def METHOD_NAME(self):
    payable = payables.get_payable(MockModel)
    MockPayable.immutable_after_payment = False
    self.assertFalse(payable.immutable_after_payment)
    MockPayable.immutable_after_payment = True
    self.assertTrue(payable.immutable_after_payment)
    self.assertFalse(Payable.immutable_after_payment)