Object based Variables for Optimization in docplex

Question

I have an implementation where I need to optimize over object variables. I am using the mathematical programming package of docplex. The problem is that all (both) object variables are being considered the same.

    from docplex.mp.model import Model
    
    test = Model()   
    
    class Node:
        def __init__(self, id):
            self.id = id
    
        x = test.integer_var(0, 50)    
    
    n1 = Node(1)
    n2 = Node(2)
    test.add_constraint(n1.x + n2.x <= 12)
    test.add_constraint(2 * n1.x + n2.x <= 16)
    test.maximize(40 * n1.x + 30 * n2.x)
    
    test.solve()
    test.print_solution()

The output is as though there is only one variable:

    objective: 350
      x1=5

The same problem occurs even if I define the variable externally (outside the class) and instantiate it with the class objects:

    class Node:
        def __init__(self, id):
            self.id = id
    
    
    x = test.integer_var(0, 50)
    Node.x = x

However, in a separate test case, when I use variables without docplex, both class and external variables are able to carry distinct values for every object.

class Node:
    def __init__(self, id):
        self.id = id
    cv = 0


ev = 0
Node.ev = ev

o1 = Node(1)
o2 = Node(2)

o1.ev = 5
o2.ev = 10

o1.cv = 6
o2.cv = 7

print(o1.ev, o2.ev)
print(o1.cv, o2.cv)

Output:

5 10
6 7

Process finished with exit code 0

Apologies for the long question. Can someone explain?

Homer512 · Accepted Answer · 2022-05-28T11:15:56.167

You are confused about how to define a class. The x attribute in your first code is a class variable (same as "static" in other languages such as Java or C++). If you want a new variable per object, put it in the constructor.

class Node:
    def __init__(self, id):
        self.id = id
        self.x = test.integer_var(0, 50)

EDIT: Relevant part of the official tutorial.

In particular, for your last test case, you change different attributes than you think.

class Node:
    def __init__(self, id):
        self.id = id
    cv = 0

This defines an instance attribute id and a class attribute cv.

Node.ev = ev

This adds a new class attribute ev.

o1 = Node(1)
print(o1.ev)

This looks up the ev attribute. o1 has no instance attribute (at this point), so the lookup checks the class. There it finds the class attribute and returns that.

o2 = Node(2)
o1.ev = 5
print(o1.ev, o2.ev)

Here you set a new instance attribute ev for the node o1. This does not overwrite the class attribute with the same name. Therefore o2.ev still refers to the class attribute.

Why does it not work with external variables? For normal variables (without docplex), why can both class variables and external variables take distinct values? — Aditya Paul, May 28 '22 at 10:24
@AdityaPaul I've extended my answer. I hope this clears up your confusion on this part. The key point is that reading a class attribute through an instance is possible. But setting the attribute instead creates a new instance attribute. It doesn't replace the class attribute — Homer512, May 28 '22 at 11:17

Object based Variables for Optimization in docplex

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