Why can't a Python class definition assign a closure variable to itself?

Question

Why doesn't the following work in Python?

def make_class(a):
    class A(object):
        a=a
    return A

Answer 1

works just fine:

>>> def make_class(a):
    class A(object):
        _a=a
    return A

>>> make_class('df')
<class '__main__.A'>
>>> make_class('df')._a
'df'

btw, function is not a reserved keyword in Python.