Pyomo: Check if solution exists and if not solve other problem

Question

I am using pyomo with gurobi (and other solvers) to solve some optimization programs. Sometimes these programs don't have a solution and throw an error such as

WARNING: Loading a SolverResults object with a warning status into
    model.name="unknown";
      - termination condition: infeasible
      - message from solver: Model was proven to be infeasible.
ERROR: evaluating object as numeric value: x[0]
        (object: <class 'pyomo.core.base.var._GeneralVarData'>)
    No value for uninitialized NumericValue object x[0]
ERROR: evaluating object as numeric value: objective
        (object: <class 'pyomo.core.base.objective.SimpleObjective'>)
    No value for uninitialized NumericValue object x[0]

...

  File "pyomo\core\expr\numvalue.pyx", line 246, in pyomo.core.expr.numvalue.value
  File "pyomo\core\expr\numvalue.pyx", line 233, in pyomo.core.expr.numvalue.value
ValueError: No value for uninitialized NumericValue object x[0]

In this case, I would like to run a simpler program without some of the constraints, so something like

solve program
if program doesn't have a solution:
   solve simpler program 

How could I do this?

Answer 1

From Pyomo's website:

from pyomo.opt import SolverStatus, TerminationCondition

…

results = opt.solve(instance) # Solving a model instance  
instance.load(results) # Loading solution into results object

if (results.solver.status == SolverStatus.ok) and (results.solver.termination_condition == TerminationCondition.optimal):
    # Do something when the solution in optimal and feasible
elif (results.solver.termination_condition == TerminationCondition.infeasible):
    # Do something when model in infeasible
else:
    # Something else is wrong
    print “Solver Status: ”,  result.solver.status

But be careful, because you want to ensure that your simpler model doesn't return an infeasible solution too.