Use the : symbol and press Enter after the expression to start a block with an increased indent. One or more statements written with the same level of indent will be executed if the Boolean expression evaluates to True .

To end the block, decrease the indentation. Subsequent statements after the block will be executed out of the if condition. The following example demonstrates the if condition.

In the above example, the expression price < 100 evaluates to True , so it will execute the block. The if block starts from the new line after : and all the statements under the if condition starts with an increased indentation, either space or tab. Above, the if block contains only one statement. The following example has multiple statements in the if condition.

Above, the if condition contains multiple statements with the same indentation. If all the statements are not in the same indentation, either space or a tab then it will raise an IdentationError .

The statements with the same indentation level as if condition will not consider in the if block. They will consider out of the if condition.

The following example demonstrates multiple if conditions.

Notice that each if block contains a statement in a different indentation, and that's valid because they are different from each other.

else Condition

Along with the if statement, the else condition can be optionally used to define an alternate block of statements to be executed if the boolean expression in the if condition evaluates to False .

As mentioned before, the indented block starts after the : symbol, after the boolean expression. It will get executed when the condition is True . We have another block that should be executed when the if condition is False . First, complete the if block by a backspace and write else , put add the : symbol in front of the new block to begin it, and add the required statements in the block.

In the above example, the if condition price >= 100 is False , so the else block will be executed. The else block can also contain multiple statements with the same indentation; otherwise, it will raise the IndentationError .

Note that you cannot have multiple else blocks, and it must be the last block.

elif Condition

Use the elif condition is used to include multiple conditional expressions after the if condition or between the if and else conditions.

The elif block is executed if the specified condition evaluates to True .

In the above example, the elif conditions are applied after the if condition. Python will evalute the if condition and if it evaluates to False then it will evalute the elif blocks and execute the elif block whose expression evaluates to True . If multiple elif conditions become True , then the first elif block will be executed.

The following example demonstrates if, elif, and else conditions.

All the if, elif, and else conditions must start from the same indentation level, otherwise it will raise the IndentationError .

Nested if, elif, else Conditions

Python supports nested if, elif, and else condition. The inner condition must be with increased indentation than the outer condition, and all the statements under the one block should be with the same indentation.

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10 if-else Practice Problems in Python

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• python programming

If you're trying to learn Python, you need to practice! These ten if-else Python practice problems provide you some hands-on experience. And don’t worry – we’ve provided full code solutions and detailed explanations!

Python is particularly good for beginners to learn. Its clear syntax can be read almost as clearly as a normal sentence. The if-else statement is a good example of this; it lets you build logic into your programs. This article will walk you through ten i f-else practice exercises in Python. Each one is specifically designed for beginners, helping you hone your understanding of if-else statements.

The exercises in this article are taken directly from our courses, including Python Basics: Part 1 . This course helps absolute beginners understand computer programming with Python; if you’ve never written a line of code, this course is a great place to start.

As the saying goes, practice makes perfect. Learning Python is no exception. As we discuss in Different Ways to Practice Python , doing online courses is a great way to practice a variety of topics. The more you practice writing code, the more comfortable and confident you'll become in your abilities. So – whether you aim to build your own software, delve into data science, or automate mundane tasks – dedicating time to practice is key.

The Python if-else Statement

Before we dive into the exercises, let's briefly discuss the workings of if-else statements in Python. An if-else statement allows your program to make decisions based on certain conditions. The syntax is straightforward:

• You provide a condition to evaluate within the if
• If that condition is true, the corresponding block of code is executed.
• If the condition is false, the code within the optional else block is executed instead.

Here's a simple example:

In this example, if the value of x is greater than 5, the program will print "x is greater than 5"; otherwise, it will print "x is not greater than 5".

This concept forms the backbone of decision-making in Python programs.

To get the most benefit from this article, attempt the problems before reading the solutions. Some of these exercises will have several possible solutions, so also try to think about alternative ways to solve the same problem. Let’s get started.

10 if-else Python Practice Exercises

Exercise 1: are you tall.

Exercise: Write a program that will ask the user for their height in centimeters. Use the input() built-in function to do this. If the height is more than 185 centimeters, print the following line of code:

Explanation: This is a similar syntax to the above template example. The height variable stores the user input as an integer and tests if it’s greater than 185 cm. If so, the message is printed. Notice there is no else; this means if the height is less than or equal to 185, nothing happens.

Exercise 2: More Options

Exercise: Write a program that will ask the user the following question:

• If the user answers y , print: Wrong! Canberra is the capital!
• If the user answers n , print: Correct!
• If the user answers anything else, print: I do not understand your answer!

Explanation: The if statement can be extended with an elif (i.e. else if) to check any answer that returns False to the first condition. If the elif statement also returns False , the indented block under the else is executed.

Exercise 3: Check If a Character Is in a String

Exercise: Write a program to ask the user to do the following:

• Provide the name of a country that does not contain any lowercase a or e letters. (Use the prompt: The country is : )
• If the user provides a correct string (i.e. one with no a or e inside it), print: You won... unless you made this name up!
• Otherwise, print: You lost!

(By the way, possible answers include Hong Kong, Cyprus, and Togo.)

Explanation: This code prompts the user to input a country name using the built-in input() function. Then, it checks if the letters 'a' or 'e' is present in the inputted country name using the in operator. If either letter is found, it prints "You lost!". Otherwise, it prints "You won... unless you made this name up!" using the else statement.

Exercise 4: Count Letter Frequency in a String

Exercise: The letter e is said to be the most frequent letter in the English language. Count and print how many times this letter appears in the poem below:

John Knox was a man of wondrous might, And his words ran high and shrill, For bold and stout was his spirit bright, And strong was his stalwart will. Kings sought in vain his mind to chain, And that giant brain to control, But naught on plain or stormy main Could daunt that mighty soul. John would sit and sigh till morning cold Its shining lamps put out, For thoughts untold on his mind lay hold, And brought but pain and doubt. But light at last on his soul was cast, Away sank pain and sorrow, His soul is gay, in a fair to-day, And looks for a bright to-morrow. (Source: "Unidentified," in Current Opinion, July 1888)

Explanation: The solution starts by initializing a counter with the value of 0. Then a for loop is used to loop through every letter in the string variable poem. The if statement tests if the current letter is an e; if it is, the counter is incremented by 1. Finally, the result of the counter is printed. Note that there is no else needed in this solution, since nothing should happen if the letter is not an e.

Exercise 5: Format Recipes

Exercise: Anne loves cooking and she tries out various recipes from the Internet. Sometimes, though, she finds recipes that are written as a single paragraph of text. For instance:

When you're in the middle of cooking, such a paragraph is difficult to read. Anne would prefer an ordered list, like this:

• Cut a slit into the chicken breast.
• Stuff it with mustard, mozzarella and cheddar.
• Secure the whole thing with rashers of bacon.
• Roast for 20 minutes at 200C.

Write a Python script that accepts a recipe string from the user ('Paste your recipe: ') and prints an ordered list of sentences, just like in the example above.

Each sentence of both the input and output should end with a single period.

Explanation: The solution takes a recipe input from the user with input() , then iterates through each character in the recipe. Whenever it encounters a period ( '.' ), it increments a counter, adds a new line character ( '\n' ) and the incremented counter followed by a period to an ordered list. If it encounters any other character, it simply adds that character to the ordered list. Finally, it calculates the length of the counter, removes the last part of the ordered list to exclude the unnecessary counter, and prints the modified ordered list, which displays the recipe with numbered steps.

Exercise 6: Nested if Statements

Exercise: A leap year is a year that consists of 366 (not 365) days. It occurs roughly every four years. More specifically, a year is considered leap if it is either divisible by 4 but not by 100 or it is divisible by 400.

Write a program that asks the user for a year and replies with either leap year or not a leap year.

Explanation: This program takes a year from the user as input. It checks if the year is divisible by 4. If it is, it then checks if the year is divisible by 100. If it is, it further checks if the year is divisible by 400. If it is, it prints 'leap year' because it satisfies all the conditions for a leap year. This is all achieved with nesting.

If it's divisible by 4, but not by 100, or if it’s not divisible by 400, the program prints 'not a leap year'. If the year is not divisible by 100, it directly prints 'leap year' because it satisfies the basic condition for a leap year (being divisible by 4). If the year is not divisible by 4, it prints 'not a leap year'.

Exercise 7: Scrabble

Exercise: Create a function called can_build_word(letters_list, word) that takes a list of letters and a word. The function should return True if the word uses only the letters from the list.

Like in real Scrabble, you cannot use one tile twice in the same word. For example, if a list of letters contains only one 'a', ‘banana’ returns False. The function should also return False if the word contains any letters not in the list.

For example …

… should return True. On the other hand …

… should return False because there is only one 'f'.

One thing to keep in mind: Neither the provided letter list nor the word should be changed by your function. It may be tempting to remove letters from the letters list to remove the used tiles. This should not be the case. Instead, you should copy the passed list. Simply use the list() function and pass it another list:

This way we can safely operate on a copy of the list without changing the contents of the original list.

Explanation: The function starts by making a copy of the list of letters to avoid modifying the original. Then, it iterates through each letter in the word. For each letter, it checks if that letter exists in the list copy. If it does, it removes that letter from the copy, which ensures each letter is used only once.

If the letter doesn't exist in the copy of the letters list, it means the word cannot be formed using the available letters; it returns False. If all the letters in the word can be found and removed from the list copy, it means the word can indeed be formed; it returns True .

If you are looking for additional material to help you learn how to work with Python lists, our article 12 Beginner-Level Python List Exercises with Solutions has got you covered.

Exercise 8: for Loops and if Statements

Exercise: Write a program that asks for a positive integer and prints all of its divisors, one by one, on separate lines in ascending order.

A divisor is a number that divides a particular number with no remainder. For example, the divisors of 4 are 1, 2, and 4.

To check if a is a divisor of b , you can use the modulo operator %. a % b returns the remainder of dividing a by b . For example, 9 % 2 is 1 because 9 = (4 * 2) + 1, and 9 % 3 is 0 because 9 divides into 3 with no remainder. If a % b == 0 , then b is a divisor of a .

Explanation: This solution takes an input number from the user and then iterates through numbers from 1 to that input number (inclusive). For each number i in this range, it checks if the input number is divisible by i with no remainder ( number % i == 0 ). If the condition is satisfied, it means that i is a factor of the input number.

Looping is another important skill in Python. This problem could also be solved with a while loop. Take a look at 10 Python Loop Exercises with Solutions to learn more about for and while loops.

Exercise 9: if Statements and Data Structures

Exercise: Write a function named get_character_frequencies() that takes a string as input and returns a dictionary of lowercase characters as keys and the number of their occurrences as values. Ignore the case of a character.

Explanation: This function initializes an empty dictionary ( freq_dict ) to store the frequencies of characters. It iterates through each character in the input word using a for loop. Within the loop, it converts each character to lowercase using the lower() method; this ensures case-insensitive counting.

The program checks if the lowercase character is already a key in the dictionary. If it's not, it adds the character as a key and sets its frequency to 1. If it is already a key, it increments the frequency count for that character by 1 and returns the dictionary.

If you’re not familiar with how Python dictionaries work, Top 10 Python Dictionary Exercises for Beginners will give you experience working with this important data structure.

Exercise 10: if Statements and Multiple Returns

Exercise: Write a function named return_bigger(a, b) that takes two numbers and returns the bigger one. If the numbers are equal, return 0.

Explanation: When using if statements in a function, you can define a return value in the indented block of code under the if-else statement. This makes your code simpler and more readable – another great example of the clear syntax which makes Python such an attractive tool for all programmers.

Do You Want More Python Practice Exercises?

Learning anything new is a challenge. Relying on a variety of resources is a good way to get a broad background. Read some books and blogs to get a theoretical understanding, watch videos online, take advantage of the active Python community on platforms like Stack Overflow . And most importantly, do some hands-on practice. We discuss these points in detail in What’s the Best Way to Practice Python?

The Python practice exercises and solutions shown here were taken directly from several of our interactive online courses, including Python Basics Practice , Python Practice: Word Games , and Working with Strings in Python . The courses build up your skills and knowledge, giving you exposure to a variety of topics.

And if you’re already motivated to take your skills to the next level, have a go at another 10 Python Practice Exercises for Beginners with Solutions . Happy coding!

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Conditional Statements 

There are reading-comprehension exercises included throughout the text. These are meant to help you put your reading to practice. Solutions for the exercises are included at the bottom of this page.

In this section, we will be introduced to the if , else , and elif statements. These allow you to specify that blocks of code are to be executed only if specified conditions are found to be true, or perhaps alternative code if the condition is found to be false. For example, the following code will square x if it is a negative number, and will cube x if it is a positive number:

Please refer to the “Basic Python Object Types” subsection to recall the basics of the “boolean” type, which represents True and False values. We will extend that discussion by introducing comparison operations and membership-checking, and then expanding on the utility of the built-in bool type.

Comparison Operations 

Comparison statements will evaluate explicitly to either of the boolean-objects: True or False . There are eight comparison operations in Python:

The first six of these operators are familiar from mathematics:

Note that = and == have very different meanings. The former is the assignment operator, and the latter is the equality operator:

Python allows you to chain comparison operators to create “compound” comparisons:

Whereas == checks to see if two objects have the same value, the is operator checks to see if two objects are actually the same object. For example, creating two lists with the same contents produces two distinct lists, that have the same “value”:

Thus the is operator is most commonly used to check if a variable references the None object, or either of the boolean objects:

Use is not to check if two objects are distinct:

bool and Truth Values of Non-Boolean Objects 

Recall that the two boolean objects True and False formally belong to the int type in addition to bool , and are associated with the values 1 and 0 , respectively:

Likewise Python ascribes boolean values to non-boolean objects. For example,the number 0 is associated with False and non-zero numbers are associated with True . The boolean values of built-in objects can be evaluated with the built-in Python command bool :

and non-zero Python integers are associated with True :

The following built-in Python objects evaluate to False via bool :

Zero of any numeric type: 0 , 0.0 , 0j

Any empty sequence, such as an empty string or list: '' , tuple() , [] , numpy.array([])

Empty dictionaries and sets

Thus non-zero numbers and non-empty sequences/collections evaluate to True via bool .

The bool function allows you to evaluate the boolean values ascribed to various non-boolean objects. For instance, bool([]) returns False wherease bool([1, 2]) returns True .

if , else , and elif 

We now introduce the simple, but powerful if , else , and elif conditional statements. This will allow us to create simple branches in our code. For instance, suppose you are writing code for a video game, and you want to update a character’s status based on her/his number of health-points (an integer). The following code is representative of this:

Each if , elif , and else statement must end in a colon character, and the body of each of these statements is delimited by whitespace .

The following pseudo-code demonstrates the general template for conditional statements:

In practice this can look like:

In its simplest form, a conditional statement requires only an if clause. else and elif clauses can only follow an if clause.

Similarly, conditional statements can have an if and an else without an elif :

Conditional statements can also have an if and an elif without an else :

Note that only one code block within a single if-elif-else statement can be executed: either the “if-block” is executed, or an “elif-block” is executed, or the “else-block” is executed. Consecutive if-statements, however, are completely independent of one another, and thus their code blocks can be executed in sequence, if their respective conditional statements resolve to True .

Reading Comprehension: Conditional statements

Assume my_list is a list. Given the following code:

What will happen if my_list is [] ? Will IndexError be raised? What will first_item be?

Assume variable my_file is a string storing a filename, where a period denotes the end of the filename and the beginning of the file-type. Write code that extracts only the filename.

my_file will have at most one period in it. Accommodate cases where my_file does not include a file-type.

"code.py" \(\rightarrow\) "code"

"doc2.pdf" \(\rightarrow\) "doc2"

"hello_world" \(\rightarrow\) "hello_world"

Inline if-else statements 

Python supports a syntax for writing a restricted version of if-else statements in a single line. The following code:

can be written in a single line as:

This is suggestive of the general underlying syntax for inline if-else statements:

The inline if-else statement :

The expression A if <condition> else B returns A if bool(<condition>) evaluates to True , otherwise this expression will return B .

This syntax is highly restricted compared to the full “if-elif-else” expressions - no “elif” statement is permitted by this inline syntax, nor are multi-line code blocks within the if/else clauses.

Inline if-else statements can be used anywhere, not just on the right side of an assignment statement, and can be quite convenient:

We will see this syntax shine when we learn about comprehension statements. That being said, this syntax should be used judiciously. For example, inline if-else statements ought not be used in arithmetic expressions, for therein lies madness:

Short-Circuiting Logical Expressions 

Armed with our newfound understanding of conditional statements, we briefly return to our discussion of Python’s logic expressions to discuss “short-circuiting”. In Python, a logical expression is evaluated from left to right and will return its boolean value as soon as it is unambiguously determined, leaving any remaining portions of the expression unevaluated . That is, the expression may be short-circuited .

For example, consider the fact that an and operation will only return True if both of its arguments evaluate to True . Thus the expression False and <anything> is guaranteed to return False ; furthermore, when executed, this expression will return False without having evaluated bool(<anything>) .

To demonstrate this behavior, consider the following example:

According to our discussion, the pattern False and short-circuits this expression without it ever evaluating bool(1/0) . Reversing the ordering of the arguments makes this clear.

In practice, short-circuiting can be leveraged in order to condense one’s code. Suppose a section of our code is processing a variable x , which may be either a number or a string . Suppose further that we want to process x in a special way if it is an all-uppercased string. The code

is problematic because isupper can only be called once we are sure that x is a string; this code will raise an error if x is a number. We could instead write

but the more elegant and concise way of handling the nestled checking is to leverage our ability to short-circuit logic expressions.

See, that if x is not a string, that isinstance(x, str) will return False ; thus isinstance(x, str) and x.isupper() will short-circuit and return False without ever evaluating bool(x.isupper()) . This is the preferable way to handle this sort of checking. This code is more concise and readable than the equivalent nested if-statements.

Reading Comprehension: short-circuited expressions

Consider the preceding example of short-circuiting, where we want to catch the case where x is an uppercased string. What is the “bug” in the following code? Why does this fail to utilize short-circuiting correctly?

Links to Official Documentation 

Truth testing

Boolean operations

Comparisons

‘if’ statements

Reading Comprehension Exercise Solutions: 

Conditional statements

If my_list is [] , then bool(my_list) will return False , and the code block will be skipped. Thus first_item will be None .

First, check to see if . is even contained in my_file . If it is, find its index-position, and slice the string up to that index. Otherwise, my_file is already the file name.

Short-circuited expressions

fails to account for the fact that expressions are always evaluated from left to right. That is, bool(x.isupper()) will always be evaluated first in this instance and will raise an error if x is not a string. Thus the following isinstance(x, str) statement is useless.

Conditional expression (ternary operator) in Python

Python has a conditional expression (sometimes called a "ternary operator"). You can write operations like if statements in one line with conditional expressions.

• 6. Expressions - Conditional expressions — Python 3.11.3 documentation

Basics of the conditional expression (ternary operator)

If ... elif ... else ... by conditional expressions, list comprehensions and conditional expressions, lambda expressions and conditional expressions.

See the following article for if statements in Python.

• Python if statements (if, elif, else)

In Python, the conditional expression is written as follows.

The condition is evaluated first. If condition is True , X is evaluated and its value is returned, and if condition is False , Y is evaluated and its value is returned.

If you want to switch the value based on a condition, simply use the desired values in the conditional expression.

If you want to switch between operations based on a condition, simply describe each corresponding expression in the conditional expression.

An expression that does not return a value (i.e., an expression that returns None ) is also acceptable in a conditional expression. Depending on the condition, either expression will be evaluated and executed.

The above example is equivalent to the following code written with an if statement.

You can also combine multiple conditions using logical operators such as and or or .

• Boolean operators in Python (and, or, not)

By combining conditional expressions, you can write an operation like if ... elif ... else ... in one line.

However, it is difficult to understand, so it may be better not to use it often.

The following two interpretations are possible, but the expression is processed as the first one.

In the sample code below, which includes three expressions, the first expression is interpreted like the second, rather than the third:

By using conditional expressions in list comprehensions, you can apply operations to the elements of the list based on the condition.

See the following article for details on list comprehensions.

• List comprehensions in Python

Conditional expressions are also useful when you want to apply an operation similar to an if statement within lambda expressions.

In the example above, the lambda expression is assigned to a variable for convenience, but this is not recommended by PEP8.

Refer to the following article for more details on lambda expressions.

• Lambda expressions in Python

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Python If Else Statements – Conditional Statements

In both real life and programming, decision-making is crucial. We often face situations where we need to make choices, and based on those choices, we determine our next actions. Similarly, in programming, we encounter scenarios where we must make decisions to control the flow of our code.

Conditional statements in Python play a key role in determining the direction of program execution. Among these, If-Else statements are fundamental, providing a way to execute different blocks of code based on specific conditions. As the name suggests, If-Else statements offer two paths, allowing for different outcomes depending on the condition evaluated.

Types of Control Flow in Python

Python If Statement

Python if else statement, python nested if statement, python elif, ternary statement | short hand if else statement.

The if statement is the most simple decision-making statement. It is used to decide whether a certain statement or block of statements will be executed or not.

Flowchart of If Statement

Let’s look at the flow of code in the Python If statements.

Flowchart of Python if statement

Syntax of If Statement in Python

Here, the condition after evaluation will be either true or false. if the statement accepts boolean values – if the value is true then it will execute the block of statements below it otherwise not.

As we know, Python uses indentation to identify a block. So the block under the Python if statements will be identified as shown in the below example:  

Example of Python if Statement

As the condition present in the if statements in Python is false. So, the block below the if statement is executed.

The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But if we want to do something else if the condition is false, we can use the else statement with the if statement Python to execute a block of code when the Python if condition is false. 

Flowchart of If Else Statement

Let’s look at the flow of code in an if else Python statement.

Syntax of If Else in Python

Example of python if else statement.

The block of code following the else if in Python, the statement is executed as the condition present in the if statement is false after calling the statement which is not in the block(without spaces).

If Else in Python using List Comprehension

In this example, we are using an Python else if statement in a list comprehension with the condition that if the element of the list is odd then its digit sum will be stored else not.

A nested if is an if statement that is the target of another if statement. Nested if statements mean an if statement inside another if statement.

Yes, Python allows us to nest if statements within if statements. i.e., we can place an if statement inside another if statement.

Flowchart of Python Nested if Statement

Flowchart of Python Nested if statement

Example of Python Nested If Statement

In this example, we are showing nested if conditions in the code, All the If condition in Python will be executed one by one.

Here, a user can decide among multiple options. The if statements are executed from the top down.

As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final “else” statement will be executed.

Flowchart of Elif Statement in Python

Let’s look at the flow of control in if-elif-else ladder:

Flowchart of if-elif-else ladder

Example of Python if-elif-else ladder

In the example, we are showing single if in Python, multiple elif conditions, and single else condition.

Whenever there is only a single statement to be executed inside the if block then shorthand if can be used. The statement can be put on the same line as the if statement. 

Example of Python If shorthand

In the given example, we have a condition that if the number is less than 15, then further code will be executed.

Example of Short Hand If Else Statements

This can be used to write the if-else statements in a single line where only one statement is needed in both the if and else blocks. 

In the given example, we are printing True if the number is 15, or else it will print False.

Similar Reads:

• Python3 – if , if..else, Nested if, if-elif statements
• Using Else Conditional Statement With For loop in Python
• How to use if, else & elif in Python Lambda Functions

Python If Else Statements – Conditional Statements – FAQs

What is the conditional statement of if-else.

The if-else statement in Python is used to control the flow of the program based on a condition. It has the following syntax: if condition: # Execute this block if condition is True else: # Execute this block if condition is False For example: x = 10 if x > 5: print("x is greater than 5") else: print("x is not greater than 5")

How many else statements can a single if condition have in Python?

A single if condition can have at most one else statement. However, you can have multiple elif (else if) statements to check additional conditions if needed: x = 10 if x > 15: print("x is greater than 15") elif x > 5: print("x is greater than 5 but not greater than 15") else: print("x is 5 or less")

What are the different types of control statements in Python?

In Python, control statements are used to alter the flow of execution based on specific conditions or looping requirements. The main types of control statements are: Conditional statements : if , else , elif Looping statements : for , while Control flow statements : break , continue , pass , return

What are the two types of control statements?

The two primary types of control statements in Python are: Conditional statements : Used to execute code based on certain conditions ( if , else , elif ). Looping statements : Used to execute code repeatedly until a condition is met ( for , while ).

Are control statements and conditional statements the same?

No, control statements and conditional statements are not exactly the same. Conditional statements ( if , else , elif ) specifically deal with checking conditions and executing code based on whether those conditions are True or False . Control statements encompass a broader category that includes both conditional statements ( if , else , elif ) and looping statements ( for , while ), as well as other statements ( break , continue , pass , return ) that control the flow of execution in a program.

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Can we have assignment in a condition?

Is it possible to have assignment in a condition?

• 3 This is the first hit on google and the top answer is incorrect. Yes you can as of python.org/dev/peps/pep-0572 if a := somefunc(): #use a –  Rqomey Commented Sep 7, 2021 at 14:09

10 Answers 10

Why not try it out?

Update: This is possible (with different syntax) in Python 3.8

• 46 this is intentionally forbidden as Guido, benevolent python dictator, finds them unnecessary and more confusing than useful. It's the same reason there's no post-increment or pre-increment operators (++). –  Matt Boehm Commented Apr 8, 2010 at 22:53
• 6 he did allow the addition of augmented assigment in 2.0 because x = x + 1 requires additional lookup time while x += 1 was somewhat faster, but i'm sure he didn't even like doing that much. :-) –  wescpy Commented Apr 8, 2010 at 23:56
• 1 hope, this will become possible soon. Python is just slow to evolve –  Nik O'Lai Commented Aug 28, 2020 at 9:07
• 55 "why not try it out" - Because who knows what the syntax might be? Maybe OP tried that and it didn't work, but that doesn't mean the syntax isn't different, or that there's not a way to do it that's not intended –  Levi H Commented Sep 22, 2020 at 17:09
• 12 Fun fact: := (the assignment expression) is also known as the "walrus operator" because if considered as a text based emoji it looks like a walrus. :) –  jlsecrest Commented Nov 6, 2021 at 4:13

UPDATE - Original answer is near the bottom

Python 3.8 will bring in PEP572

Abstract This is a proposal for creating a way to assign to variables within an expression using the notation NAME := expr . A new exception, TargetScopeError is added, and there is one change to evaluation order.

https://lwn.net/Articles/757713/

The "PEP 572 mess" was the topic of a 2018 Python Language Summit session led by benevolent dictator for life (BDFL) Guido van Rossum. PEP 572 seeks to add assignment expressions (or "inline assignments") to the language, but it has seen a prolonged discussion over multiple huge threads on the python-dev mailing list—even after multiple rounds on python-ideas. Those threads were often contentious and were clearly voluminous to the point where many probably just tuned them out. At the summit, Van Rossum gave an overview of the feature proposal, which he seems inclined toward accepting, but he also wanted to discuss how to avoid this kind of thread explosion in the future.

https://www.python.org/dev/peps/pep-0572/#examples-from-the-python-standard-library

Examples from the Python standard library site.py 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 _pydecimal.py 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 copy.py 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) datetime.py 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 sysconfig.py 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 Simplifying list comprehensions A list comprehension can map and filter efficiently by capturing the condition: results = [(x, y, x/y) for x in input_data if (y := f(x)) > 0] Similarly, a subexpression can be reused within the main expression, by giving it a name on first use: stuff = [[y := f(x), x/y] for x in range(5)] Note that in both cases the variable y is bound in the containing scope (i.e. at the same level as results or stuff). Capturing condition values Assignment expressions can be used to good effect in the header of an if or while statement: # Loop-and-a-half while (command := input("> ")) != "quit": print("You entered:", command) # Capturing regular expression match objects # See, for instance, Lib/pydoc.py, which uses a multiline spelling # of this effect if match := re.search(pat, text): print("Found:", match.group(0)) # The same syntax chains nicely into 'elif' statements, unlike the # equivalent using assignment statements. elif match := re.search(otherpat, text): print("Alternate found:", match.group(0)) elif match := re.search(third, text): print("Fallback found:", match.group(0)) # Reading socket data until an empty string is returned while data := sock.recv(8192): print("Received data:", data) 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. Fork An example from the low-level UNIX world: if pid := os.fork(): # Parent code else: # Child code

Original answer

http://docs.python.org/tutorial/datastructures.html

Note that in Python, unlike C, assignment cannot occur inside expressions. C programmers may grumble about this, but it avoids a common class of problems encountered in C programs: typing = in an expression when == was intended.

http://effbot.org/pyfaq/why-can-t-i-use-an-assignment-in-an-expression.htm

• 1 I like this answer because it actually points out why such a "feature" might have been deliberately left out of Python. When teaching beginner's programming, I have seen many make this mistake if (foo = 'bar') while intending to test the value of foo . –  Jonathan Cross Commented Feb 19, 2019 at 23:25
• 3 @JonathanCross, This "feature" is actually going to be added in 3.8. It is unlikely to be used as sparingly as it should - but at least it is not a plain = –  John La Rooy Commented Feb 19, 2019 at 23:52
• @JohnLaRooy: Looking at the examples, I think "unlikely to be used as sparingly as it should" was spot-on; Out of the ~10 examples, I find that only two actually improve the code. (Namely, as the sole expression either in a while condition, to avoid duplicating the line or having the loop condition in the body, or in an elif-chain to avoid nesting) –  Aleksi Torhamo Commented Apr 10, 2020 at 11:07
• Doesn't work with ternary a if a := 1 == 1 else False → Invalid syntax :c So apparently still have to resort to the old awkward (lambda: (ret := foo(), ret if ret else None))()[-1] (i.e. as a way to avoid the repeated call to foo() in ternary) . –  Hi-Angel Commented Nov 25, 2020 at 21:20
• 3 @Hi-Angel This actually works fine (Python 3.10.8). You just have to add parentheses according to what you actually want to express, i.e. either a if (a := 1) == 1 else False (yields 1 ) or a if (a := 1 == 1) else False (yields True ). –  user686249 Commented Nov 29, 2022 at 15:54

Nope, the BDFL didn't like that feature.

From where I sit, Guido van Rossum, "Benevolent Dictator For Life”, has fought hard to keep Python as simple as it can be. We can quibble with some of the decisions he's made -- I'd have preferred he said 'No' more often. But the fact that there hasn't been a committee designing Python, but instead a trusted "advisory board", based largely on merit, filtering through one designer's sensibilities, has produced one hell of a nice language, IMHO.

• 20 Simple? This feature could simplified quite some of my code because it could have made it more compact and therefor more readable. Now I need two lines where I used to need one. I never got the point why Python rejected features other programming languages have for many years (and often for a very good reason). Especially this feature we're talking about here is very, very useful. –  Regis May Commented Sep 10, 2017 at 8:09
• 7 Less code isn't always simpler or morde readable. Take a recursive function for example. It's loop-equivalent is often more readable. –  F.M.F. Commented Apr 9, 2018 at 8:50
• 1 I don't like like the C version of it, but I really miss having something like rust's if let when I have an if elif chain, but need to store and use the value of the condition in each case. –  Thayne Commented Jul 10, 2018 at 4:57
• 1 I have to say the code I am writing now (the reason I searched this issue) is MUCH uglier without this feature. Instead of using if followed by lots of else ifs, I need to keep indenting the next if under the last else. –  MikeKulls Commented Oct 24, 2018 at 3:34

Yes, but only from Python 3.8 and onwards.

PEP 572 proposes Assignment Expressions and has already been accepted.

Quoting the Syntax and semantics part of the PEP:

In your specific case, you will be able to write

Not directly, per this old recipe of mine -- but as the recipe says it's easy to build the semantic equivalent, e.g. if you need to transliterate directly from a C-coded reference algorithm (before refactoring to more-idiomatic Python, of course;-). I.e.:

BTW, a very idiomatic Pythonic form for your specific case, if you know exactly what falsish value somefunc may return when it does return a falsish value (e.g. 0 ), is

so in this specific case the refactoring would be pretty easy;-).

If the return could be any kind of falsish value (0, None , '' , ...), one possibility is:

but you might prefer a simple custom generator:

• I would vote this up twice if I could. This is a great solution for those times when something like this is really needed. I adapted your solution to a regex Matcher class, which is instantiated once and then .check() is used in the if statement and .result() used inside its body to retrieve the match, if there was one. Thanks! :) –  Teekin Commented Jul 26, 2018 at 15:23

Thanks to Python 3.8 new feature it will be possible to do such a thing from this version, although not using = but Ada-like assignment operator := . Example from the docs:

No. Assignment in Python is a statement, not an expression.

• And Guido wouldn't have it any other way. –  Mark Ransom Commented Apr 8, 2010 at 22:51
• 1 @MarkRansom All hail Guido. Right .. sigh. –  WestCoastProjects Commented Dec 11, 2017 at 4:49
• @javadba the guy has been right much more often than he's been wrong. I appreciate that having a single person in charge of the vision results in a much more coherent strategy than design by committee; I can compare and contrast with C++ which is my main bread and butter. –  Mark Ransom Commented Dec 11, 2017 at 4:58
• I feel both ruby and scala ( v different languages) get it right significantly moreso than python: but in any case here is not the place.. –  WestCoastProjects Commented Dec 11, 2017 at 6:11
• @MarkRansom "And Guido wouldn't have it any other way" - and yet, 10 years later we have Assignment Expressions ... –  Tomerikoo Commented Apr 10, 2021 at 14:48

You can define a function to do the assigning for you:

One of the reasons why assignments are illegal in conditions is that it's easier to make a mistake and assign True or False:

In Python 3 True and False are keywords, so no risk anymore.

• 1 In [161]: l_empty==[] Out[161]: True In [162]: []==[] Out[162]: True I do not think that is the reason –  volcano Commented Jan 2, 2014 at 13:23
• Pretty sure most people put == True on the right side anyway. –  numbermaniac Commented Mar 11, 2019 at 7:27

The assignment operator - also known informally as the the walrus operator - was created at 28-Feb-2018 in PEP572 .

For the sake of completeness, I'll post the relevant parts so you can compare the differences between 3.7 and 3.8:

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NPTEL Assignment Answers and Solutions 2024 (July-Dec). Get Answers of Week 1 2 3 4 5 6 7 8 8 10 11 12 for all courses. This guide offers clear and accurate answers for your all assignments across various NPTEL courses

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• Artificial Intelligence Search Methods for Problem Solving
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• Deep Learning for Computer Vision
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• Introduction to Machine Learning IIT KGP
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• Problem Solving Through Programming in C
• Programming DSA Using Python
• Programming in Java
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• The Joy of Computation Using Python
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Contributors 3

Defining a symbolic syntax for referring to assignment targets

Full disclosure: I’m not sure this is a good idea (since it is seriously cryptic when you first encounter it, and hard to look up on your own), so I have precisely zero plans to pursue it any further myself. I do, however, find it to be an intriguing idea (prompted by this post in the PEP 736 discussion thread), so it seemed worth sharing in case someone else liked it enough to pick it up and run with it.

The core of the idea:

• A standalone @ symbol on the right hand side of an assignment expression or a regular (not augmented) assignment statement with a single target becomes a shortand that means “assignment target”. Exactly what that means depends on the nature of the assignment target (more details on that below).
• When the assignment target is an identifier, @'' or @"" can be used to mean “the assignment target as a string” (useful for APIs like collections.NamedTuple and typing.NewType )
• In function calls using keyword arguments, @ and @'' are resolved as if the parameter name were a local variable (so param_name=@ would pass a local variable named param_name , and param_name=ns[@''] would be equivalent to param_name=ns["param_name"]

Handling different kinds of assignment targets:

• identifiers: @ is an ordinary variable lookup for that name
• dotted attributes: @ looks up the corresponding attribute rather than setting it. The target expression up to the last dot is only evaluated once for the entire statement rather than being evaluated multiple times
• subscript or slice: @ looks up the corresponding container subscript rather than setting it. The target container expression and subscript expression are only evaluated once for the entire statement rather than being evaluated multiple times
• tuple unpacking: not allowed (specifically due to star unpacking)
• multiple targets: not allowed (which target should @ refer to? Leftmost? Rightmost? Tuple of all targets?)

Disallowed due to the potential for ambiguity:

Note that it isn’t that there’s no reasonable meaning for these (see the various comments about that later in the thread), it’s that the use cases for them are weak and they’d necessarily be harder to interpret than the simpler cases above. If they’re left out initially, it’s straightforward to add them later. If they’re included up front and are later found to be problematic, they’re hard to get rid of.

The mnemonic for remembering this use of the @ (“at”) symbol is the acronym “AT” for “Assignment Target”. Aside from the shared symbol, it has no connection to function decorators or matrix multiplication. As a style recommendation, it would be worth pointing out that combining assignment target references and the matrix multiplication operator in the same expression is intrinsically hard to read (due to the potential confusion between the two uses).

Edit: added a clarifying note to the list of arguably ambiguous targets

If this doesn’t comply with the presented idea, please disregard as off-topic.

I would find assignment targets useful in a for-loop where the assignment is implicit.

(I made-up the syntax).

The = @.lower() examples reminded me of the previous .= assignment discussion . Still not sure whether it would be a good idea, but the @ form is more general than the .= special case. Definitely intriguing …

I’d write that numlist = [num + 100 for num in numlist]

The more I think about it, the more I like it. The matrix multiplication thing does seem a bit of an issue though.

Also, I would consider allowing multiple targets on rightmost.

Given it can provide a significant decrease in verbosity, it would be a shame to not be able to take full advantage of it:

Or could specify which one (also, exclamation mark on its own looks pretty bad, adding a number to it makes it a bit better):

That’s semantically different, though, creating a new list instead of modifying an existing list.

I was pretty skeptical when I started reading, but the examples section really sold it to me!

So many of these are things I’ve had to do many times and shrugged about the verbosity.

Especially these:

(and similar)

Infact, the function call examples (PEP 736-like) seemed the least useful and intuitive to me, and I’m not quite sure I follow on how they’re consistent with the other assignment targets

The idea wouldn’t be useful in a regular for loop, but could reasonably be defined in comprehensions and generator expressions:

Less useful though, since it’s already acceptable style to use one letter iteration variable names in such cases.

I listed those separately because they’re genuinely different from the pure assignment case. Specifically, they’re looking up the given name in a different context from the one where the name is getting set.

Both tuple unpacking and multi-assigment could be allowed by having @ be a tuple in those cases.

My problem with that for multi-assigment is that it feels hard to remember (since rightmost and leftmost target remain plausible alternative meanings)

For tuple unpacking, the restriction could be limited to cases without star unpacking rather than all tuple targets, but it just doesn’t seem useful enough to justify that complexity.

I like the idea a lot, and I don’t quite see what can be potentially ambiguous about the two usages quoted above. The reversed example in particular makes the intent of the code that much more pronounced.

I do think that we should disallow mixed usage of matrix multiplication and the proposed assignment target reference as a special case though.

Maybe I’m missing something obvious, but I don’t even see what can be ambiguous about allowing a starred expression in the assignment targets, where the quoted example would intuitively translate into:

Also note that we should preserve the data types of tuples and/or lists in the assignment targets when evaluating them on the RHS:

This would be ambiguous if the function returns a value to an assignment target:

Meng Qinyuan

Aug 27, 2023

Hi @ncoghlan ,

Thanks for sharing this interesting idea! I think the use of @ to refer to assignment targets could be a neat feature, especially for scenarios where we need to reference the target itself, like in the examples with collections.NamedTuple and typing.NewType .

However, I do agree that the initial learning curve might be steep for newcomers, and the potential for confusion with existing uses of @ (decorators and matrix multiplication) is a concern.

It would be great to see more community feedback on this proposal, especially regarding the specific cases where it’s disallowed and how those could be handled differently.

Looking forward to seeing how this evolves!

MengQinyuan

It looks prety Perlish to me.

I’m not sure about this idea but I can see applications of it. One that comes up a lot in SymPy:

It is very easy to make a mistake with that and can easily cause bugs. This would be better:

Or maybe that is excluded because the assignment target is not an identifier…

My current summary of my own feelings: “Wow, that’s cool… I think I hate it”. Super useful (so it would get used), but super cryptic and not pretty to read (so I’d feel bad every time I used it, or saw someone else use it). I want to like it because of its utility and practicality, but the way it looks… ugh.

I only left out string forms for the non-identifier cases because I couldn’t think of a reasonable use case for them.

Defining it to be the literal source code target string regardless of the target kind wouldn’t be ambiguous though (and wouldn’t change the meaning for identifiers). That way APIs that accept comma separated strings that are expected to match the names of tuple assignment targets could be passed @'' as shown, while those that accept iterables could be passed @''.split(',') .

It would also strengthen the case for allowing @ for target tuples in general (since it would be odd if @'' was allowed, but @ wasn’t).

TensorFlow has the same design.

In my opinion, Jax solved this problem through better design. Instead of creating the symbols by binding a name, you do:

The decorator replaces the passed-in value x with a symbol x , produces the expression graph, compiles it, and then calls it with the passed-in arguments.

Similarly, SymPy could have been designed this way:

That is not a nice design for something that is often used interactively or in notebooks, short scripts etc. I guess jax has a more limited scope for how the symbols are used but needing to have all code in functions would be quite limiting for most SymPy users (including many who are not really “programmers” as such).

Other computer algebra systems often just allow symbols to be appear automagically without needing to be declared (which is both good and bad). Some like Matlab and Julia which are programming languages that have symbolic capability allow a nicer declaration something like:

Those languages could be extended to support this in a nicer way but Python can’t because it lacks macros and it would be considered out of scope for this sort of thing to be built in. The duplication in x, y = symbols('x, y') is of course not unique to SymPy and applies to the other situations mentioned in this thread as well which might equally be solved by macros in other languages.

Fair enough, I haven’t seen enough real life SymPy code to know.

Anyway, I think this usage in SymPy and similarly in (now-deprecated) type variables ( TypeVar ) is the most compelling use case to me. (Are there any other similar cases?)

The other examples seem to just illustrate how this features makes typing code easier, but reading code slightly harder (at least to my eyes). I think it should be the other way around: we should try to make reading code easier.

I’m going to heart this idea since I think it’s a worthwhile and interesting discussion even though I don’t think it would be a good addition.

This is way off topic, and I’m not trying to suggest how SymPy should be designed, but I’m surprised no-one has mentioned abusing sys._getframe(1).f_locals to have a function that sets variables in the parent frame. With that you could simply do something like

Part of it is the symbol @ itself, which is fine having that story of @ => AT => Assignment Target, but that is a story long to remember and one has to know it first.

An alternative is to use instead a keyword that reads in English as what is meant, such one can guess the meaning relying on knowledge that is more common.

I wouldn’t know which, for not knowing English enough: thesame , idem , ditto ?

Using the last one, for example, would turn

some.nested.attribute = @.lower()

some.nested.attribute = ditto.lower()

This has long existed but its use is discouraged because experience has shown that there are various ways that it can’t fully work:

Python Tutorial

File handling, python modules, python numpy, python pandas, python matplotlib, python scipy, machine learning, python mysql, python mongodb, python reference, module reference, python how to, python examples, python if else.

The else keyword catches anything which isn't caught by the preceding conditions.

In this example a is greater than b , so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".

You can also have an else without the elif :

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