python variable assignment based on condition

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! 😊

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

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A Comprehensive Guide to Using Conditionals in Python with Real-World Examples

Conditionals are a fundamental concept in programming that allow code to execute differently based on certain conditions. In Python, conditionals take the form of if , elif , and else statements. Mastering conditionals is key to writing dynamic, flexible programs that can handle different scenarios and make decisions.

This comprehensive guide will provide a deep dive into using conditionals in Python for real-world applications. We will cover the following topics:

Table of Contents

Basic syntax and structure of conditionals, comparison operators, logic operators, if statements, if-else statements, if-elif-else statements, nested conditionals, ternary operator, common errors and mistakes, user input validation, handling different user types, recommendation systems, data analysis and visualization, game design and gameplay logic.

The basic syntax for an if statement in Python is:

The condition can be any expression that evaluates to True or False. The code block indented under the if statement runs only when the condition is True.

Some key points:

• The condition follows the if keyword and ends with a colon (:)
• The code block after the condition is indented (usually 4 spaces)
• if , elif , and else are lowercase
• Code blocks end when the indentation returns to the left margin

Let’s look at a simple example:

Here we check if the value of age is greater than or equal to 18. If so, we print a message saying the person can vote. The print statement is indented under the if to indicate it runs conditionally.

Comparison operators allow us to compare two values and evaluate to True or False. They are essential for writing conditional expressions.

Here are some examples of using comparison operators in conditional statements:

We can also chain multiple comparisons using logic operators like and and or .

Logic operators allow us to combine multiple conditional expressions and evaluate the overall logic.

The two main logic operators are:

• and - Both conditions must be True for overall expression to be True
• or - Either one condition must be True for overall expression to be True

Both age >= 18 and citizen must be True for the print statement to execute.

Other logical operators include:

• not - Negates or flips the Boolean value
• in - Checks if a value is present in a sequence
• not in - Checks if a value is not present

Logic operators allow us to handle complex conditional logic in a concise way.

The if statement is used when we want to execute code only when some condition is fulfilled. For example:

Here we only want to print “Great job!” when the score is 80 or higher. The if statement allows us to specify this condition.

Some things to note about if statements:

• They execute the code block only when condition evaluates to True
• The condition can use any comparison or logical operators
• We can use complex logic by chaining multiple conditions with and , or , not
• The code block must be indented under the if statement

Let’s look at some more examples:

The if statement allows us to execute code conditioned on any criteria we specify in the conditional expression.

The if-else statement extends the simple if by allowing us to specify code that executes when the condition evaluates to False.

The syntax is:

Let’s look at an example:

Here if age is less than 18, we print a different message using the else block.

Key points on if-else :

• The else can only be used after an if statement
• The else block runs when the if condition is False
• We can chain multiple elif blocks for more conditions (see next section)
• Only one code block will execute - either if or else

More examples:

The if-else statement allows us to conditionally run different code blocks based on the evaluation of the condition expression.

The elif statement is used to chain multiple conditional checks. Using elif we can have multiple conditions evaluated in order.

This allows us to check many conditions and selectively run code for each case. For example:

Here we check the score against multiple grade thresholds. First if to check for A, then elif to check for B, etc. The final else acts as a default case if none match.

Some key points on if-elif-else :

• Only one block will execute
• Each condition is checked in order
• elif lets us chain multiple conditions
• The else block is optional

The elif conditionals allow us to concisely handle multiple scenarios without writing nested if statements.

Nested conditionals refer to if statements within if statements. We can nest conditionals indefinitely to handle complex logic.

For example:

The outer if checks age, and the inner if-else selectively prints messages for students vs non-students.

Nested conditionals are useful when:

• We want to check secondary conditions after initial condition passes
• Breaking down complex conditional logic into simple steps
• Handling specific cases before handling general cases

However, deeply nested conditionals can make code hard to read. In those cases, functions may be better for readability.

The ternary operator provides a compact syntax for basic conditional logic:

This condenses a basic if-else check into one line.

Some points on ternary operator usage:

• Best for simple one line conditionals
• Hard to read for complex logic
• Can be nested but not recommended
• Has form value_if_true if condition else value_if_false

The ternary operator is ideal for quick conditional assignments or returning values conditionally from functions.

Some common errors when using conditionals include:

• Forgetting colons : after conditionals
• Indentation errors with code blocks
• Using assignment = instead of comparisons ==
• Misspellings in conditionals like adn , ro , etc.
• Missing parentheses around conditions
• Checking equality on two different types

These often cause syntax errors or unexpected logic errors. Always double check the condition expressions and indentations when debugging conditional issues.

Proper code commenting and leaving notes during coding can help identify issues with complex conditional statements. Start small and test conditionals thoroughly when chaining many elif clauses.

Real-World Examples and Exercises

Next we’ll explore some real-world examples to illustrate how conditionals are used in Python programming for tasks like user input validation, handling different user types, recommendation engines, data analysis, game design, and more.

Validating user input is crucial for many programs. For example:

We first check if the input is a digit, then convert to an integer. Next we check if age meets the 18+ requirement for access. The else handles any non-digit input.

Here are some other user input validation examples:

Careful input validation prevents bugs and errors down the line.

We can use conditionals to handle different features or pricing for various user types:

Different access levels can also be handled:

Conditionals allow flexible user handling in large applications.

Many recommendation systems use conditional logic to provide personalized suggestions based on certain factors. For example:

Products can be intelligently recommended using if-elif conditional chains.

When analyzing and visualizing data in Python, we can use conditionals to handle missing data or special cases:

Conditionals help account for incomplete data and customize data visualization.

Games make heavy use of conditionals to implement gameplay mechanics, physics, ballistics, animations, etc.

This implements a basic combat loop with damage dealt conditionally based on hit chance rolls. The end condition checks remaining health to determine winner.

Many other gameplay elements can be implemented using conditionals - physics, animations, resource management, abilities, etc.

Conditionals allow us to execute code selectively based on Boolean logic and are a core programming concept in any language. Python provides an intuitive syntax using if , else , elif for implementing conditional code execution.

In this guide, we covered the basics of conditionals in Python including operators, complex conditional chains, nesting conditionals, ternary expressions, and common errors. We examined real-world examples of using conditional logic for input validation, handling user types, recommendation systems, data analysis, and game mechanics.

Conditionals enable you to write dynamic, flexible programs that can make intelligent decisions and handle varying scenarios. Mastering their usage takes practice, but being comfortable with conditional logic will enable you to take on more advanced programming tasks.

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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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Understanding and mastering Python’s conditional statements is fundamental for any programmer aspiring to write efficient and robust code. In this guide, we’ll delve into the intricacies of conditional statements in Python, covering the basics, advanced techniques, and best practices.

• What are Conditional Statements?

Conditional Statements are statements in Python that provide a choice for the control flow based on a condition. It means that the control flow of the Python program will be decided based on the outcome of the condition.

Now let us see how Conditional Statements are implemented in Python.

Types of Conditional Statements in Python

Table of Content

• Types of Conditional Statement in Python

1. If Conditional Statement in Python

• 2. If else Conditional Statement in Python
• 3. Nested if..else Conditional Statement in Python
• 4. If-elif-else Conditional Statement in Python
• 5. Ternary Expression Conditional Statement in Python

Best Practices for Using Conditional Statements

If the simple code of block is to be performed if the condition holds then the if statement is used. Here the condition mentioned holds then the code of the block runs otherwise not.

Syntax of If Statement :

2. If else Conditional Statements in Python

In a conditional if Statement the additional block of code is merged as an else statement which is performed when if condition is false. 

Syntax of Python If-Else : 

3. Nested if..else Conditional Statements in Python

Nested if..else means an if-else statement inside another if statement. Or in simple words first, there is an outer if statement, and inside it another if – else statement is present and such type of statement is known as nested if statement. We can use one if or else if statement inside another if or else if statements.

4. If-elif-else Conditional Statements in Python

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.

5. Ternary Expression Conditional Statements in Python

The Python ternary Expression determines if a condition is true or false and then returns the appropriate value in accordance with the result. The ternary Expression is useful in cases where we need to assign a value to a variable based on a simple condition, and we want to keep our code more concise — all in just one line of code.

Syntax of Ternary Expression

• Keep conditions simple and expressive for better readability.
• Avoid deeply nested conditional blocks; refactor complex logic into smaller, more manageable functions.
• Comment on complex conditions to clarify their purpose.
• Prefer the ternary operator for simple conditional assignments.
• Advanced Techniques:
• Using short-circuit evaluation for efficiency in complex conditions.
• Leveraging the any() and all() functions with conditions applied to iterables.
• Employing conditional expressions within list comprehensions and generator expressions.

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How To Assign Values To Variables In Python?

Harsh Pandey

Last updated on 16 Apr 2024

Assigning values in Python to variables is a fundamental and straightforward process. This action forms the basis for storing and manipulating Python data. The various methods for assigning values to variables in Python are given below.

Assign Values To Variables Direct Initialisation Method

Assigning values to variables in Python using the Direct Initialization Method involves a simple, single-line statement. This method is essential for efficiently setting up variables with initial values.

To use this method, you directly assign the desired value to a variable. The format follows variable_name = value . For instance, age = 21 assigns the integer 21 to the variable age.

This method is not just limited to integers. For example, assigning a string to a variable would be name = "Alice" . An example of this implementation in Python is.

Python Variables – Assign Multiple Values

In Python, assigning multiple values to variables can be done in a single, efficient line of code. This feature streamlines the initializing of several variables at once, making the code more concise and readable.

Python allows you to assign values to multiple variables simultaneously by separating each variable and value with commas. For example, x, y, z = 1, 2, 3 simultaneously assigns 1 to x, 2 to y, and 3 to z.

Additionally, Python supports unpacking a collection of values into variables. For example, if you have a list values = [1, 2, 3] , you can assign these values to a, b, c by writing a, b, c = values .

Python’s capability to assign multiple values to multiple variables in a single line enhances code efficiency and clarity. This feature is applicable across various data types and includes unpacking collections into multiple variables, as shown in the examples.

Assign Values To Variables Using Conditional Operator

Assigning values to variables in Python using a conditional operator allows for more dynamic and flexible value assignments based on certain conditions. This method employs the ternary operator, a concise way to assign values based on a condition's truth value.

The syntax for using the conditional operator in Python follows the pattern: variable = value_if_true if condition else value_if_false . For example, status = 'Adult' if age >= 18 else 'Minor' assigns 'Adult' to status if age is 18 or more, and 'Minor' otherwise.

This method can also be used with more complex conditions and various data types. For example, you can assign different strings to a variable based on a numerical comparison

Using the conditional operator for variable assignment in Python enables more nuanced and condition-dependent variable initialization. It is particularly useful for creating readable one-liners that eliminate the need for longer if-else statements, as illustrated in the examples.

Python One Liner Conditional Statement Assigning

Python allows for one-liner conditional statements to assign values to variables, providing a compact and efficient way of handling conditional assignments. This approach utilizes the ternary operator for conditional expressions in a single line of code.

The ternary operator syntax in Python is variable = value_if_true if condition else value_if_false . For instance, message = 'High' if temperature > 20 else 'Low' assigns 'High' to message if temperature is greater than 20, and 'Low' otherwise.

This method can also be applied to more complex conditions.

Using one-liner conditional statements in Python for variable assignment streamlines the process, especially when dealing with simple conditions. It replaces the need for multi-line if-else statements, making the code more concise and readable.

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Conditional Assignment Operator in Python

• Python How-To's
• Conditional Assignment Operator in …

Meaning of ||= Operator in Ruby

Implement ruby’s ||= conditional assignment operator in python using the try...except statement, implement ruby’s ||= conditional assignment operator in python using local and global variables.

There isn’t any exact equivalent of Ruby’s ||= operator in Python. However, we can use the try...except method and concepts of local and global variables to emulate Ruby’s conditional assignment operator ||= in Python.

The basic meaning of this operator is to assign the value of the variable y to variable x if variable x is undefined or is falsy value, otherwise no assignment operation is performed.

But this operator is much more complex and confusing than other simpler conditional operators like += , -= because whenever any variable is encountered as undefined, the console throws out NameError .

a+=b evaluates to a=a+b .

a||=b looks as a=a||b but actually behaves as a||a=b .

We use try...except to catch and handle errors. Whenever the try except block runs, at first, the code lying within the try block executes. If the block of code within the try block successfully executes, then the except block is ignored; otherwise, the except block code will be executed, and the error is handled. Ruby’s ||= operator can roughly be translated in Python’s try-catch method as :

Here, if the variable x is defined, the try block will execute smoothly with no NameError exception. Hence, no assignment operation is performed. If x is not defined, the try block will generate NameError , then the except block gets executed, and variable x is assigned to 10 .

The scope of local variables is confined within a specific code scope, whereas global variables have their scope defined in the entire code space.

All the local variables in a particular scope are available as keys of the locals dictionary in that particular scope. All the global variables are stored as keys of the globals dictionary. We can access those variables whenever necessary using the locals and the globals dictionary.

We can check if a variable exists in any of the dictionaries and set its value only if it does not exist to translate Ruby’s ||= conditional assignment operator in Python.

Here, if the variable x is present in either global or local scope, we don’t perform any assignment operation; otherwise, we assign the value of x to 10 . It is similar to x||=10 in Ruby.

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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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Set Pandas Conditional Column Based on Values of Another Column

• August 9, 2021 February 22, 2022

There are many times when you may need to set a Pandas column value based on the condition of another column. In this post, you’ll learn all the different ways in which you can create Pandas conditional columns.

Table of Contents

Video Tutorial

If you prefer to follow along with a video tutorial, check out my video below:

Loading a Sample Dataframe

Let’s begin by loading a sample Pandas dataframe that we can use throughout this tutorial.

We’ll begin by import pandas and loading a dataframe using the .from_dict() method:

This returns the following dataframe:

Using Pandas loc to Set Pandas Conditional Column

Pandas loc is incredibly powerful! If you need a refresher on loc (or iloc), check out my tutorial here . Pandas’ loc creates a boolean mask, based on a condition. Sometimes, that condition can just be selecting rows and columns, but it can also be used to filter dataframes. These filtered dataframes can then have values applied to them.

Let’s explore the syntax a little bit:

With the syntax above, we filter the dataframe using .loc and then assign a value to any row in the column (or columns) where the condition is met.

Let’s try this out by assigning the string ‘Under 30’ to anyone with an age less than 30, and ‘Over 30’ to anyone 30 or older.

Let's take a look at what we did here:

• We assigned the string 'Over 30' to every record in the dataframe. To learn more about this, check out my post here or creating new columns.
• We then use .loc to create a boolean mask on the Age column to filter down to rows where the age is less than 30. When this condition is met, the Age Category column is assigned the new value 'Under 30'

But what happens when you have multiple conditions? You could, of course, use .loc multiple times, but this is difficult to read and fairly unpleasant to write. Let's see how we can accomplish this using numpy's .select() method.

Using Numpy Select to Set Values using Multiple Conditions

Similar to the method above to use .loc to create a conditional column in Pandas, we can use the numpy .select() method.

Let's begin by importing numpy and we'll give it the conventional alias np :

Now, say we wanted to apply a number of different age groups, as below:

• <20 years old,
• 20-39 years old,
• 40-59 years old,
• 60+ years old

In order to do this, we'll create a list of conditions and corresponding values to fill:

Running this returns the following dataframe:

Let's break down what happens here:

• We first define a list of conditions in which the criteria are specified. Recall that lists are ordered meaning that they should be in the order in which you would like the corresponding values to appear.
• We then define a list of values to use , which corresponds to the values you'd like applied in your new column.

Something to consider here is that this can be a bit counterintuitive to write. You can similarly define a function to apply different values. We'll cover this off in the section of using the Pandas .apply() method below .

One of the key benefits is that using numpy as is very fast, especially when compared to using the .apply() method.

Using Pandas Map to Set Values in Another Column

The Pandas .map() method is very helpful when you're applying labels to another column. In order to use this method, you define a dictionary to apply to the column.

For our sample dataframe, let's imagine that we have offices in America, Canada, and France. We want to map the cities to their corresponding countries and apply and "Other" value for any other city.

When we print this out, we get the following dataframe returned:

What we can see here, is that there is a NaN value associated with any City that doesn't have a corresponding country. If we want to apply "Other" to any missing values, we can chain the .fillna() method:

Using Pandas Apply to Apply a function to a column

Finally, you can apply built-in or custom functions to a dataframe using the Pandas .apply() method.

Let's take a look at both applying built-in functions such as len() and even applying custom functions.

Applying Python Built-in Functions to a Column

We can easily apply a built-in function using the .apply() method. Let's see how we can use the len() function to count how long a string of a given column.

Take note of a few things here:

• We apply the .apply() method to a particular column,
• We omit the parentheses "()"

Using Third-Party Packages in Pandas Apply

Similarly, you can use functions from using packages. Let's use numpy to apply the .sqrt() method to find the scare root of a person's age.

Using Custom Functions with Pandas Apply

Something that makes the .apply() method extremely powerful is the ability to define and apply your own functions.

Let's revisit how we could use an if-else statement to create age categories as in our earlier example:

In this post, you learned a number of ways in which you can apply values to a dataframe column to create a Pandas conditional column, including using .loc , .np.select() , Pandas .map() and Pandas .apply() . Each of these methods has a different use case that we explored throughout this post.

Learn more about Pandas methods covered here by checking out their official documentation:

• Pandas Apply
• Numpy Select

Nik Piepenbreier

Nik is the author of datagy.io and has over a decade of experience working with data analytics, data science, and Python. He specializes in teaching developers how to use Python for data science using hands-on tutorials. View Author posts

2 thoughts on “Set Pandas Conditional Column Based on Values of Another Column”

Thank you so much! Brilliantly explained!!!

Thanks Aisha!

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Python's "Ternary Operator"

Welcome to another Python snippet post . This week we're going to take a brief look at conditional expressions in Python, sometimes referred to as Python's ternary operator.

Conditional expressions are a slightly obscure bit of syntax in Python, but they essentially allow us to assign values to variables based on some condition.

Let's take a look at a quick example:

In this case we have some value bound to the variable x , and we check if the value of x is less than 10 . If it is, we assign the number to value ; otherwise, we assign the string, "Invalid value" .

We can see a case where the value of x is not less than 10 below:

Let's break down the syntax.

First we start with the value to return if the condition is True . In our case, this is x . We then have the if keyword followed by some condition. In our case this is a comparison using the less than operator, but any expression which can evaluated to a Boolean value is fine. After the condition, we use the else keyword, followed by the value to return if the condition evaluates to False .

One thing to keep in mind when it comes to conditional expressions is that we actually need all of the parts. We can't simply do away with the else clause if we don't care about it. We have to explicitly define a value for if the condition evaluates to False .

Failing to add an else clause results in a SyntaxError .

Conditional expressions can be chained by appending to the else clause, but the syntax is already confusing enough that I wouldn't ever recommend doing this:

So, should you be using conditionally expressions all over your own code? Probably not.

Personally, I think the order of the conditions and the return values is pretty unintuitive, and it can be hard to follow the logic of these conditional expressions. Often it's much clearer to just use an if statement, even if it's a little longer:

There are, however, plenty of examples of this structure being used in the wild, so it's important to be able to recognise it and understand it when it's used in other people's code.

Wrapping up

That's it for this snippet post. Hopefully you learnt something new, and I hope you find some use cases for conditional expressions in your own code. Sometimes they can be very useful.

If you want to upgrade your Python skills even further, I'd recommend checking out our Complete Python Course over on Udemy! There's over 35 hours of material, as well as dozens of exercises and projects to get you really comfortable working with Python.

I'd also recommend signing up to your mailing list below, as we post regular discount codes for our courses, ensuring you get the best deal.

• Module 2: The Essentials of Python »
• Variables & Assignment
• View page source

Variables & Assignment 

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.

Variables permit us to write code that is flexible and amendable to repurpose. Suppose we want to write code that logs a student’s grade on an exam. The logic behind this process should not depend on whether we are logging Brian’s score of 92% versus Ashley’s score of 94%. As such, we can utilize variables, say name and grade , to serve as placeholders for this information. In this subsection, we will demonstrate how to define variables in Python.

In Python, the = symbol represents the “assignment” operator. The variable goes to the left of = , and the object that is being assigned to the variable goes to the right:

Attempting to reverse the assignment order (e.g. 92 = name ) will result in a syntax error. When a variable is assigned an object (like a number or a string), it is common to say that the variable is a reference to that object. For example, the variable name references the string "Brian" . This means that, once a variable is assigned an object, it can be used elsewhere in your code as a reference to (or placeholder for) that object:

Valid Names for Variables 

A variable name may consist of alphanumeric characters ( a-z , A-Z , 0-9 ) and the underscore symbol ( _ ); a valid name cannot begin with a numerical value.

var : valid

_var2 : valid

ApplePie_Yum_Yum : valid

2cool : invalid (begins with a numerical character)

I.am.the.best : invalid (contains . )

They also cannot conflict with character sequences that are reserved by the Python language. As such, the following cannot be used as variable names:

for , while , break , pass , continue

in , is , not

if , else , elif

def , class , return , yield , raises

import , from , as , with

try , except , finally

There are other unicode characters that are permitted as valid characters in a Python variable name, but it is not worthwhile to delve into those details here.

Mutable and Immutable Objects 

The mutability of an object refers to its ability to have its state changed. A mutable object can have its state changed, whereas an immutable object cannot. For instance, a list is an example of a mutable object. Once formed, we are able to update the contents of a list - replacing, adding to, and removing its elements.

To spell out what is transpiring here, we:

Create (initialize) a list with the state [1, 2, 3] .

Assign this list to the variable x ; x is now a reference to that list.

Using our referencing variable, x , update element-0 of the list to store the integer -4 .

This does not create a new list object, rather it mutates our original list. This is why printing x in the console displays [-4, 2, 3] and not [1, 2, 3] .

A tuple is an example of an immutable object. Once formed, there is no mechanism by which one can change of the state of a tuple; and any code that appears to be updating a tuple is in fact creating an entirely new tuple.

Mutable & Immutable Types of Objects 

The following are some common immutable and mutable objects in Python. These will be important to have in mind as we start to work with dictionaries and sets.

Some immutable objects

numbers (integers, floating-point numbers, complex numbers)

“frozen”-sets

Some mutable objects

dictionaries

NumPy arrays

Referencing a Mutable Object with Multiple Variables 

It is possible to assign variables to other, existing variables. Doing so will cause the variables to reference the same object:

What this entails is that these common variables will reference the same instance of the list. Meaning that if the list changes, all of the variables referencing that list will reflect this change:

We can see that list2 is still assigned to reference the same, updated list as list1 :

In general, assigning a variable b to a variable a will cause the variables to reference the same object in the system’s memory, and assigning c to a or b will simply have a third variable reference this same object. Then any change (a.k.a mutation ) of the object will be reflected in all of the variables that reference it ( a , b , and c ).

Of course, assigning two variables to identical but distinct lists means that a change to one list will not affect the other:

Reading Comprehension: Does slicing a list produce a reference to that list?

Suppose x is assigned a list, and that y is assigned a “slice” of x . Do x and y reference the same list? That is, if you update part of the subsequence common to x and y , does that change show up in both of them? Write some simple code to investigate this.

Reading Comprehension: Understanding References

Based on our discussion of mutable and immutable objects, predict what the value of y will be in the following circumstance:

Reading Comprehension Exercise Solutions: 

Does slicing a list produce a reference to that list?: Solution

Based on the following behavior, we can conclude that slicing a list does not produce a reference to the original list. Rather, slicing a list produces a copy of the appropriate subsequence of the list:

Understanding References: Solutions

Integers are immutable, thus x must reference an entirely new object ( 9 ), and y still references 3 .