How to use the executing.PY3 function in executing

Source.decode_source(encoded)
            else:
                decoded = Source.decode_source(encoded)
                if matches:
                    self.assertEqual(decoded, source)
                else:
                    self.assertNotEqual(decoded, source)

        check(u'# coding=utf8\né', 'utf8')
        check(u'# coding=gbk\né', 'gbk')

        check(u'# coding=utf8\né', 'gbk', exception=UnicodeDecodeError)
        check(u'# coding=gbk\né', 'utf8', matches=False)

        # In Python 3 the default encoding is assumed to be UTF8
        if PY3:
            check(u'é', 'utf8')
            check(u'é', 'gbk', exception=SyntaxError)