>>> import pyomo.environ as pyo
>>> from pyomo.opt import SolverFactory

>>> model = pyo.ConcreteModel()

>>> model.x = pyo.Var(within=pyo.PositiveReals)
>>> model.y = pyo.Var(within=pyo.PositiveReals)

>>> model.sillybound = pyo.Constraint(expr = model.x + model.y <= 2)

>>> model.obj = pyo.Objective(expr = 20 * model.x)

>>> opt = SolverFactory('glpk') 
>>> opt.solve(model) 

>>> model.pprint() 

>>> print ("------------- extend obj --------------") 
>>> model.obj.expr += 10 * model.y

>>> opt = SolverFactory('cplex') 
>>> opt.solve(model) 
>>> model.pprint()