Why nested functions can access variables from outer functions, but are not allowed to modify them [duplicate]

In Python 3.x this is possible:

def f1():
        x = 5
        def f2():
                nonlocal x
                x+=1
        return f2

The problem and a solution to it, for Python 2.x as well, are given in this post. Additionally, please read PEP 3104 for more information on this subject.