routing: pybind11 fixup

Author: Mizux

ortools/routing/python/model.cc

@@ -202,8 +202,8 @@ PYBIND11_MODULE(model, m) {
          pybind11::return_value_policy::reference_internal, arg("index"));
   rm.def("get_arc_cost_for_vehicle", &Model::GetArcCostForVehicle,
          arg("from_index"), arg("to_index"), arg("vehicle"));
-  rm.def("solver", &Model::solver,
-         pybind11::return_value_policy::reference_internal);
+  rm.def_property_readonly("solver", &Model::solver,
+                           pybind11::return_value_policy::reference_internal);
 
   pybind11::enum_<Model::PenaltyCostBehavior>(rm, "PenaltyCostBehavior")
       .value("PENALIZE_ONCE", Model::PenaltyCostBehavior::PENALIZE_ONCE)

ortools/routing/python/model_test.py

@@ -656,7 +656,7 @@ def testRoutingModelParameters(self):
         routing_model = model.Model(manager, parameters)
         self.assertIsNotNone(routing_model)
         self.assertEqual(1, routing_model.vehicles())
-        self.assertTrue(routing_model.solver().parameters.trace_propagation)
+        self.assertTrue(routing_model.solver.parameters.trace_propagation)
 
     def testRoutingLocalSearchFiltering(self):
         parameters = model.default_routing_model_parameters()
@@ -666,7 +666,7 @@ def testRoutingLocalSearchFiltering(self):
         routing_model = model.Model(manager, parameters)
         self.assertIsNotNone(routing_model)
         routing_model.solve()
-        profile = routing_model.solver().local_search_profile()
+        profile = routing_model.solver.local_search_profile()
         print(profile)
         self.assertIsInstance(profile, str)
         self.assertTrue(profile)  # Verify it's not empty.