Python decorator parameters value not updating

Question

from functools import wraps

def panel(title=None, footer=None):
    def decorator(func):
        @wraps(func)
        def inner(*args, **kwargs):
            heading = ''
            if title:
                heading = title
            if footer:
               footer = footer + '\n'
            return heading + footer
        return inner
    return decorator


@panel(title='Hello', footer='World')
def hello_foo():
    print 'Executing'

hello_foo()

So, I am working on a code sample which follows the same logic . On executing this code I get an error:

UnboundLocalError: local variable 'footer' referenced before assignment

After debugging, I found that if try to update the value of footer it throws me an error as it is a decorator parameter.

So, the two code samples given below work

def panel(title=None, footer=None):
    def decorator(func):
        @wraps(func)
        def inner(*args, **kwargs):
            heading = ''
            if title:
                heading = title
            if footer:
               print footer
            print heading + footer
        return inner
    return decorator

In the above sample I am just printing the value of footer and not doing any update operation

def panel(title=None, footer_t=None):
    def decorator(func):
        @wraps(func)
        def inner(*args, **kwargs):
            heading = ''
            footer = ''
            if title:
                heading = title
            if footer_t:
                footer = footer_t + '\n'
            return heading + footer
        return inner
    return decorator

Here I changed the decorator parameter from footer to footer_t.

My question is what could be the possible reason behind the the error when I try to update the decorator parameter?