Referenced before assignment error

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:

# Variable decleration outside function (global scope)
myVar = 10


def myFunction():
      if myVar == 5:
            print("5")
      elif myVar == 10:
            print("10")
      elif myVar >= 15:
            print("greater than 15 ")
     # reassigning myVar in the function
      myVar = 0

myFunction()

Output

Explanation

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

Solutions

Using Global Variables

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

myVar = 10
def myFunction():
     # Telling the function to consider myVar as global
      global myVar
      if myVar == 5:
            print("5")
      elif myVar == 10:
            print("10")
      elif myVar >= 15:
            print("greater than 15 ")
      myVar = 0

myFunction()

Output

10

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.

# Passing myVar as a parameter in the function
def myFunction(myVar):
      if myVar == 5:
            print("5")
      elif myVar == 10:
            print("10")
      elif myVar >= 15:
            print("greater than 15 ")
      myVar = 0

myFunction(10)

Output

10

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.

$ anaconda-navigator          
Traceback (most recent call last):
  File "/home/user/anaconda3/lib/python3.7/site-packages/anaconda_navigator/widgets/main_window.py", line 541, in setup
    self.post_setup(conda_data=conda_data)
...
...
UnboundLocalError: local variable 'DISTRO_NAME' referenced before assignment

Solution 1

Try and update your Anaconda Navigator with the following command.

conda update anaconda-navigator

Solution 2

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

.../anaconda3//lib/python3.7/site-packages/anaconda_navigator/api/external_apps/vscode.py

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.

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

from django import forms


# Creating a Class for django forms
class MyForm(forms.Form):
    name = forms.CharField(required=True)
    email = forms.EmailField(required=True)
    msg = forms.CharField(
        required=True,
        widget=forms.Textarea
    )


...
# Create a function to retrieve info from the form
def GetContact(request):
    form_class = ContactForm
    if request.method == 'POST':
        form = form_class(request.POST)

        # Upon verifying validity, get following info and email with said info
        if form.is_valid():
            name = request.POST.get('name')
            email = request.POST.get('email')
            msg = request.POST.get('msg')

            send_mail('Subject here', msg, email, ['[email protected]'], fail_silently=False)
            return HttpResponseRedirect('blog/inicio')
            return render(request, 'blog/inicio.html', {'form': form})
...

Upon running you get the following error:

local variable 'form' referenced before assignment

Explanation

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.

FAQs on Local Variable Referenced Before Assignment

Conclusion

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.

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:

test_var = 0

def test_func(test_var):
    test_var += 1
    return test_var

test_func(test_var)

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

test_var = 0

def test_func():
    global test_var
    test_var += 1
    return test_var

test_func()

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

What is Scope in Python?

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: local variable referenced before assignment

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:

var = 10

def test_func():
    print(var)

test_func()

10

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

var = 10

def test_func():
    var += 1
    print(var)
test_func()

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
Input In [17], in <cell line: 6>()
      4     var += 1
      5     print(var)
----> 6 test_func()

Input In [17], in test_func()
      3 def test_func():
----> 4     var += 1
      5     print(var)

UnboundLocalError: local variable 'var' referenced before assignment

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.

number = 10

def increment_func():
    number += 1
    return number

print(increment_func())

Let’s run the code to see what happens:

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
Input In [19], in <cell line: 7>()
      4     number += 1
      5     return number
----> 7 print(increment_func())

Input In [19], in increment_func()
      3 def increment_func():
----> 4     number += 1
      5     return number

UnboundLocalError: local variable 'number' referenced before assignment

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

Solution #1: Passing Parameters to the Function

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:

number = 10

def increment_func(number):

    number += 1

    return number

print(increment_func(number))

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:

11

We successfully printed the value to the console.

Solution #2: Use Global Keyword

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:

number = 10

def increment_func():

    global number

    number += 1

    return number

print(increment_func())

Let’s run the code to see the result:

11

We successfully printed the value to the console.

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.

score = int(input("Enter your score between 0 and 100: "))

def calculate_level(score):

    if score > 90:

        level = 'expert'

    elif score > 70:

        level = 'advanced'

    elif score > 55:

        level = 'intermediate'

    return 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:

print(f'Your level is: {calculate_level(score)}')

Enter your score between 0 and 100: 40

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
Input In [12], in <cell line: 1>()
----> 1 print(f'Your level is: {calculate_level(score)}')

Input In [11], in calculate_level(score)
      7 elif score > 55:
      8     level = 'intermediate'
----> 9 return level

UnboundLocalError: local variable 'level' referenced before assignment

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.

Solution #1: Include else statement

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:

score = int(input("Enter your score between 0 and 100: "))

def calculate_level(score):

    if score > 90:

        level = 'expert'

    elif score > 70:

        level = 'advanced'

    elif score > 55:

        level = 'intermediate'

    else:

        level = 'beginner'

    return level

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

print(f'Your level is: {calculate_level(score)}')

Enter your score between 0 and 100: 40

Your level is: beginner

Solution #2: Use global keyword

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.

score = int(input("Enter your score between 0 and 100: "))

level = 'beginner'

def calculate_level(score):

    global level

    if score > 90:

        level = 'expert'

    elif score > 70:

        level = 'advanced'

    elif score > 55:

        level = 'intermediate'

    return level

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:

print(f'Your level is: {calculate_level(score)}')

40 

Your level is: beginner

Summary

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.

Have fun and happy researching!

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.

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:

UnboundLocalError: local variable referenced before assignment

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:

numerical = 36
letter = "F"

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:

def calculate_grade(grade):
	if grade > 80:
		letter = "A"
	elif grade > 70:
		letter = "B"
	elif grade > 60:
		letter = "C"
	elif grade > 50:
		letter = "D"
	return letter

Finally, we call our function:

print(calculate_grade(numerical))

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:

Traceback (most recent call last):
  File "main.py", line 15, in <module>
	print(calculate_grade(numerical))
  File "main.py", line 13, in calculate_grade
	return letter
UnboundLocalError: local variable 'letter' referenced before assignment

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:

def calculate_grade(grade):
	if grade > 80:
		letter = "A"
	elif grade > 70:
		letter = "B"
	elif grade > 60:
		letter = "C"
    elif grade > 50:
        letter = "D"
    else:
        letter = "F"
    return letter

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.

Conclusion

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!