def METHOD_NAME(self):
    utime_mock = Mock(
        netValues = [('NULL',None,12345)],
        netPrevValues = [('NULL',None,12354)]
    )
    group = {'kernel.all.cpu.user':utime_mock}
    m_repo = ReportingMetricRepository(group)
    c_utime = m_repo.previous_value('kernel.all.cpu.guest',None)
    self.assertIsNone(c_utime)