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Python Conditional Assignment

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

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

1. Using Ternary Operator

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

It goes like this:

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

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

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

2. Using if-else statement

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

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

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

3. Using Logical Short Circuit Evaluation

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

The format of logical short circuit evaluation is:

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

Let's see an example:

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

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

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

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

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

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

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

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

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

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

Happy coding! 😊

How to assign a variable in Python

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How can you assign a variable in Python?

An equals sign assigns in Python

In Python, the equal sign ( = ) assigns a variable to a value :

This is called an assignment statement . We've pointed the variable count to the value 4 .

We don't have declarations or initializations

Some programming languages have an idea of declaring variables .

Declaring a variable says a variable exists with this name, but it doesn't have a value yet . After you've declared the variable, you then have to initialize it with a value.

Python doesn't have the concept of declaring a variable or initializing variables . Other programming languages sometimes do, but we don't.

In Python, a variable either exists or it doesn't:

If it doesn't exist, assigning to that variable will make it exist:

Valid variable names in Python

Variables in Python can be made up of letters, numbers, and underscores:

The first character in a variable cannot be a number :

And a small handful of names are reserved , so they can't be used as variable names (as noted in SyntaxError: invalid syntax ):

Reassigning a variable in Python

What if you want to change the value of a variable ?

The equal sign is used to assign a variable to a value, but it's also used to reassign a variable:

In Python, there's no distinction between assignment and reassignment .

Whenever you assign a variable in Python, if a variable with that name doesn't exist yet, Python makes a new variable with that name . But if a variable does exist with that name, Python points that variable to the value that we're assigning it to .

Variables don't have types in Python

Note that in Python, variables don't care about the type of an object .

Our amount variable currently points to an integer:

But there's nothing stopping us from pointing it to a string instead:

Variables in Python don't have types associated with them. Objects have types but variables don't . You can point a variable to any object that you'd like.

Type annotations are really type hints

You might have seen a variable that seems to have a type. This is called a type annotation (a.k.a. a "type hint"):

But when you run code like this, Python pretty much ignores these type hints. These are useful as documentation , and they can be introspected at runtime. But type annotations are not enforced by Python . Meaning, if we were to assign this variable to a different type, Python won't care:

What's the point of that?

Well, there's a number of code analysis tools that will check type annotations and show errors if our annotations don't match.

One of these tools is called MyPy .

Type annotations are something that you can opt into in Python, but Python won't do type-checking for you . If you want to enforce type annotations in your code, you'll need to specifically run a type-checker (like MyPy) before your code runs.

Use = to assign a variable in Python

Assignment in Python is pretty simple on its face, but there's a bit of complexity below the surface.

For example, Python's variables are not buckets that contain objects : Python's variables are pointers . Also you can assign into data structures in Python.

Also, it's actually possible to assign without using an equal sign in Python. But the equal sign ( = ) is the quick and easy way to assign a variable in Python.

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  • Python if...else Statement

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 if the specified condition is met.

Here, if the condition of the if statement is:

  • True - the body of the if statement executes.
  • False - the body of the if statement is skipped from execution.

Let's look at an example.

Working of if Statement

Note: Be mindful of the indentation while writing the if statements. Indentation is the whitespace at the beginning of the code.

Here, the spaces before the print() statement denote that it's the body of the if statement.

  • Example: Python if Statement

Sample Output 1

In the above example, we have created a variable named number . Notice the test condition ,

As the number is greater than 0 , the condition evaluates True . Hence, the body of the if statement executes.

Sample Output 2

Now, let's change the value of the number to a negative integer, say -5 .

Now, when we run the program, the output will be:

This is because the value of the number is less than 0 . Hence, the condition evaluates to False . And, the body of the if statement is skipped.

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

Working of if…else Statement

  • Example: Python if…else Statement

In the above example, we have created a variable named number .

Since the value of the number is 10 , the condition evaluates to True . Hence, code inside the body of if is executed.

If we change the value of the variable to a negative integer, let's say -5 , our output will be:

Here, the test condition evaluates to False . Hence code inside 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.

  • if condition1 - This checks if condition1 is True . If it is, the program executes code block 1 .
  • elif condition2 - If condition1 is not True , the program checks condition2 . If condition2 is True , it executes code block 2 .
  • else - If neither condition1 nor condition2 is True , the program defaults to executing code block 3 .

Working of if…elif…else Statement

  • Example: Python if…elif…else Statement

Since the value of the number is 0 , both the test conditions evaluate to False .

Hence, the statement inside the body of else is 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.

Working of Nested if Statement

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

We can use logical operators such as and and or within 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.

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  • Introduction

Video: Python if...else Statement

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Add an item if the key does not exist in dict with setdefault in Python

In Python, you can add a new item to the dictionary ( dict ) with dict_object[key] = new_value . If the key already exists, the value is updated (overwritten) with the new value.

The setdefault() method allows you to add new keys with new values, without changing the values of existing keys.

  • Built-in Types - dict.setdefault() — Python 3.11.3 documentation

This method is useful when you want to avoid modifying existing items.

Add and update an item in the dictionary by specifying the key

How to use the setdefault() method, return value of the setdefault() method.

Use the in keyword to check if a key exists in a dictionary. For more information, refer to the following article.

  • Check if a key/value exists in a dictionary in Python

To add or update items in a dictionary, use the following syntax:

If you specify a non-existent key, a new item is added. If you specify an existing key, the current value is updated (overwritten).

For more information on adding multiple items at once or merging multiple dictionaries, see the following articles.

  • Add and update an item in a dictionary in Python
  • Merge dictionaries in Python

In the setdefault() method, the first argument is the key and the second is the value.

If the key specified in the first argument does not exist, a new item is added.

The default value of the second argument is None . If the second argument is omitted, the item is added with a value of None .

If the key specified as the first argument already exists, the existing item remains unchanged, regardless of the value specified as the second argument.

The setdefault() method returns the value for the specified key.

If the key does not exist, a new item is added with the value from the second argument, and that value is then returned.

If the second argument is omitted, the item whose value is None is added, and None is returned.

If the key already exists, the value for that key is returned as-is.

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How to Check if a Variable Exists in Python

In this article, we will learn to check the existence of variables in Python . We will use some built-in functions in Python and some custom codes including exceptions of the functions as well. Let's first have a quick look over what are variables and then how many types of variables are there in Python.

What is Variable in Python?

Variables are containers and reserved memory locations for storing data values. Variables store data that can be used when evaluating an expression. Variables in Python can store any type of data or value say, integer type, string type, float type, or a boolean value, etc. There is no need to mention the type of the variable while defining it in the program. In the Python programming language, it is necessary for the variables to be defined before they are used in any function or in the program.

Variable Example

x is a variable of integer type becuase it holds integer value. No data type like int is used before the variable name.

In Python, all variables are expected to be defined before use. The None object is a value you often assign to signify that you have no real value for a variable, as shown below.

Then it’s easy to test whether a variable is bound to None or not.

Python Variable Exist or Not?

Python doesn’t have a specific function to test whether a variable is defined, since all variables are expected to have been defined before use, even if we initially assigned the variable to None object. Attempting to access a variable that hasn’t previously been defined raises a NameError exception. This NameError exception can be handled with a try/except statement, as you can do for any other Python exception.

We can use try-except block to check the existence of a variable that is not defined earlier before use.

Instead of ensuring that a variable is initialized like we see above that variable was assigned none value, you may prefer to test whether it’s defined where you want to use it. Let us see the example below.

Now, Python brings two functions locals() and globals() to overcome this situation. These two functions will help in checking whether the two variables i.e. local variable and global variable exists or not.

Checking Local Variable Existence in Python

To check the existence of a variable locally we are going to use the locals() function to get the dictionary of the current local symbol table. It returns true if the variable is found in the local entry system else returns false.

Checking Global Variable Existence in Python

Now, To check the existence of a variable globally we are going to use the globals() function to get the dictionary of the current global symbol table.

This way the programmer can use the locals() and globals() function provided by Python language to check the existence of the variable. The program will print TRUE if the variable exists and FALSE if the variable does not exist.

In this article, we learned to check the existence of variables in Python by using two built-in functions such as locals() and globals() . We used some custom codes as well. We learned about exceptions too. For example, we used None object and assigned it to a new variable, and checked what exceptions we are getting using examples.

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Python If Else Statements – Conditional Statements

If-Else statements in Python are part of conditional statements, which decide the control of code. As you can notice from the name If-Else, you can notice the code has two ways of directions.

There are situations in real life when we need to make some decisions and based on these decisions, we decide what we should do next. Similar situations arise in programming also where we need to make some decisions and based on these decisions we will execute the next block of code.

Conditional statements in Python languages decide the direction(Control Flow) of the flow of program execution.

Types of Control Flow in Python

Python control flow statements are as follows:

  • The if statement
  • The if-else statement
  • The nested-if statement
  • The if-elif-else ladder

Python if 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.

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 an if statement will be identified as shown in the below example:  

Flowchart of Python if statement

Let’s look at the flow of code in the If statement

Flowchart of Python if statement

Example of Python if Statement

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

Python If-Else Statement

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 to execute a block of code when the if condition is false. 

Syntax of Python If-Else : 

Flowchart of Python if-else statement

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

Flowchart of Python is-else statement

Flowchart of Python is-else statement

Using Python if-else statement

The block of code following the else 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).

Python if else statement in a List Comprehension

In this example, we are using an 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.

Nested-If Statement in Python

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

Let’s look at the flow of control in Nested if Statements

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 conditions will be executed one by one.

Python if-elif-else Ladder

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 Python if-elif-else ladder

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

python assign if exists

Flowchart of if-elif-else ladder

Example of Python if-elif-else ladder

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

Short Hand if statement

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.

Short Hand if-else statement

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. 

Example of Python if else shorthand  

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

Python if-else Statements Exercise Questions

Below are two Exercise Questions on Python if-else statements. We have covered 2 important exercise questions based on the odd-even program and the eligible age-to-vote program.

Q1. Odd-even practice exercise using if-else statements

Q2. Eligible to vote exercise questions using if-else statements

In this article, we have covered all the variations of if-else statements. Conditional statements are a very important concept in Programming as it is used in loops and many programs. You can use any of the if-else variations depending on your needs.

Similar Reads:

  • Python3 – if , if..else, Nested if, if-elif statements
  • Using Else Conditional Statement With For loop in Python
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Check if the variable exists in Python

In this Python tutorial, I will explain how to check if the variable exists in Python . We will see different methods that can be used to Python check if a variable is defined . In this process, we will see the need for a Python variable existence check .

When working with Python, it’s common to find ourselves in a situation where we want Python to check if a variable is defined before using it. This is especially important in large projects or scripts where a variable might be defined conditionally.

Variable in Python : In Python, a variable is used to store data that can be used and manipulated throughout a program. A variable provides a way of labeling data with a descriptive name, so our programs can be understood more clearly by the reader and ourselves.

Global and Local Variables : Variables declared inside a function or block are local to that function or block. Those declared outside are global .

Remember, in Python, everything is an object. When we create a variable, we’re essentially labeling an object, which is stored in a specific memory location.

Table of Contents

Why Checking Variable Existence is Important

Two main reasons for Python check if variable exists are:

  • Reassign of variable: In Python, reassigning a variable means assigning a new value to a variable that already exists. When you do this, the variable no longer refers to its previous value (unless another variable or structure still holds a reference to it), and instead, it refers to the new value.
  • Exceptions occur: Trying to access this non-existent variable will cause a NameError in Python. So, it becomes crucial to verify if a variable exists before using it.

For example:

Case-1: Reassign of variable in Python

To avoid reassigning a variable in a Python code we need to check if the variable is already present or not.

For instance, Imagine we’re tracking the location of a tourist in Python as they visit various landmarks across the country. We have a Python variable landmark_visited that we update in Python as they visit new places:

The output is: Only the last value of the variable is the output, others are lost.

Check if the variable exists in Python

We need to verify variable existence in Python before the declaration.

Case-2: Exception occurs while calling a variable in Python

In Python, a NameError is raised when we try to access a variable name that has not been defined or has not been imported into the current namespace. Essentially, it’s Python’s way of telling us that it doesn’t recognize the name we’re trying to use.

For instance, Imagine we’re a Python data analyst at the National Park Service. Our responsibility includes tracking monthly visitor numbers at various National Parks to help with planning resources and understanding tourism trends in Python.

The output is: In this scenario related to U.S. National Parks, the Python analyst tried to access data (visitor count) for a specific park without ensuring the respective variable was defined first. This action led to a NameError when Python couldn’t recognize the variable name being referenced.

python check if variable is defined

We need to verify variable existence in Python before using the code.

Different Methods to Check if the variable exists in Python

There are five different ways to check if variable exists in Python :

  • if statement
  • Using try and except
  • the globals() Function
  • the locals() Function
  • the dir() Function

Let’s see them one by one using demonstrative examples

Method-1: How to see if a variable exists Python using if statement

Sometimes we might want to check if a Python variable is not only existing, but also has been assigned a meaningful value (i.e., not None or some empty structure).

This method is useful for situations where a variable might exist, but it might be None , an empty list, an empty string, etc. Remember, in Python, structures like empty lists, strings, dictionaries, etc., are considered False in a boolean context.

For instance, We’re a resident in New York and we’re trying to list down the events we’ll be attending this month. We want to Python conditionally check variable if we’ve added events for this month or if the Python list is empty.

The output is: As the ‘ events_this_month ‘ stores an empty Python list. So, else statement got triggered.

python detect variable declaration

This way we can use if conditional statement to check if variable has been defined Python .

Method-2: Python check if variable exists using try and except

In Python, it’s common to use the try and except approach to handle possible errors. This can be used to gracefully manage the error if a variable does not exist.

For instance, Imagine a tourist agency in Florida. Tourists can choose different packages, and not all packages are available every day. If a user opts for the beach_package, the agency can use:

The output is:

check if the variable exists Python

This way we can use try and except block for Python to check variable existence .

Method-3: Python check variable before use with the globals() Function

The globals() function in Python returns a dictionary of the current global symbol table, which includes all global variables and hence, helps Python check if variable defined.

For example, We’re compiling a Python list of national parks and their average visitor count.

check if variable is initialized in python

This way we can use the globals() method to check if var exists in Python .

Method-4: Python check if var exists before use with the locals() Function

While globals() gives us global variables, locals() provides a Python dictionary of the current namespace. It can be used inside a function to check the existence of a local Python variable .

For instance, Inside a function, we want to check if a local Python variable for ‘Grand Canyon’ visitor count exists.

Python variable check if exists

This way we use locals() function in Python to check variable if exists .

Method-5: Python var check if exists using dir() function

The dir() method returns a list of names in the current local scope or a list of attributes of an object. It’s another way for the Python method to check variable existence .

For instance, We’re verifying the existence of a variable in Python that stores the number of states in the USA.

Python check if the var is present

This way we can use the dir() function to check a variable in Python if present .

In this tutorial, we have seen how to check if the variable exists in Python using different methods like if conditional statement, try and except block , globals() , locals() , or dir() functions with illustrative examples.

Python variable existence check is essential to prevent errors and manage uncertainties in our Python code.

You may also like to read:

  • Check if a variable is None in Python
  • How to add two variables in Python
  • Isidentifier method in String Python
  • Variable in Python

Bijay - Python Expert

I am Bijay Kumar, a Microsoft MVP in SharePoint. Apart from SharePoint, I started working on Python, Machine learning, and artificial intelligence for the last 5 years. During this time I got expertise in various Python libraries also like Tkinter, Pandas, NumPy, Turtle, Django, Matplotlib, Tensorflow, Scipy, Scikit-Learn, etc… for various clients in the United States, Canada, the United Kingdom, Australia, New Zealand, etc. Check out my profile .

How to Return a default value if None in Python

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Last updated: Feb 18, 2023 Reading time · 4 min

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# Table of Contents

  • Return a default value if None in Python
  • Using None as a default argument in Python

# Return a default value if None in Python

Use a conditional expression to return a default value if None in Python.

The conditional expression will return the default value if the variable stores None , otherwise the variable is returned.

return default value if none

Conditional expressions are very similar to an if/else statement.

In the examples, we check if the variable stores None , and if it does, we return the string default value , otherwise, the variable is returned.

Alternatively, you can switch the order of conditions.

switching the order of conditions

The code sample first checks if the variable is not None , in which case it returns it.

Otherwise, a default value is returned.

# Return a default value if None using the boolean OR operator

An alternative approach is to use the boolean OR operator.

return default value if none using boolean or

The expression x or y returns the value to the left if it's truthy, otherwise, the value to the right is returned.

However, this approach does not explicitly check for None .

All values that are not truthy are considered falsy. The falsy values in Python are:

  • constants defined to be falsy: None and False .
  • 0 (zero) of any numeric type
  • empty sequences and collections: "" (empty string), () (empty tuple), [] (empty list), {} (empty dictionary), set() (empty set), range(0) (empty range).

So if the value to the left is any of the aforementioned falsy values, the value to the right is returned.

This might or might not suit your use case.

If you only want to check for None , use the conditional expression from the first code snippet.

# Using None as a default argument in Python

None is often used as a default argument value in Python because it allows us to call the function without providing a value for an argument that isn't required on each function invocation.

None is especially useful for list and dict arguments.

using none as default argument

We declared a function with 2 default parameters set to None .

Default parameter values have the form parameter = expression .

When we declare a function with one or more default parameter values, the corresponding arguments can be omitted when the function is invoked.

A None value is often used for default parameters that are not essential to the function.

None is used when the function could still run even if a value for the parameter wasn't provided.

# Use None as default arguments, not Objects

Here is an example of how using an empty dictionary as a default value for a parameter can cause issues and how we can fix it with a None value.

The get_address function has a parameter with a default value of an empty dictionary.

We called the function 2 times and stored the results in variables.

Notice that we only set the country key on one of the dictionaries but both of them got updated.

They are not evaluated each time the function is called.

When a non-primitive default parameter value, such as a dictionary or a list is mutated, it is mutated for all function calls.

To resolve the issue, set the default parameter value to None and conditionally update it in the body of the function.

using none as default argument value

The body of the function is run every time it is invoked, so the issue is resolved.

I've also written an article on how to return multiple values and only use one .

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

  • Why does list.reverse() return None in Python
  • Purpose of 'return self' from a class method in Python
  • Why does my function print None in Python [Solved]
  • How to check if a variable is a Function in Python

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Python One Line If Not None

To assign the result of a function get_value() to variable x if it is different from None , use the Walrus operator if tmp := get_value(): x = tmp within a single-line if block. The Walrus operator assigns the function’s return value to the variable tmp and returns it at the same time, so that you can check and assign it to variable x subsequently.

Problem : How to assign a value to a variable if it is not equal to None —using only a single line of Python code?

Example : Say, you want to assign the return value of a function get_value() , but only if it doesn’t return None . Otherwise, you want to leave the value as it is.

Here’s a code example:

While this works, you need to execute the function get_value() twice which is not optimal. An alternative would be to assign the result of the get_value() function to a temporary variable to avoid repeated function execution:

However, this seems clunky and ineffective. Is there a better way?

Let’s have an overview of the one-liners that conditionally assign a value to a given variable:

Exercise : Run the code. Does it always generate the same result?

Method 1: Ternary Operator + Semicolon

The most basic ternary operator x if c else y consists of three operands x , c , and y . It is an expression with a return value. The ternary operator returns x if the Boolean expression c evaluates to True . Otherwise, if the expression c evaluates to False , the ternary operator returns the alternative y .

You can use the ternary operator to solve this problem in combination with the semicolon to write multiple lines of code as a Python one-liner.

You cannot run the get_value() function twice—to check whether it returns True and to assign the return value to the variable x . Why? Because it’s nondeterministic and may return different values for different executions.

Therefore, the following code would be a blunt mistake:

The variable x may still be None —even after the ternary operator has seemingly checked the condition.

The Python Ternary Operator -- And a Surprising One-Liner Hack

Related articles:

  • Python Ternary
  • Python Single-Line If Statement
  • Python Semicolon

Method 2: Walrus + One-Line-If

A beautiful extension of Python 3.8 is the Walrus operator . The Walrus operator := is an assignment operator with return value. Thus, it allows you to check a condition and assign a value at the same time:

This is a very clean, readable, and Pythonic way. Also, you don’t have the redundant identity assignment in case the if condition is not fulfilled.

Python 3.8 Walrus Operator (Assignment Expression)

Related Article: The Walrus Operator in Python 3.8

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 .

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  • Leverage data structures to solve real-world problems , like using Boolean indexing to find cities with above-average pollution
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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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To help students reach higher levels of Python success, he founded the programming education website Finxter.com that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a computer science enthusiast, freelancer , and owner of one of the top 10 largest Python blogs worldwide.

His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.

Data to Fish

Data to Fish

5 ways to apply an IF condition in Pandas DataFrame

In this guide, you’ll see 5 different ways to apply an IF condition in Pandas DataFrame.

Specifically, you’ll see how to apply an IF condition for:

  • Set of numbers
  • Set of numbers and lambda
  • Strings and lambda
  • OR condition

Applying an IF condition in Pandas DataFrame

Let’s now review the following 5 cases:

(1) IF condition – Set of numbers

Suppose that you created a DataFrame in Python that has 10 numbers (from 1 to 10). You then want to apply the following IF conditions:

  • If the number is equal or lower than 4, then assign the value of ‘True’
  • Otherwise, if the number is greater than 4, then assign the value of ‘False’

This is the general structure that you may use to create the IF condition:

For our example, the Python code would look like this:

Here is the result that you’ll get in Python:

(2) IF condition – set of numbers and  lambda

You’ll now see how to get the same results as in case 1 by using lambda, where the conditions are:

Here is the generic structure that you may apply in Python:

And for our example:

This is the result that you’ll get, which matches with case 1:

(3) IF condition – strings

Now, let’s create a DataFrame that contains only strings/text with 4  names : Jon, Bill, Maria and Emma.

The conditions are:

  • If the name is equal to ‘Bill,’ then assign the value of ‘Match’
  • Otherwise, if the name is not  ‘Bill,’ then assign the value of ‘Mismatch’

Once you run the above Python code, you’ll see:

(4) IF condition – strings and lambda 

You’ll get the same results as in case 3 by using lambda:

And here is the output from Python:

(5) IF condition with OR

Now let’s apply these conditions:

  • If the name is ‘Bill’  or ‘Emma,’ then assign the value of ‘Match’
  • Otherwise, if the name is neither ‘Bill’ nor ‘Emma,’ then assign the value of ‘Mismatch’

Run the Python code, and you’ll get the following result:

Applying an IF condition under an existing DataFrame column

So far you have seen how to apply an IF condition by creating a new column.

Alternatively, you may store the results under an existing DataFrame column.

For example, let’s say that you created a DataFrame that has 12 numbers, where the last two numbers are zeros:

‘set_of_numbers’: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0 , 0 ]

You may then apply the following IF conditions, and then store the results under the existing ‘set_of_numbers’ column:

  • If the number is equal to 0, then change the value to 999
  • If the number is equal to 5, then change the value to 555

Here are the before and after results, where the ‘5’ became ‘555’ and the 0’s became ‘999’ under the existing ‘set_of_numbers’ column:

On another instance, you may have a DataFrame that contains NaN values . You can then apply an IF condition to replace those values with zeros , as in the example below:

Before you’ll see the NaN values, and after you’ll see the zero values:

You just saw how to apply an IF condition in Pandas DataFrame . There are indeed multiple ways to apply such a condition in Python. You can achieve the same results by using either lambda, or just by sticking with Pandas.

At the end, it boils down to working with the method that is best suited to your needs.

Finally, you may want to check the following external source for additional information about Pandas DataFrame .

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COMMENTS

  1. python

    python - One line if-condition-assignment - Stack Overflow One line if-condition-assignment Ask Question Asked 12 years, 3 months ago Modified 11 months ago Viewed 482k times 208 I have the following code num1 = 10 someBoolValue = True I need to set the value of num1 to 20 if someBoolValue is True; and do nothing otherwise.

  2. Python Conditional Assignment (in 3 Ways)

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

  3. Python's Assignment Operator: Write Robust Assignments

    The Assignment Statement Syntax The Assignment Operator Assignments and Variables Other Assignment Syntax Assignment Statements in Action Initializing and Updating Variables Making Multiple Variables Refer to the Same Object Updating Lists Through Indices and Slices Adding and Updating Dictionary Keys Doing Parallel Assignments Unpacking Iterables

  4. How to assign a variable in Python

    The equal sign is used to assign a variable to a value, but it's also used to reassign a variable: >>> amount = 6 >>> amount = 7 >>> amount 7. In Python, there's no distinction between assignment and reassignment. Whenever you assign a variable in Python, if a variable with that name doesn't exist yet, Python makes a new variable with that name .

  5. The Walrus Operator: Python 3.8 Assignment Expressions

    Each new version of Python adds new features to the language. For Python 3.8, the biggest change is the addition of assignment expressions.Specifically, the := operator gives you a new syntax for assigning variables in the middle of expressions. This operator is colloquially known as the walrus operator.. This tutorial is an in-depth introduction to the walrus operator.

  6. How to Use IF Statements in Python (if, else, elif, and more

    March 3, 2022 Tutorial: Using If Statements in Python Our life is full of conditions even if we don't notice them most of the time. Let's look at a few examples: If tomorrow it doesn't rain, I'll go out with my friends in the park. Otherwise, I'll stay home with a cup of hot tea and watch TV.

  7. Python if, if...else Statement (With Examples)

    Python if Statement An if statement executes a block of code only if the specified condition is met. Syntax if condition: # body of if statement Here, if the condition of the if statement is: True - the body of the if statement executes. False - the body of the if statement is skipped from execution. Let's look at an example.

  8. Python: Assign Value if None Exists

    In Python, we can use the or operator to assign a default value if none exists. For example, let's say we have a variable x that may or may not have a value assigned to it. We can assign a default value of 0 to x if it is None using the following syntax: x = x or 0 This code snippet first checks if x is None or False. If it is, then x is set to 0.

  9. Python Variable Assignment If Variable Exists else None in One Line

    Python Variable Assignment If Variable Exists else None in One Line Asked 9 years, 7 months ago Modified 9 years, 7 months ago Viewed 2k times 0 Can this be done in one line? Something that looks like this data ['x'] = (x if x else "") (except doesn't raise an exception) Otherwise I often end up doing the following:

  10. Add an item if the key does not exist in dict with setdefault in Python

    Use the in keyword to check if a key exists in a dictionary. For more information, refer to the following article. Check if a key/value exists in a dictionary in Python Add and update an item in the dictionary by specifying the key To add or update items in a dictionary, use the following syntax: dict_object[key] = new_value

  11. Python if statements with multiple conditions (and + or) · Kodify

    Python's if statements test multiple conditions with and and or. Those logical operators combine several conditions into a single True or False value. ... See path exists See path is file See path is directory Validate path Path components ... To assign the right staff member to the order, we have to know if the customer wants an additional ...

  12. How to check if a Python variable exists?

    Method 1: Checking the existence of a local variable To check the existence of variables locally we are going to use the locals () function to get the dictionary of the current local symbol table. Python3 def func (): a_variable = 0 if 'a_variable' in locals(): return True func () Output: True Method 2: Checking the existence of a global variable

  13. How to Check if a Variable Exists in Python

    def func (): # defining local variable local_var = 0 # using locals () function for checking existence in symbol table is_local = "local_var" in locals () # printing result print (is_local) # driver code func () TRUE Checking Global Variable Existence in Python

  14. Python If Else Statements

    Practice. If-Else statements in Python are part of conditional statements, which decide the control of code. As you can notice from the name If-Else, you can notice the code has two ways of directions. There are situations in real life when we need to make some decisions and based on these decisions, we decide what we should do next.

  15. Check if the variable exists in Python

    Reassign of variable: In Python, reassigning a variable means assigning a new value to a variable that already exists. When you do this, the variable no longer refers to its previous value (unless another variable or structure still holds a reference to it), and instead, it refers to the new value.

  16. How to Return a default value if None in Python

    None is often used as a default argument value in Python because it allows us to call the function without providing a value for an argument that isn't required on each function invocation. None is especially useful for list and dict arguments. main.py. def get_employee(name=None, age=None): return {'name': name, 'age': age} # 👇 {'name ...

  17. python

    How do I check if a variable exists? Ask Question Asked 14 years, 9 months ago Modified 5 days ago Viewed 1.7m times 1369 I want to check if a variable exists. Now I'm doing something like this: try: myVar except NameError: # Do something. Are there other ways without exceptions? python exception variables Share Follow edited Apr 9, 2022 at 9:48

  18. Python One Line If Not None

    Related articles: Python Ternary; Python Single-Line If Statement; Python Semicolon; Method 2: Walrus + One-Line-If. A beautiful extension of Python 3.8 is the Walrus operator. The Walrus operator := is an assignment operator with return value. Thus, it allows you to check a condition and assign a value at the same time:

  19. python

    22 Is there a way to do an assignment only if the assigned value is not None, and otherwise do nothing? Of course we can do: x = get_value () if get_value () is not None but this will read the value twice. We can cache it to a local variable: v = get_value () x = v if v is not None but now we have made two statements for a simple thing.

  20. 5 ways to apply an IF condition in Pandas DataFrame

    June 25, 2022 In this guide, you'll see 5 different ways to apply an IF condition in Pandas DataFrame. Specifically, you'll see how to apply an IF condition for: Set of numbers Set of numbers and lambda Strings Strings and lambda OR condition Applying an IF condition in Pandas DataFrame Let's now review the following 5 cases:

  21. python

    Short way to write if exists else Asked 9 years, 1 month ago Viewed 9k times 5 What I want to do is : if myObject: # (not None) attr = myObject.someAttr else: attr = '' And avoiding if possible, ternary expressions. Is there something like : attr = myObject.someAttr || '' ? I was thinking of creating my own function such as :