UnboundLocalError in Python with Example

UnboundLocalError in Python occurs when variable is identified as Local but it is used prior of creation.

This article explains UnboundLocalError with Example.

Consider a following simple Python program in which variable var is created in global scope and accessed from my_function:

var = 20 # var is global variable

def my_function():
    print(var) # It access global

my_function() # Calling function

Output

20

Above output is straight forward and is expected.

But, when we have assignment for var within my_function after print then things will be different. See following python program which illustrates UnboundLocalError:

# UnboundLocalError in Python
var = 20 # a is global variable

def my_function():
    print(var)
    var = 'Hello' # This causes UnboundLocalError
    print(var)

my_function() # Calling function

Output

UnboundLocalError: local variable 'var' referenced before assignment

So, what happened here? When we create & execute function definition def my_function():, without calling it, it does not create any local scope for my_function.

Creating local scope for my_function in this example happens at compile time. At compile time Python sees var = 'Hello' which is creating local variable var. So Python determines var will be local but at the same time it also sees print(var) which is used before assignment. Which means variable var is local but not yet bounded or created giving UnboundLocalError but it is determined that var will be local.