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Best way to change the value of an indexed parameter


I am new to Pyomo, and am wanting to know how to change the value of an already existing model parameter that has one or more index.

I have seen some examples for scalar parameters, i.e. no index. For example:

model5 = ConcreteModel()
model5.data2 = Param(initialize=10.0, mutable=True)
print("print data2 before")
model5.data2.pprint()
model5.data2 = 999
print("print data2 after")
model5.data2.pprint()

This produces the output:

print data2 before
data2 : Size=1, Index=None, Domain=Any, Default=None, Mutable=True
    Key  : Value
    None :  10.0
print data2 after
data2 : Size=1, Index=None, Domain=Any, Default=None, Mutable=True
    Key  : Value
    None :   999

But if I try and do it with a parameter that has an index I get an error. The following code fails, but probably no surprise because I am trying to assign a Python object to a Pyomo object. What is the correct way to update a parameter with an index (or more than one index)?

model5 = ConcreteModel()
# Make a small set
myList = ['i1', 'i2', 'i3', 'i4']
model5.i = Set(dimen=1, initialize=myList)
# Make a dict for each element in the set and give it the value 10
dataDict = {}
for  element in myList:
  dataDict[element] = 10
print("print dataDict")
print(dataDict)
# Make the data into a model Param
model5.data = Param(model5.i, initialize=dataDict, mutable=True)
print("print data parameter")
model5.data.pprint()
# Change a values for each element to 999
for  element in myList:
  dataDict[element] = 999
# Try and update the Param
model5.data = dataDict # THIS FAILS <-- how do I do this?

Solution

  • tl,dr: use the reconstruct method of the (mutable!) parameter you want to update.

    First, my suggestion is to put the procedure to initialize your model into a function, so that you can call it from different places and reuse it.

    from pyomo import environ as pe
    
    def create_model(d: dict) -> pe.ConcreteModel:
        """Create Pyomo Concrete Model.
    
        Parameters
        ----------
        d : Dictionary with keys corresponding to components names.
        """
        model = pe.ConcreteModel()
        model.I = pe.Set(initialize=d['I'])
        model.data = pe.Param(model.I, mutable=True, initialize=d['data'])
        return model
    

    Then you can initialize the model with whatever data you want:

    d = {}
    d['I'] = ['i1', 'i2', 'i3', 'i4']
    d['data'] = {i : 10 for i in d['I']}
    
    model = create_model(d)
    ​
    model.data.pprint()
    
    data : Size=4, Index=I, Domain=Any, Default=None, Mutable=True
        Key : Value
         i1 :    10
         i2 :    10
         i3 :    10
         i4 :    10
    

    Now update the values using reconstruct:

    new_values = {i: 5 for i in d['I']} # 5 here is arbitrary, you
    model.data.reconstruct(new_values) 
    ​
    model.data.pprint()
    data : Size=4, Index=I, Domain=Any, Default=None, Mutable=True
        Key : Value
         i1 :     5
         i2 :     5
         i3 :     5
         i4 :     5
    

    As a side note, data is a really confusing name for a parameter, you should find something more specific.