UnboundLocalError: local variable referenced before assignment

How to Reproduce the Error

Following is a simple example of reproducing the UnboundLocalError in Python.

# declare variable
text = "Tutorial"

def print_text():
    print(text)
    text = "Python Tutorial"

print_text()

Output

UnboundLocalError: local variable 'text' referenced before assignment

Python does not have a variable declaration. Hence it determines the variable's scope when you assign a value to it.

If you assign a value to a variable inside a function, then it's considered a local scope; otherwise, it's considered a global scope.

In the above example, we have created a variable text outside the function, which is a global variable.

We also have created a local variable text and assigned a value inside the function print_text() that shadows the global variable scope, and hence it results in UnboundLocalError: local variable 'text' referenced before assignment.

How to Fix the Error

Solution 1: Change the scope of the variable to global

The global variables created can be accessed inside a function, but you cannot change or modify its value unless you change the variable's scope.

In our example, we were modifying the value of a global variable inside a function which resulted in UnboundLocalError

To resolve the error, simply mark the variable as global inside the function, which changes the variable's scope.

# declare variable
text = "Tutorial"

def print_text():
    # change the scope to global
    global text
    print(text)
    text = "Python Tutorial"

print_text()
print_text()

Output

Tutorial
Python Tutorial

Solution 2: Passing global variable as an argument to the function

The alternate solution is to pass the global variable as an argument to the function, as shown below.

# declare variable
text = "Tutorial"

def print_text(text):
   
    print(text)
    text = "Python Tutorial"

print_text(text)

Output

Tutorial

Rules for Global and Local variables in Python

To avoid UnboundLocalError you should also know the general guidelines of local and global variables in Python.

• In Python, variables that are only referenced inside a function are implicitly global.
• If the value is assigned to a variable inside a function, then the scope changes to local unless it's explicitly declared as global.

Conclusion

In Python, UnboundLocalError: local variable referenced before assignment occurs when you reference a local variable before assigning any value to it inside a function.

To solve the error, you need to make it a global variable inside a function or pass the global variable as a function argument to the method.