'Attempt an open, but if the file is /dev/net/tun and it does not exist,
translate the error into L{SkipTest} so that tests that require
platform support for tuntap devices are skipped instead of failed.'

def open(self, filename, *args, **kwargs):

    try:
        return super(TestRealSystem, self).open(filename, *args, **kwargs)
    except OSError as e:
        if ((e.errno in (ENOENT, ENODEV)) and (filename == '/dev/net/tun')):
            raise SkipTest('Platform    lacks    /dev/net/tun')
        raise
