def METHOD_NAME(self, block, port, number):
    """assert that the given block has the given number of subscriptions
    on the given port
    :param block: a block sptr
    :param string port: the port name
    :param number: the number of subscriptions expected
    """
    subs = block.message_subscribers(pmt.intern(port))
    self.assertEqual(pmt.length(subs), number)