add back mutable vs non mutable handling

Author: quantresearch1

Diff for: pyomo/contrib/solver/solvers/highs.py

@@ -600,6 +600,7 @@ def _set_objective(self, obj):
         if self._last_results_object is not None:
             self._last_results_object.solution_loader.invalidate()
         n = len(self._pyomo_var_to_solver_var_map)
+        indices = np.arange(n)
         costs = np.zeros(n, dtype=np.double)
 
         # Initialize empty lists for all coefficient types
@@ -658,6 +659,7 @@ def _set_objective(self, obj):
                     )
 
         self._solver_model.changeObjectiveSense(sense)
+        self._solver_model.changeColsCost(n, indices, costs)
         self._mutable_objective = _MutableObjective(
             self._solver_model,
             mutable_constant,

Diff for: pyomo/contrib/solver/tests/solvers/test_highs.py

@@ -9,15 +9,10 @@
 #  This software is distributed under the 3-clause BSD License.
 #  ___________________________________________________________________________
 
-import subprocess
-import sys
-
 import pyomo.common.unittest as unittest
 import pyomo.environ as pe
 
 from pyomo.contrib.solver.solvers.highs import Highs
-from pyomo.common.log import LoggingIntercept
-from pyomo.common.tee import capture_output
 
 opt = Highs()
 if not opt.available():