Solver disregards previous set objectives

[yaccos]

I have discovered a bug with the Gurobi solver interface. GurobiSolver.solve() calls GurobiSolver.set_objective() internally (

reframed/reframed/solvers/gurobi_solver.py

Line 240 in 15a52a9

self.set_objective(linear, quadratic, minimize)

), but doing so will wipe out previous set objectives (

reframed/reframed/solvers/gurobi_solver.py

Line 197 in 15a52a9

obj_expr = quicksum(quad_obj + lin_obj)

) to the solver even if objectives to GurobiSolver.Solve() are unspecified. In effect, calling set_objective() before a call to solve() has no effect even when the model is updating after setting the objective.