Constraint issue with pyomo involving a scalar - constraints

Question

working on an economic optimization problem with pyomo, I would like to add a constraint to prevent the product of the commodity quantity and its price to go below zero (<0), avoiding a negative revenue. It appears that all the data are in a dataframe and I can't setup a constraint like:
def positive_revenue(model, t)
return model.P * model.C >=0
model.positive_rev = Constraint(model.T, rule=positive_revenue)
The system returns the error that the price is a scalar and it cannot process it. Indeed the price is set as such in the model:
model.T = Set(doc='quarter of year', initialize=df.quarter.tolist(), ordered=True)
model.P = Param(initialize=df.price.tolist(), doc='Price for each quarter')
##while the commodity is:
model.C = Var(model.T, domain=NonNegativeReals)
I just would like to apply that for each timestep (quarter of hour here) that:
price(t) * model.C(t) >=0
Can someone help me to spot the issue ? Thanks
Here are more information:
df dataframe:
df time_stamp price Status imbalance
quarter
0 2021-01-01 00:00:00 64.84 Final 16
1 2021-01-01 00:15:00 13.96 Final 38
2 2021-01-01 00:30:00 12.40 Final 46
index = quarter from 0 till 35049, so it is ok
Here is the df.info()
# Column Non-Null Count Dtype
--- ------ -------------- -----
0 time_stamp 35040 non-null datetime64[ns]
1 price 35040 non-null float64
2 Status 35040 non-null object
3 imbalance 35040 non-null int64
I modified the to_list() > to_dict() in model.T but still facing the same issue:
KeyError: "Cannot treat the scalar component 'P' as an indexed component" at the time model.T is defined in the model parameter, set and variables.
Here is the constraint where the system issues the error:
def revenue_positive(model,t):
for t in model.T:
return (model.C[t] * model.P[t]) >= 0
model.positive_revenue = Constraint(model.T,rule=revenue_positive)
Can't figure it out...any idea ?
UPDATE
Model works after dropping an unfortunate 'quarter' column somewhere...after I renamed the index as quarter.
It runs but i still get negative revenues, so the constraints seems not working at present, here is how it is written:
def revenue_positive(model,t):
for t in model.T:
return (model.C[t] * model.P[t]) >= 0
model.positive_revenue = Constraint(model.T,rule=revenue_positive)
What am I missing here ? Thanks for help, just beginning

Answer 1

Welcome to the site.
The problem you appear to be having is that you are not building your model parameter model.P as an indexed component. I believe you likely want it to be indexed by your set model.T.
When you make indexed params in pyomo you need to initialize it with some key:value pairing, like a python dictionary. You can make that from your data frame by re-indexing your data frame so that the quarter labels are the index values.
Caution: The construction you have for model.T and this assume there are no duplicates in the quarter names.
If you have duplicates (or get a warning) then you'll need to do something else. If the quarter labels are unique you can do this:
import pandas as pd
import pyomo.environ as pyo
df = pd.DataFrame({'qtr':['Q5', 'Q6', 'Q7'], 'price':[12.80, 11.50, 8.12]})
df.set_index('qtr', inplace=True)
print(df)
m = pyo.ConcreteModel()
m.T = pyo.Set(initialize=df.index.to_list())
m.price = pyo.Param(m.T, initialize=df['price'].to_dict())
m.pprint()
which should get you:
price
qtr
Q5 12.80
Q6 11.50
Q7 8.12
1 Set Declarations
T : Size=1, Index=None, Ordered=Insertion
Key : Dimen : Domain : Size : Members
None : 1 : Any : 3 : {'Q5', 'Q6', 'Q7'}
1 Param Declarations
price : Size=3, Index=T, Domain=Any, Default=None, Mutable=False
Key : Value
Q5 : 12.8
Q6 : 11.5
Q7 : 8.12
2 Declarations: T price
edit for clarity...
NOTE:
The first argument when you create a pyomo parameter is the indexing set. If this is not provided, pyomo assumes that it is a scalar. You are missing the set as shown in my example and highlighted with arrow here: :)
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m.price = pyo.Param(m.T, initialize=df['price'].to_dict())
Also note, you will need to initialize model.P with a dictionary as I have in the example, not a list.