Referencing of sets in Pyomo objectives

Question

I have a question about referecing set in Pyomo objectives. Basically I have 2 options and I do not understand why one of it works whereas the other yields an error. Here you see them:

model.set_timeslots = pyo.RangeSet(0,95)

#Option1
def ObjectiveRule (model):
    return sum(model.variable_heatGeneration[t]  for t in model.set_timeslots)

model.objective_costs = pyo.Objective( rule=ObjectiveRule, sense =pyo.minimize)

#Option 2
def ObjectiveRule_2 (model, t):
    return sum(model.variable_heatGeneration[t])

model.objective_costs = pyo.Objective(model.set_timeslots, rule=ObjectiveRule_2, sense =pyo.minimize)

The second options leads to the error

TypeError: '_GeneralVarData' object is not iterable

Can you tell me what the difference is and why the second option does not work?

Reminder: Can nobody tell me what the difference is?

Answer 1

The first option makes sense to me : you are returning a sum such as : $$ \sum_{t \in T} h_t $$

where $h_t$ represents your variable_heatGeneration and $T$ your set_timeslots.

The second option I do not understand and am not surprised an error is raised. You are returning the sum of a constant $h_t$. I believe the error

TypeError: '_GeneralVarData' object is not iterable

is raised because the object model.variable_heatGeneration[t] is constant and not an iterable object such as list. It is trying to sum over a constant which makes no sense.

In a sum, you need an iterable object, for example :

sum(x for x in range(10)) # sum over an iterable
sum(x for x in L) # if L is a list 
sum([x for x in range(10)]) # sum over a list

In option 2 you are doing something like

sum(x) # where x is a constant
# or 
sum(3)

To get rid of this specific error, try removing the sum :

#Option 2
def ObjectiveRule_2 (model, t):
    return model.variable_heatGeneration[t]

Note that this will probably create another error somewhere else, and it will not model the wanted cost function. Sticking with option $1$ is a better idea.