How to access objective function value in pyomo?

I am trying to output the objective value from my pyomo model. I did access to the variable values but I cannot get access to the objective function value. My codes are:

   instance = model.create_instance(data)

opt = SolverFactory('cplex')
results = opt.solve(instance)
instance.solutions.store_to(results)
results.write()
# instance.display()

# output the solution
var_val = []
for v in instance.component_data_objects(Var):
    var_val.append(int(v.value))

obj_val = value(instance.obj)

And the last line gives error info:

    obj_val = value(instance.obj)
NameError: name 'value' is not defined

But I can clearly see the value from result.write() :

 Message: None
  Objective:
    obj:
      Value: 104728.80233047833
  Variable:
    x[0,1]:
      Value: 1569
    x[1,0]:
      Value: 1569
    x[1,1]:
      Value: 206
    x[2,2]:
      Value: 230
    x[2,3]:
      Value: 213
    x[3,2]:
      Value: 213

Solution

How are you importing Pyomo? If you're using from pyomo.environ import * the value function will be included. If you're importing each thing you're using individually then you just need to make sure you import the value function: from pyomo.environ import value