python function variable referenced before assignment

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Python local variable referenced before assignment Solution

When you start introducing functions into your code, you’re bound to encounter an UnboundLocalError at some point. This error is raised when you try to use a variable before it has been assigned in the local context .

In this guide, we talk about what this error means and why it is raised. We walk through an example of this error in action to help you understand how you can solve it.

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What is unboundlocalerror: local variable referenced before assignment.

Trying to assign a value to a variable that does not have local scope can result in this error:

Python has a simple rule to determine the scope of a variable. If a variable is assigned in a function , that variable is local. This is because it is assumed that when you define a variable inside a function you only need to access it inside that function.

There are two variable scopes in Python: local and global. Global variables are accessible throughout an entire program; local variables are only accessible within the function in which they are originally defined.

Let’s take a look at how to solve this error.

An Example Scenario

We’re going to write a program that calculates the grade a student has earned in class.

We start by declaring two variables:

These variables store the numerical and letter grades a student has earned, respectively. By default, the value of “letter” is “F”. Next, we write a function that calculates a student’s letter grade based on their numerical grade using an “if” statement :

Finally, we call our function:

This line of code prints out the value returned by the calculate_grade() function to the console. We pass through one parameter into our function: numerical. This is the numerical value of the grade a student has earned.

Let’s run our code and see what happens:

An error has been raised.

The Solution

Our code returns an error because we reference “letter” before we assign it.

We have set the value of “numerical” to 42. Our if statement does not set a value for any grade over 50. This means that when we call our calculate_grade() function, our return statement does not know the value to which we are referring.

We do define “letter” at the start of our program. However, we define it in the global context. Python treats “return letter” as trying to return a local variable called “letter”, not a global variable.

We solve this problem in two ways. First, we can add an else statement to our code. This ensures we declare “letter” before we try to return it:

Let’s try to run our code again:

Our code successfully prints out the student’s grade.

If you are using an “if” statement where you declare a variable, you should make sure there is an “else” statement in place. This will make sure that even if none of your if statements evaluate to True, you can still set a value for the variable with which you are going to work.

Alternatively, we could use the “global” keyword to make our global keyword available in the local context in our calculate_grade() function. However, this approach is likely to lead to more confusing code and other issues. In general, variables should not be declared using “global” unless absolutely necessary . Your first, and main, port of call should always be to make sure that a variable is correctly defined.

In the example above, for instance, we did not check that the variable “letter” was defined in all use cases.

That’s it! We have fixed the local variable error in our code.

The UnboundLocalError: local variable referenced before assignment error is raised when you try to assign a value to a local variable before it has been declared. You can solve this error by ensuring that a local variable is declared before you assign it a value.

Now you’re ready to solve UnboundLocalError Python errors like a professional developer !

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Fix "local variable referenced before assignment" in Python

Introduction

If you're a Python developer, you've probably come across a variety of errors, like the "local variable referenced before assignment" error. This error can be a bit puzzling, especially for beginners and when it involves local/global variables.

Today, we'll explain this error, understand why it occurs, and see how you can fix it.

The "local variable referenced before assignment" Error

The "local variable referenced before assignment" error in Python is a common error that occurs when a local variable is referenced before it has been assigned a value. This error is a type of UnboundLocalError , which is raised when a local variable is referenced before it has been assigned in the local scope.

Here's a simple example:

Running this code will throw the "local variable 'x' referenced before assignment" error. This is because the variable x is referenced in the print(x) statement before it is assigned a value in the local scope of the foo function.

Even more confusing is when it involves global variables. For example, the following code also produces the error:

But wait, why does this also produce the error? Isn't x assigned before it's used in the say_hello function? The problem here is that x is a global variable when assigned "Hello ". However, in the say_hello function, it's a different local variable, which has not yet been assigned.

We'll see later in this Byte how you can fix these cases as well.

Fixing the Error: Initialization

One way to fix this error is to initialize the variable before using it. This ensures that the variable exists in the local scope before it is referenced.

Let's correct the error from our first example:

In this revised code, we initialize x with a value of 1 before printing it. Now, when you run the function, it will print 1 without any errors.

Fixing the Error: Global Keyword

Another way to fix this error, depending on your specific scenario, is by using the global keyword. This is especially useful when you want to use a global variable inside a function.

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Here's how:

In this snippet, we declare x as a global variable inside the function foo . This tells Python to look for x in the global scope, not the local one . Now, when you run the function, it will increment the global x by 1 and print 1 .

Similar Error: NameError

An error that's similar to the "local variable referenced before assignment" error is the NameError . This is raised when you try to use a variable or a function name that has not been defined yet.

Running this code will result in a NameError :

In this case, we're trying to print the value of y , but y has not been defined anywhere in the code. Hence, Python raises a NameError . This is similar in that we are trying to use an uninitialized/undefined variable, but the main difference is that we didn't try to initialize y anywhere else in our code.

Variable Scope in Python

Understanding the concept of variable scope can help avoid many common errors in Python, including the main error of interest in this Byte. But what exactly is variable scope?

In Python, variables have two types of scope - global and local. A variable declared inside a function is known as a local variable, while a variable declared outside a function is a global variable.

Consider this example:

In this code, x is a global variable, and y is a local variable. x can be accessed anywhere in the code, but y can only be accessed within my_function . Confusion surrounding this is one of the most common causes for the "variable referenced before assignment" error.

In this Byte, we've taken a look at the "local variable referenced before assignment" error and another similar error, NameError . We also delved into the concept of variable scope in Python, which is an important concept to understand to avoid these errors. If you're seeing one of these errors, check the scope of your variables and make sure they're being assigned before they're being used.

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Local variable referenced before assignment in Python

Last updated: Apr 8, 2024 Reading time · 4 min

# Local variable referenced before assignment in Python

The Python "UnboundLocalError: Local variable referenced before assignment" occurs when we reference a local variable before assigning a value to it in a function.

To solve the error, mark the variable as global in the function definition, e.g. global my_var .

Here is an example of how the error occurs.

We assign a value to the name variable in the function.

# Mark the variable as global to solve the error

To solve the error, mark the variable as global in your function definition.

If a variable is assigned a value in a function's body, it is a local variable unless explicitly declared as global .

# Local variables shadow global ones with the same name

You could reference the global name variable from inside the function but if you assign a value to the variable in the function's body, the local variable shadows the global one.

Accessing the name variable in the function is perfectly fine.

On the other hand, variables declared in a function cannot be accessed from the global scope.

The name variable is declared in the function, so trying to access it from outside causes an error.

Make sure you don't try to access the variable before using the global keyword, otherwise, you'd get the SyntaxError: name 'X' is used prior to global declaration error.

# Returning a value from the function instead

An alternative solution to using the global keyword is to return a value from the function and use the value to reassign the global variable.

We simply return the value that we eventually use to assign to the name global variable.

# Passing the global variable as an argument to the function

You should also consider passing the global variable as an argument to the function.

We passed the name global variable as an argument to the function.

If we assign a value to a variable in a function, the variable is assumed to be local unless explicitly declared as global .

# Assigning a value to a local variable from an outer scope

If you have a nested function and are trying to assign a value to the local variables from the outer function, use the nonlocal keyword.

The nonlocal keyword allows us to work with the local variables of enclosing functions.

Had we not used the nonlocal statement, the call to the print() function would have returned an empty string.

Printing the message variable on the last line of the function shows an empty string because the inner() function has its own scope.

Changing the value of the variable in the inner scope is not possible unless we use the nonlocal keyword.

Instead, the message variable in the inner function simply shadows the variable with the same name from the outer scope.

# Discussion

As shown in this section of the documentation, when you assign a value to a variable inside a function, the variable:

• Becomes local to the scope.
• Shadows any variables from the outer scope that have the same name.

The last line in the example function assigns a value to the name variable, marking it as a local variable and shadowing the name variable from the outer scope.

At the time the print(name) line runs, the name variable is not yet initialized, which causes the error.

The most intuitive way to solve the error is to use the global keyword.

The global keyword is used to indicate to Python that we are actually modifying the value of the name variable from the outer scope.

• If a variable is only referenced inside a function, it is implicitly global.
• If a variable is assigned a value inside a function's body, it is assumed to be local, unless explicitly marked as global .

If you want to read more about why this error occurs, check out [this section] ( this section ) of the docs.

# Additional Resources

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

• SyntaxError: name 'X' is used prior to global declaration

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

[SOLVED] Local Variable Referenced Before Assignment

Python treats variables referenced only inside a function as global variables. Any variable assigned to a function’s body is assumed to be a local variable unless explicitly declared as global.

Why Does This Error Occur?

Unboundlocalerror: local variable referenced before assignment occurs when a variable is used before its created. Python does not have the concept of variable declarations. Hence it searches for the variable whenever used. When not found, it throws the error.

Before we hop into the solutions, let’s have a look at what is the global and local variables.

Local Variable Declarations vs. Global Variable Declarations

Local Variable Referenced Before Assignment Error with Explanation

Try these examples yourself using our Online Compiler.

Let’s look at the following function:

Explanation

The variable myVar has been assigned a value twice. Once before the declaration of myFunction and within myFunction itself.

Using Global Variables

Passing the variable as global allows the function to recognize the variable outside the function.

Create Functions that Take in Parameters

Instead of initializing myVar as a global or local variable, it can be passed to the function as a parameter. This removes the need to create a variable in memory.

UnboundLocalError: local variable ‘DISTRO_NAME’

This error may occur when trying to launch the Anaconda Navigator in Linux Systems.

Upon launching Anaconda Navigator, the opening screen freezes and doesn’t proceed to load.

Try and update your Anaconda Navigator with the following command.

If solution one doesn’t work, you have to edit a file located at

After finding and opening the Python file, make the following changes:

In the function on line 159, simply add the line:

DISTRO_NAME = None

Save the file and re-launch Anaconda Navigator.

DJANGO – Local Variable Referenced Before Assignment [Form]

The program takes information from a form filled out by a user. Accordingly, an email is sent using the information.

Upon running you get the following error:

We have created a class myForm that creates instances of Django forms. It extracts the user’s name, email, and message to be sent.

A function GetContact is created to use the information from the Django form and produce an email. It takes one request parameter. Prior to sending the email, the function verifies the validity of the form. Upon True , .get() function is passed to fetch the name, email, and message. Finally, the email sent via the send_mail function

Why does the error occur?

We are initializing form under the if request.method == “POST” condition statement. Using the GET request, our variable form doesn’t get defined.

Local variable Referenced before assignment but it is global

This is a common error that happens when we don’t provide a value to a variable and reference it. This can happen with local variables. Global variables can’t be assigned.

This error message is raised when a variable is referenced before it has been assigned a value within the local scope of a function, even though it is a global variable.

Here’s an example to help illustrate the problem:

In this example, x is a global variable that is defined outside of the function my_func(). However, when we try to print the value of x inside the function, we get a UnboundLocalError with the message “local variable ‘x’ referenced before assignment”.

This is because the += operator implicitly creates a local variable within the function’s scope, which shadows the global variable of the same name. Since we’re trying to access the value of x before it’s been assigned a value within the local scope, the interpreter raises an error.

To fix this, you can use the global keyword to explicitly refer to the global variable within the function’s scope:

However, in the above example, the global keyword tells Python that we want to modify the value of the global variable x, rather than creating a new local variable. This allows us to access and modify the global variable within the function’s scope, without causing any errors.

Local variable ‘version’ referenced before assignment ubuntu-drivers

This error occurs with Ubuntu version drivers. To solve this error, you can re-specify the version information and give a split as 2 –

Here, p_name means package name.

With the help of the threading module, you can avoid using global variables in multi-threading. Make sure you lock and release your threads correctly to avoid the race condition.

When a variable that is created locally is called before assigning, it results in Unbound Local Error in Python. The interpreter can’t track the variable.

Therefore, we have examined the local variable referenced before the assignment Exception in Python. The differences between a local and global variable declaration have been explained, and multiple solutions regarding the issue have been provided.

Trending Python Articles

How to Fix Local Variable Referenced Before Assignment Error in Python

Table of Contents

Fixing local variable referenced before assignment error.

In Python , when you try to reference a variable that hasn't yet been given a value (assigned), it will throw an error.

That error will look like this:

In this post, we'll see examples of what causes this and how to fix it.

Let's begin by looking at an example of this error:

If you run this code, you'll get

The issue is that in this line:

We are defining a local variable called value and then trying to use it before it has been assigned a value, instead of using the variable that we defined in the first line.

If we want to refer the variable that was defined in the first line, we can make use of the global keyword.

The global keyword is used to refer to a variable that is defined outside of a function.

Let's look at how using global can fix our issue here:

Global variables have global scope, so you can referenced them anywhere in your code, thus avoiding the error.

If you run this code, you'll get this output:

In this post, we learned at how to avoid the local variable referenced before assignment error in Python.

The error stems from trying to refer to a variable without an assigned value, so either make use of a global variable using the global keyword, or assign the variable a value before using it.

Thanks for reading!

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Local variable referenced before assignment in Python

The “local variable referenced before assignment” error occurs in Python when you try to use a local variable before it has been assigned a value.

This error typically arises in situations where you declare a variable within a function but then try to access or modify it before actually assigning a value to it.

Here’s an example to illustrate this error:

In this example, you would encounter the “local variable ‘x’ referenced before assignment” error because you’re trying to print the value of x before it has been assigned a value. To fix this, you should assign a value to x before attempting to access it:

In the corrected version, the local variable x is assigned a value before it’s used, preventing the error.

Keep in mind that Python treats variables inside functions as local unless explicitly stated otherwise using the global keyword (for global variables) or the nonlocal keyword (for variables in nested functions).

If you encounter this error and you’re sure that the variable should have been assigned a value before its use, double-check your code for any logical errors or typos that might be causing the variable to not be assigned properly.

Using the global keyword

If you have a global variable named letter and you try to modify it inside a function without declaring it as global, you will get error.

This is because Python assumes that any variable that is assigned a value inside a function is a local variable, unless you explicitly tell it otherwise.

To fix this error, you can use the global keyword to indicate that you want to use the global variable:

Using nonlocal keyword

The nonlocal keyword is used to work with variables inside nested functions, where the variable should not belong to the inner function. It allows you to modify the value of a non-local variable in the outer scope.

For example, if you have a function outer that defines a variable x , and another function inner inside outer that tries to change the value of x , you need to use the nonlocal keyword to tell Python that you are referring to the x defined in outer , not a new local variable in inner .

Here is an example of how to use the nonlocal keyword:

If you don’t use the nonlocal keyword, Python will create a new local variable x in inner , and the value of x in outer will not be changed:

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How to Solve Error - Local Variable Referenced Before Assignment in Python

• Python How-To's
• How to Solve Error - Local Variable …

Check the Variable Scope to Fix the local variable referenced before assignment Error in Python

Initialize the variable before use to fix the local variable referenced before assignment error in python, use conditional assignment to fix the local variable referenced before assignment error in python.

This article delves into various strategies to resolve the common local variable referenced before assignment error. By exploring methods such as checking variable scope, initializing variables before use, conditional assignments, and more, we aim to equip both novice and seasoned programmers with practical solutions.

Each method is dissected with examples, demonstrating how subtle changes in code can prevent this frequent error, enhancing the robustness and readability of your Python projects.

The local variable referenced before assignment occurs when some variable is referenced before assignment within a function’s body. The error usually occurs when the code is trying to access the global variable.

The primary purpose of managing variable scope is to ensure that variables are accessible where they are needed while maintaining code modularity and preventing unexpected modifications to global variables.

We can declare the variable as global using the global keyword in Python. Once the variable is declared global, the program can access the variable within a function, and no error will occur.

The below example code demonstrates the code scenario where the program will end up with the local variable referenced before assignment error.

In this example, my_var is a global variable. Inside update_var , we attempt to modify it without declaring its scope, leading to the Local Variable Referenced Before Assignment error.

We need to declare the my_var variable as global using the global keyword to resolve this error. The below example code demonstrates how the error can be resolved using the global keyword in the above code scenario.

In the corrected code, we use the global keyword to inform Python that my_var references the global variable.

When we first print my_var , it displays the original value from the global scope.

After assigning a new value to my_var , it updates the global variable, not a local one. This way, we effectively tell Python the scope of our variable, thus avoiding any conflicts between local and global variables with the same name.

Ensure that the variable is initialized with some value before using it. This can be done by assigning a default value to the variable at the beginning of the function or code block.

The main purpose of initializing variables before use is to ensure that they have a defined state before any operations are performed on them. This practice is not only crucial for avoiding the aforementioned error but also promotes writing clear and predictable code, which is essential in both simple scripts and complex applications.

In this example, the variable total is used in the function calculate_total without prior initialization, leading to the Local Variable Referenced Before Assignment error. The below example code demonstrates how the error can be resolved in the above code scenario.

In our corrected code, we initialize the variable total with 0 before using it in the loop. This ensures that when we start adding item values to total , it already has a defined state (in this case, 0).

This initialization is crucial because it provides a starting point for accumulation within the loop. Without this step, Python does not know the initial state of total , leading to the error.

Conditional assignment allows variables to be assigned values based on certain conditions or logical expressions. This method is particularly useful when a variable’s value depends on certain prerequisites or states, ensuring that a variable is always initialized before it’s used, thereby avoiding the common error.

In this example, message is only assigned within the if and elif blocks. If neither condition is met (as with guest ), the variable message remains uninitialized, leading to the Local Variable Referenced Before Assignment error when trying to print it.

The below example code demonstrates how the error can be resolved in the above code scenario.

In the revised code, we’ve included an else statement as part of our conditional logic. This guarantees that no matter what value user_type holds, the variable message will be assigned some value before it is used in the print function.

This conditional assignment ensures that the message is always initialized, thereby eliminating the possibility of encountering the Local Variable Referenced Before Assignment error.

Throughout this article, we have explored multiple approaches to address the Local Variable Referenced Before Assignment error in Python. From the nuances of variable scope to the effectiveness of initializations and conditional assignments, these strategies are instrumental in developing error-free code.

The key takeaway is the importance of understanding variable scope and initialization in Python. By applying these methods appropriately, programmers can not only resolve this specific error but also enhance the overall quality and maintainability of their code, making their programming journey smoother and more rewarding.

Python UnboundLocalError: local variable referenced before assignment

by Suf | Programming , Python , Tips

If you try to reference a local variable before assigning a value to it within the body of a function, you will encounter the UnboundLocalError: local variable referenced before assignment.

The preferable way to solve this error is to pass parameters to your function, for example:

Alternatively, you can declare the variable as global to access it while inside a function. For example,

This tutorial will go through the error in detail and how to solve it with code examples .

Table of contents

What is scope in python, unboundlocalerror: local variable referenced before assignment, solution #1: passing parameters to the function, solution #2: use global keyword, solution #1: include else statement, solution #2: use global keyword.

Scope refers to a variable being only available inside the region where it was created. A variable created inside a function belongs to the local scope of that function, and we can only use that variable inside that function.

A variable created in the main body of the Python code is a global variable and belongs to the global scope. Global variables are available within any scope, global and local.

UnboundLocalError occurs when we try to modify a variable defined as local before creating it. If we only need to read a variable within a function, we can do so without using the global keyword. Consider the following example that demonstrates a variable var created with global scope and accessed from test_func :

If we try to assign a value to var within test_func , the Python interpreter will raise the UnboundLocalError:

This error occurs because when we make an assignment to a variable in a scope, that variable becomes local to that scope and overrides any variable with the same name in the global or outer scope.

var +=1 is similar to var = var + 1 , therefore the Python interpreter should first read var , perform the addition and assign the value back to var .

var is a variable local to test_func , so the variable is read or referenced before we have assigned it. As a result, the Python interpreter raises the UnboundLocalError.

Example #1: Accessing a Local Variable

Let’s look at an example where we define a global variable number. We will use the increment_func to increase the numerical value of number by 1.

Let’s run the code to see what happens:

The error occurs because we tried to read a local variable before assigning a value to it.

We can solve this error by passing a parameter to increment_func . This solution is the preferred approach. Typically Python developers avoid declaring global variables unless they are necessary. Let’s look at the revised code:

We have assigned a value to number and passed it to the increment_func , which will resolve the UnboundLocalError. Let’s run the code to see the result:

We successfully printed the value to the console.

We also can solve this error by using the global keyword. The global statement tells the Python interpreter that inside increment_func , the variable number is a global variable even if we assign to it in increment_func . Let’s look at the revised code:

Let’s run the code to see the result:

Example #2: Function with if-elif statements

Let’s look at an example where we collect a score from a player of a game to rank their level of expertise. The variable we will use is called score and the calculate_level function takes in score as a parameter and returns a string containing the player’s level .

In the above code, we have a series of if-elif statements for assigning a string to the level variable. Let’s run the code to see what happens:

The error occurs because we input a score equal to 40 . The conditional statements in the function do not account for a value below 55 , therefore when we call the calculate_level function, Python will attempt to return level without any value assigned to it.

We can solve this error by completing the set of conditions with an else statement. The else statement will provide an assignment to level for all scores lower than 55 . Let’s look at the revised code:

In the above code, all scores below 55 are given the beginner level. Let’s run the code to see what happens:

We can also create a global variable level and then use the global keyword inside calculate_level . Using the global keyword will ensure that the variable is available in the local scope of the calculate_level function. Let’s look at the revised code.

In the above code, we put the global statement inside the function and at the beginning. Note that the “default” value of level is beginner and we do not include the else statement in the function. Let’s run the code to see the result:

Congratulations on reading to the end of this tutorial! The UnboundLocalError: local variable referenced before assignment occurs when you try to reference a local variable before assigning a value to it. Preferably, you can solve this error by passing parameters to your function. Alternatively, you can use the global keyword.

If you have if-elif statements in your code where you assign a value to a local variable and do not account for all outcomes, you may encounter this error. In which case, you must include an else statement to account for the missing outcome.

For further reading on Python code blocks and structure, go to the article: How to Solve Python IndentationError: unindent does not match any outer indentation level .

Go to the  online courses page on Python  to learn more about Python for data science and machine learning.

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4 ways to fix local variable referenced before assignment error in python, resolving the local variable referenced before assignment error in python.

Python is one of the world’s most popular programming languages due to its simplicity, readability, and versatility. Despite its many advantages, when coding in Python, one may encounter various errors, with the most common being the “local variable referenced before assignment” error.

Even the most experienced Python developers have encountered this error at some point in their programming career. In this article, we will look at four effective strategies for resolving the local variable referenced before assignment error in Python.

Strategy 1: Assigning a Value before Referencing

The first strategy is to assign a value to a variable before referencing it. The error occurs when the variable is referenced before it is assigned a value.

This problem can be avoided by initializing the variable before referencing it. For example, let us consider the snippet below:

“`python

add_numbers():

print(x + y)

add_numbers()

In the snippet above, the variables `x` and `y` are not assigned values before they are referenced in the `print` statement. Therefore, we will get a local variable “referenced before assignment” error.

To resolve this error, we must initialize the variables before referencing them. We can avoid this error by assigning a value to `x` and `y` before they are referenced, as shown below:

Strategy 2: Using the Global Keyword

In Python, variables declared inside a function are considered local variables. Thus, they are separate from other variables declared outside of the function.

If we want to use a variable outside of the function, we must use the global keyword. Using the global keyword tells Python that you want to use the variable that was defined globally, not locally.

For example:

In the code snippet above, the `global` keyword tells Python to use the variable `x` defined outside of the function rather than a local variable named `x`. Thus, Python will output 30.

Strategy 3: Adding Input Parameters for Functions

Another way to avoid the local variable referenced before assignment error is by adding input parameters to functions.

def add_numbers(x, y):

add_numbers(10, 20)

In the code snippet above, `x` and `y` are variables that are passed into the `add_numbers` function as arguments.

This approach allows us to avoid the local variable referenced before assignment error because the variables are being passed into the function as input parameters. Strategy 4: Initializing Variables before Loops or Conditionals

Finally, it’s also a good practice to initialize the variables before loops or conditionals.

If you are defining a variable within a loop, you must initialize it before the loop starts. This way, the variable already exists, and we can update the value inside the loop.

my_list = [1, 2, 3, 4, 5]

for number in my_list:

sum += number

In the code snippet above, the variable `sum` has been initialized with the value of 0 before the loop runs. Thus, we can update and use the variable inside the loop.

In conclusion, the “local variable referenced before assignment” error is a common issue in Python. However, with the strategies discussed in this article, you can avoid the error and write clean Python code.

Remember to initialize your variables, use the global keyword, add input parameters in functions, and initialize variables before loops or conditionals. By following these techniques, your Python code will be error-free and much easier to manage.

In essence, this article has provided four key strategies for resolving the “local variable referenced before assignment” error that is common in Python. These strategies include initializing variables before referencing, using the global keyword, adding input parameters to functions, and initializing variables before loops or conditionals.

These techniques help to ensure clean code that is free from errors. By implementing these strategies, developers can improve their code quality and avoid time-wasting errors that can occur in their work.

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What is UnboundLocalError: local variable referenced before assignment?

Trying to assign a value to a variable that does not have local scope can result in this error:

Python has a simple rule to determine the scope of a variable. To clarify, a variable is assigned in a function, that variable is local. Because it is assumed that when you define a variable inside a function, you only need to access it inside that function.

There are two variable scopes in Python: local and global. Global variables are accessible throughout an entire program. Whereas, local variables are only accessible within the function in which they are originally defined.

An example of Local variable referenced before assignment

We’re going to write a program that calculates the grade a student has earned in class.

Firstly, we start by declaring two variables:

These variables store the numerical and letter grades a student has earned, respectively. By default, the value of “letter” is “F”. Then, we write a function that calculates a student’s letter grade based on their numerical grade using an “if” statement:

Finally, we call our function:

This line of code prints out the value returned by the  calculate_grade()  function to the console. We pass through one parameter into our function: numerical. This is the numerical value of the grade a student has earned.

Let’s run our code of Local variable referenced before assignment and see what happens:

Here is an error!

The Solution of Local variable referenced before assignment

The code returns an error: Unboundlocalerror local variable referenced before assignment because we reference “letter” before we assign it.

We have set the value of “numerical” to 42. Our  if  statement does not set a value for any grade over 50. This means that when we call our  calculate_grade()  function, our return statement does not know the value to which we are referring.

Moreover, we do define “letter” at the start of our program. However, we define it in the global context. Because Python treats “return letter” as trying to return a local variable called “letter”, not a global variable.

Therefore, this problem of variable referenced before assignment could be solved in two ways. Firstly, we can add an  else  statement to our code. This ensures we declare “letter” before we try to return it:

Let’s try to run our code again:

Our code successfully prints out the student’s grade. This approach is good because it lets us keep “letter” in the local context. To clarify, we could even remove the “letter = “F”” statement from the top of our code because we do not use it in the global context.

Alternatively, we could use the “global” keyword to make our global keyword available in the local context in our  calculate_grade()  function:

We use the “global” keyword at the start of our function.

This keyword changes the scope of our variable to a global variable. This means the “return” statement will no longer treat “letter” like a local variable. Let’s run our code. Our code returns: F.

The code works successfully! Let’s try it using a different grade number by setting the value of “numerical” to a new number:

Our code returns: B.

Finally, we have fixed the local variable referenced before assignment error in the code.

To sum up, as you can see, the UnboundLocalError: local variable referenced before assignment error is raised when you try to assign a value to a local variable before it has been declared. Then, you can solve this error by ensuring that a local variable is declared before you assign it a value. Moreover, if a variable is declared globally that you want to access in a function, you can use the “global” keyword to change its value. In case you have any inquiry, let’s CONTACT US . With a lot of experience in Mobile app development services , we will surely solve it for you instantly.

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Function parameters and naming

Do I understand well that def function parameters allow the communication of values between the function scope and the rest of the code? So, for example, if I have:

I don’t need to set the my_function() param because I don’t need any parameter within print() . While if I define something like this:

{name} within the print() cannot retrieve value outside that function scope so I do have to set greeting parameter?

And why do the function parameter placeholder sometimes differ from variables in or outside the scope? For example, I define:

The first run of the outer_function does not need to retrieve any values from outside the function scope, and on the second run, just the inner_function is performed. So why is there a whatever parameter? Do we somehow use whatever ?

Hmm, did you intend to use a closure? Otherwise this function only works if input_string is a global variable.

The inner function can use the parameters of the outer function.

The way you use the terminology is a bit strange, but your understanding seems fine.

Because that’s the point . When you write a function that has a parameter, you are consciously expecting the calling code to give you the value that the code will work with. Because of this, you don’t need to know what the outside code calls it, because you won’t go looking for it - it’s given to you. You can call it whatever makes sense locally, and use that name to write the code for the function. That also means that multiple other places can use your function, and give it differently named - or even unnamed things:

This example is a lot more complicated. But first you should understand that there is no “second run” of outer_function . Instead, calling outer_function creates a new inner_function ; and when the button is pressed, Tkinter will call that separately created inner_function .

The intended technique looks like this instead:

Now, the inner_function will look up the whatever name when it runs. But when it was compiled, Python knew “oh, this function is nested, and the other function that it’s nested inside, has its own whatever name”. So Python will compile this a little differently. When the inner_function runs, it will look for a closure for whatever - a special storage area that remembers the locals from the outer_function call, even after the outer_function has finished running. (And it only creates this storage because it saw the nested function during compilation, and it only creates storage for whatever .)

The idea is, when we write

, now the outer_function can create a button callback that works with any tk.StringVar , not just the input_string global. You just have to pass it to outer_function , and it sets up this special “binding” for the inner_function , that lets it remember which tk.StringVar to use when the button is clicked.

So this means we can make multiple tk.StringVar s and multiple ttk.Button s, and use the same outer_function logic to explain how the buttons should work. When we call outer_function , we say which StringVar to use for which Button :

When we click each button, it shows the message from the corresponding entry.

Do I understand well that def function parameters allow the communication of values between the function scope and the rest of the code?

Yes. In particular, you write the code for the function to work with some fixed variable name, eg name , and the parameters assign a value to name for a particular call:

In this way the function does not need to know anything about the code which called it or what variable names that code might be using.

def greeting(name): print(f"Hi, my name is {name}.") greeting("Peter") {name} within the print() cannot retrieve value outside that function scope so I do have to set greeting parameter?

Broadly, we do not access variables from outside the function directly. We try to write “pure functions” which receive values only through their parameters and produce results only by the return statement. These functions do not have side effects, so they are easy to reason able and use safely and correctly.

However, a function has access to its outer scopes. When you use a variable name such as name in your greeting() function Python looks for the variable in the local scope i.e. the function’s local variables, which include the parameters, and then in outer scopes. Typically that outer scope is the module namespace, which is where what you think of as “global variables” live.

Here we have a function foo() with a parameter p . If you don’t supply the parameter is defaults to None and the function recognises that and sets p=DEFAULT_VALUE in that case. This kind of this is quite common. So: foo(3) would print foo: 3 and foo() would print foo: 10 .

Let’s look at the variables. When you access a variable Python looks for its name in the local variables and then in the outer scopes (which are just the module.global scope here). For p it finds it in the local variables because it is a parameter. For DEFAULT_VALUE it does not find it in the local variables but does find it in the outer scope.

You’ll have noticed that we don’t “declare” variables in Python. When you define a function Python scans the code for assignment statements. If you assign to the name anywhere in a function , that name is a local variable. The parameters are assigned to. So Python knows p is local because it’s a parameter. DEFAULT_VALUE is not assigned to, so it must be found elsewhere. Note that the assignment does not need to actually be run:

The variable x is a local variable because there’s an assignment statement in the function code. When you run the above you’ll get a UnboundLocalError because x has not yet been set, but Python still knows that it is a local variable, even if the outer scope also had an x variable.

And why do the function parameter placeholder sometimes differ from variables in or outside the scope? For example, I define: def outer_function(whatever): def inner_function(): print("Button pressed.") print(input_string.get()) return inner_function ... input_string = tk.StringVar(value = "test") entry = ttk.Entry(window, textvariable = input_string) entry.pack() ... button = ttk.Button(window, text = "Button 1", command = outer_function(input_string)) button.pack() The first run of the outer_function does not need to retrieve any values from outside the function scope, and on the second run, just the inner_function is performed. So why is there a whatever parameter? Do we somehow use whatever ?

This is more complex than you might think. And, I think, more complex than it needs to be.

To answer your question: you’re not using the whatever variable. I don’t think you need it at all. As written above . But I think the code above isn’t what might ordinarily be done.

I was thinking that Nice Zombies had missed the point, but now I think they’re spot on.

Do I gather you’ve adapted this code from some example?

So, you’re defining a button:

Here we make a button with the label Button 1 and arrange that when it is pushed it calls a function - that function is passed as the command parameter to the button setup. The idea is that pressing the button prints the current value of the input string, so the function you pass must access the object holding the input string.

Now, you’ve also defined an Entry widget which stores its stae, the input string, in a StringVar object which we refer to with the input_string variable:

So your button wants a callback function (the command parameter) which accesses the StringVar to get the current input string value. And that is when you call outer_function() : outer_function is a function which returns a function . The function it returns is what accesses and prints the value. Let’s look at how it is defined:

What does it do? It defines inner_function() , which runs a couple of print() calls. The second print directly accesses input_string to obtain the value - that’s the StringVar object. The whatever parameter is not used.

This works, but only because you’ve got exactly one Entry widget, so you get to name it directly in the function. What if you had more, for example two entries and two buttons:

As written, the call to outer_function(input_string) or outer_function(input_string2) ignores the parameter ( whatever ) and returns a function which always accesses input_string , the first one. Not so good: both buttons report on the same StringVar object.

I’d be naming these functions differently:

so that it is more clear that you have a function report() which reports on the value of some StringVar , and a function get_report_on_var() which returns a report function. So we would set up the buttons like this:

But the report() function still reaches for the hardwired input_string object. The intent of the button setups above is more clear: we want to report on input_string from button 1 and report on input_string2 from button 2. But the get_report_on_var() function ignores its argument!

What we now want is for get_report_on_var() to return a function which reports on whatever StringVar we hand it. So it needs a further change:

Now it reports on the particular StringVar object you passed in when you set up the button, and the parameter (formerly whatever ) is not ignored !

So: how does this work?

What you have above is called a “closure”, where the inner function report() has access to the variables in its immediate outer scope: those local variables of get_report_on_var() . When it accesses some_var , that is not a local variable of report() . So Python looks in the next outer scope, which is the local variables of get_report_on_var() at the time you called it . That’s important.

When you set up the buttons, you call get_report_on_var() to obtain the callback function for the button. The get_report_on_var() function defines a new function called report() :

Functions are just another type of object in Python, so this is a kind of assignment statement: the name report is bound to the function as a local variable in get_report_on_var() . And the new function you just made has access to get_report_on_var() local variables, particularly some_var , which is a local variable because it came from the parameters.

When you run a function, the local variables are new , unrelated to the ones some any earlier or later call: you get a fresh set every time.

So when you define the first button, you call get_report_on_var() once, and it returns a function which accesses some_var , which on that call is input_string .

When you define the second button, you call get_report_on_var() once again, and it returns another function which accesses some_var , which on that call is input_string2 , the other StringVar object`.

These “callback unctions” are not run until you press one of the buttons.

I think someone’s trying to teach your about closures, which is why the functions had the names outer_function and inner_function , but they’re poorly named for the programme as a whole.

Finally, returning your first question about scopes: the inner function report() (which gets defines anew on every call to get_report_on_var() ) has 3 scopes to access names:

• its local variables
• the local variables of get_report_on_var()
• the module/global variables

A function parameter is a local variable, but one that can be defined in a special way. For example,

Both x and y are local variables, defined in the scope of foo . But while y is defined explicitly by an assignment statement, x is defined implicitly via a function call. That is,

basically performs an assignment x = 3 before entering the body of the function.

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Python Functions

Python Functions is a block of statements that return the specific task. The idea is to put some commonly or repeatedly done tasks together and make a function so that instead of writing the same code again and again for different inputs, we can do the function calls to reuse code contained in it over and over again.

Some Benefits of Using Functions

• Increase Code Readability 
• Increase Code Reusability

Python Function Declaration

The syntax to declare a function is:

Syntax of Python Function Declaration

Types of Functions in Python

Below are the different types of functions in Python :

• Built-in library function: These are Standard functions in Python that are available to use.
• User-defined function: We can create our own functions based on our requirements.

Creating a Function in Python

We can define a function in Python, using the def keyword. We can add any type of functionalities and properties to it as we require. By the following example, we can understand how to write a function in Python. In this way we can create Python function definition by using def keyword.

Calling a Function in Python

After creating a function in Python we can call it by using the name of the functions Python followed by parenthesis containing parameters of that particular function. Below is the example for calling def function Python.

Python Function with Parameters

If you have experience in C/C++ or Java then you must be thinking about the return type of the function and data type of arguments. That is possible in Python as well (specifically for Python 3.5 and above).

Python Function Syntax with Parameters

The following example uses arguments and parameters that you will learn later in this article so you can come back to it again if not understood.

Note: The following examples are defined using syntax 1, try to convert them in syntax 2 for practice.

Python Function Arguments

Arguments are the values passed inside the parenthesis of the function. A function can have any number of arguments separated by a comma.

In this example, we will create a simple function in Python to check whether the number passed as an argument to the function is even or odd.

Types of Python Function Arguments

Python supports various types of arguments that can be passed at the time of the function call. In Python, we have the following function argument types in Python:

• Default argument
• Keyword arguments (named arguments)
• Positional arguments
• Arbitrary arguments (variable-length arguments *args and **kwargs)

Let’s discuss each type in detail. 

Default Arguments

A default argument is a parameter that assumes a default value if a value is not provided in the function call for that argument. The following example illustrates Default arguments to write functions in Python.

Like C++ default arguments, any number of arguments in a function can have a default value. But once we have a default argument, all the arguments to its right must also have default values.

Keyword Arguments

The idea is to allow the caller to specify the argument name with values so that the caller does not need to remember the order of parameters.

Positional Arguments

We used the Position argument during the function call so that the first argument (or value) is assigned to name and the second argument (or value) is assigned to age. By changing the position, or if you forget the order of the positions, the values can be used in the wrong places, as shown in the Case-2 example below, where 27 is assigned to the name and Suraj is assigned to the age.

Arbitrary Keyword  Arguments

In Python Arbitrary Keyword Arguments, *args, and **kwargs can pass a variable number of arguments to a function using special symbols. There are two special symbols:

• *args in Python (Non-Keyword Arguments)
• **kwargs in Python (Keyword Arguments)

Example 1: Variable length non-keywords argument

Example 2: Variable length keyword arguments

The first string after the function is called the Document string or Docstring in short. This is used to describe the functionality of the function. The use of docstring in functions is optional but it is considered a good practice.

The below syntax can be used to print out the docstring of a function.

Example: Adding Docstring to the function

Python Function within Functions

A function that is defined inside another function is known as the inner function or nested function . Nested functions can access variables of the enclosing scope. Inner functions are used so that they can be protected from everything happening outside the function.

Anonymous Functions in Python

In Python, an anonymous function means that a function is without a name. As we already know the def keyword is used to define the normal functions and the lambda keyword is used to create anonymous functions.

Recursive Functions in Python

Recursion in Python refers to when a function calls itself. There are many instances when you have to build a recursive function to solve Mathematical and Recursive Problems.

Using a recursive function should be done with caution, as a recursive function can become like a non-terminating loop. It is better to check your exit statement while creating a recursive function.

Here we have created a recursive function to calculate the factorial of the number. You can see the end statement for this function is when n is equal to 0. 

Return Statement in Python Function

The function return statement is used to exit from a function and go back to the function caller and return the specified value or data item to the caller. The syntax for the return statement is:

The return statement can consist of a variable, an expression, or a constant which is returned at the end of the function execution. If none of the above is present with the return statement a None object is returned.

Example: Python Function Return Statement

Pass by Reference and Pass by Value

One important thing to note is, in Python every variable name is a reference. When we pass a variable to a function Python, a new reference to the object is created. Parameter passing in Python is the same as reference passing in Java.

When we pass a reference and change the received reference to something else, the connection between the passed and received parameters is broken. For example, consider the below program as follows:

Another example demonstrates that the reference link is broken if we assign a new value (inside the function). 

Exercise: Try to guess the output of the following code. 

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FAQs- Python Functions

Q1. what is function in python.

Python function is a block of code, that runs only when it is called. It is programmed to return the specific task. You can pass values in functions called parameters. It helps in performing repetitive tasks.

Q2. What are the 4 types of Functions in Python?

The main types of functions in Python are: Built-in function User-defined function Lambda functions Recursive functions

Q3. H ow to Write a Function in Python ?

To write a function in Python you can use the def keyword and then write the function name. You can provide the function code after using ‘:’. Basic syntax to define a function is: def function_name(): #statement

Q4. What are the parameters of a function in Python?

Parameters in Python are the variables that take the values passed as arguments when calling the functions. A function can have any number of parameters. You can also set default value to a parameter in Python.

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