How to retrieve the sense of an objective function in pyomo

Question

So the problem is as follows:

I am working on an algorithm where an argument is whether the optimization problem is a maximization or a minimization problem. From the optimization model, the code looks like this:

model.obj = pyo.Objective(expr= sum(model.c[i]*model.x[i] for i in model.i),
                      sense=maximize)

And the output from pprint() looks like this:

obj : Size=1, Index=None, Active=True
Key  : Active : Sense    : Expression
None :   True : maximize : 1.2*x[1] + 2*x[2]

How can I retrieve the element "maximize" from model.obj?

Answer 1

It appears that the pyo.Objective model object holds the attribute you are looking for. After a little spelunking in ipython:

In [1]: import pyomo.environ as pyo

In [2]: m = pyo.ConcreteModel()

In [3]: m.obj = pyo.Objective(expr=1, sense=pyo.maximize)

In [4]: m.pprint()
1 Objective Declarations
    obj : Size=1, Index=None, Active=True
        Key  : Active : Sense    : Expression
        None :   True : maximize :        1.0

1 Declarations: obj

In [5]: m.obj.sense
Out[5]: -1

In [6]: m.obj = pyo.Objective(expr=1, sense=pyo.minimize)
WARNING: Implicitly replacing the Component attribute obj (type=<class
    'pyomo.core.base.objective.ScalarObjective'>) on block unknown with a new
    Component (type=<class 'pyomo.core.base.objective.ScalarObjective'>). This
    is usually indicative of a modelling error. To avoid this warning, use
    block.del_component() and block.add_component().

In [7]: m.obj.sense
Out[7]: 1