add model cloning to model_builder python + sample

ortools/linear_solver/python/model_builder.py

@@ -631,6 +631,12 @@ class ModelBuilder:
     def __init__(self):
         self.__helper: mbh.ModelBuilderHelper = mbh.ModelBuilderHelper()
 
+    def clone(self) -> "ModelBuilder":
+        """Returns a clone of the current model."""
+        clone = ModelBuilder()
+        clone.helper.overwrite_model(self.helper)
+        return clone
+
     @typing.overload
     def _get_linear_constraints(self, constraints: Optional[pd.Index]) -> pd.Index:
         ...

ortools/linear_solver/python/model_builder_helper.cc

@@ -169,6 +169,7 @@ PYBIND11_MODULE(model_builder_helper, m) {
 
   py::class_<ModelBuilderHelper>(m, "ModelBuilderHelper")
       .def(py::init<>())
+      .def("overwrite_model", &ModelBuilderHelper::OverwriteModel, arg("other_helper"))
       .def("export_to_mps_string", &ModelBuilderHelper::ExportToMpsString,
            arg("options") = MPModelExportOptions())
       .def("export_to_lp_string", &ModelBuilderHelper::ExportToLpString,

ortools/linear_solver/samples/BUILD.bazel

@@ -44,6 +44,8 @@ code_sample_py(name = "assignment_mb")
 
 code_sample_py(name = "bin_packing_mb")
 
+code_sample_py(name = "copy_model_mb")
+
 code_sample_py(name = "simple_lp_program_mb")
 
 code_sample_py(name = "simple_mip_program_mb")

ortools/linear_solver/samples/copy_model_mb.py

@@ -0,0 +1,92 @@
+#!/usr/bin/env python3
+# Copyright 2010-2022 Google LLC
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# [START program]
+"""Integer programming examples that show how to use the APIs."""
+# [START import]
+import math
+
+from ortools.linear_solver.python import model_builder
+# [END import]
+
+
+def main():
+    # [START model]
+    # Create the model.
+    model = model_builder.ModelBuilder()
+    # [END model]
+
+    # [START variables]
+    # x and y are integer non-negative variables.
+    x = model.new_int_var(0.0, math.inf, "x")
+    y = model.new_int_var(0.0, math.inf, "y")
+    # [END variables]
+
+    # [START constraints]
+    # x + 7 * y <= 17.5.
+    c1 = model.add(x + 7 * y <= 17.5)
+
+    # x <= 3.5.
+    c2 = model.add(x <= 3.5)
+    # [END constraints]
+
+    # [START objective]
+    # Maximize x + 10 * y.
+    model.maximize(x + 10 * y)
+    # [END objective]
+
+    # Deep copy.
+    print("Cloning the model.")
+    model_copy = model.clone()
+    x_copy = model_copy.var_from_index(x.index)
+    y_copy = model_copy.var_from_index(y.index)
+    z_copy = model_copy.new_bool_var("z")
+    c2_copy = model_copy.linear_constraint_from_index(c2.index)
+
+    # Add new constraint.
+    model_copy.add(x_copy >= 1)
+    print("Number of constraints in original model =", model.num_constraints)
+    print("Number of constraints in cloned model =", model_copy.num_constraints)
+
+    # Modify a constraint.
+    c2_copy.add_term(z_copy, 2.0)
+
+    print(model_copy.export_to_lp_string())
+
+    # [START solve]
+    # Create the solver with the SCIP backend, and solve the model.
+    solver = model_builder.ModelSolver("scip")
+    status = solver.solve(model_copy)
+    # [END solve]
+
+    # [START print_solution]
+    if status == model_builder.SolveStatus.OPTIMAL:
+        print("Solution:")
+        print("Objective value =", solver.objective_value)
+        print("x =", solver.value(x_copy))
+        print("y =", solver.value(y_copy))
+        print("z =", solver.value(z_copy))        
+    else:
+        print("The problem does not have an optimal solution.")
+    # [END print_solution]
+
+    # [START advanced]
+    print("\nAdvanced usage:")
+    print("Problem solved in %f seconds" % solver.wall_time)
+    # [END advanced]
+
+
+if __name__ == "__main__":
+    main()
+# [END program]

ortools/linear_solver/wrappers/model_builder_helper.cc

@@ -40,6 +40,10 @@
 
 namespace operations_research {
 
+void ModelBuilderHelper::OverwriteModel(const ModelBuilderHelper& other_helper) {
+  model_ = other_helper.model();
+}
+
 std::string ModelBuilderHelper::ExportToMpsString(
     const MPModelExportOptions& options) {
   return operations_research::ExportModelAsMpsFormat(model_, options)

ortools/linear_solver/wrappers/model_builder_helper.h

@@ -45,6 +45,7 @@ namespace operations_research {
 // absl::Status or absl::StatusOr.
 class ModelBuilderHelper {
  public:
+  void OverwriteModel(const ModelBuilderHelper& other_helper);
   std::string ExportToMpsString(const operations_research::MPModelExportOptions&
                                     options = MPModelExportOptions());
   std::string ExportToLpString(const operations_research::MPModelExportOptions&