Fix Python UnboundLocalError: local variable 'x' referenced before assignment (2025 Guide)

Posted on: March 22, 2025

Encountered an "UnboundLocalError: local variable 'x' referenced before assignment" in Python? This error occurs when a variable is used in a function before being assigned a value locally. Let’s fix it fast in this 2025 guide!

What Causes "UnboundLocalError: local variable 'x' referenced before assignment"?

This error happens due to Python’s scope rules (LEGB: Local, Enclosing, Global, Built-in). If a variable is assigned within a function, Python treats it as local, but referencing it before assignment triggers the error. Common causes include:

• Scope Confusion: Using a global variable without declaring it.
• Missing Assignment: Referencing a variable before defining it locally.
• Conditional Assignment: Assigning in one branch but not all.

# This triggers "UnboundLocalError"
x = 10
def increment():
    x += 1  # x is treated as local but not yet assigned
    print(x)
increment()

How to Fix It: 3 Solutions

(Diagram: Developer adjusts scope or assignment, resolves error, runs successfully.)

Solution 1: Use the Global Keyword

# Wrong
x = 10
def increment():
    x += 1

# Fixed
x = 10
def increment():
    global x  # Declare x as global
    x += 1
    print(x)
increment()  # 11

Use `global` to tell Python to use the outer scope variable.

Solution 2: Assign Locally First

# Wrong
def increment():
    x += 1

# Fixed
def increment():
    x = 0  # Assign locally first
    x += 1
    print(x)
increment()  # 1

Initialize the variable within the function before use.

Solution 3: Pass as Parameter

# Wrong
x = 10
def increment():
    x += 1

# Fixed
def increment(x):
    x += 1
    return x
x = 10
x = increment(x)
print(x)  # 11

Pass the variable as an argument to avoid scope issues.

Quick Checklist

• Global variable? (Use `global`)
• No assignment? (Define it first)
• Cleaner code? (Pass as parameter)

Conclusion

The "UnboundLocalError: local variable 'x' referenced before assignment" in Python is a scope-related issue you can fix with proper variable handling. With these 2025 solutions, your code will run smoothly. Got another Python error? Let us know in the comments!