How to Fix UnboundLocalError

What is UnboundLocalError?

An UnboundLocalError occurs when you try to use a local variable in a function before assigning a value to it. This often happens when a variable with the same name exists in the global scope, but you assign to it inside a function without declaring it as global; Python then treats it as a new local variable, which hasn't been given a value yet.

Common Causes

• Assigning to a variable inside a function that also exists in the global scope, without using the 'global' keyword.
• Referencing a local variable before it is assigned within a function.
• Confusing global and local variable scopes in nested functions.

How to Fix

1. If you intend to use the global variable inside the function, declare it as 'global' at the beginning of the function.
2. Ensure that you assign a value to the variable before referencing it within the function.

Wrong Code

x = 10

def foo():
    print(x)  # UnboundLocalError here
    x = 5

foo()

Correct Code

x = 10

def foo():
    global x
    print(x)  # Works fine
    x = 5

foo()