Python属性是否已损坏？

import unittest

class PropTest(unittest.TestCase):
    def test(self):
        class C():
            val = 'initial val'

            def get_p(self):
                return self.val

            def set_p(self,prop):
                if prop == 'legal val':
                    self.val = prop

            prop=property(fget=get_p,fset=set_p)

        c=C()
        self.assertEqual('initial val',c.prop)

        c.prop='legal val'
        self.assertEqual('legal val',c.prop)

        c.prop='illegal val'
        self.assertNotEqual('illegal val',c.prop)

失败如下？

Failure
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/unittest.py",line 279,in run
    testMethod()
  File "/Users/jacob/aau/admissions_proj/admissions/plain_old_unit_tests.py",line 24,in test
    self.assertNotEqual('illegal val',c.prop)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/unittest.py",line 358,in failIfEqual
    (msg or '%r == %r' % (first,second))
AssertionError: 'illegal val' == 'illegal val'