Adding a variable to an existing constraint on pyomo

Question

I have the constraint z_1 + z_2 = 1 in a concrete model. I want to add another variable z_n at the nth step of solving. E.g. the constraint would become z_1 + z_2 + z_3 = 1 in the first step of solving and z_1 + z_2 + z_3 + z_4 = 1 in the next step, and so on.

I really appreciate it if anyone has a suggestion

Thanks a lot!

I am using ConstraintList() method and one way that came to my mind was deleting the constraint in each step and rewriting that. But I am not sure how to do it and if it is efficient or not.

Thanks,

Answer 1

There are a lot of ways to do what you want. The simplest is to update the constraint in question with a new relational expression:

>>> import pyomo.environ as pyo
>>> m = pyo.ConcreteModel()
>>> m.z = pyo.Var(range(5))
>>> m.c = pyo.Constraint(expr=m.z[0] + m.z[1] == 1)
>>> m.c.pprint()
c : Size=1, Index=None, Active=True
    Key  : Lower : Body        : Upper : Active
    None :   1.0 : z[0] + z[1] :   1.0 :   True
>>> m.c.set_value(m.c.body + m.z[2] == m.c.upper)
>>> m.c.pprint()
c : Size=1, Index=None, Active=True
    Key  : Lower : Body               : Upper : Active
    None :   1.0 : z[0] + z[1] + z[2] :   1.0 :   True
>>> m.c.set_value(m.c.body + m.z[3] == m.c.upper)
>>> m.c.pprint()
c : Size=1, Index=None, Active=True
    Key  : Lower : Body                      : Upper : Active
    None :   1.0 : z[0] + z[1] + z[2] + z[3] :   1.0 :   True

Alternatively, an approach that requires less manipulation of the model is to use an Expression (named expression) component to hold the constraint body and just update that expression:

>>> m = pyo.ConcreteModel()
>>> m.z = pyo.Var(range(5))
>>> m.e = pyo.Expression(expr=m.z[0] + m.z[1])
>>> m.c = pyo.Constraint(expr=m.e == 1)
>>> m.c.pprint()
c : Size=1, Index=None, Active=True
    Key  : Lower : Body : Upper : Active
    None :   1.0 :    e :   1.0 :   True
>>> m.e.pprint()
e : Size=1, Index=None
    Key  : Expression
    None : z[0] + z[1]
>>> m.e.expr += m.z[2]
>>> m.c.pprint()
c : Size=1, Index=None, Active=True
    Key  : Lower : Body : Upper : Active
    None :   1.0 :    e :   1.0 :   True
>>> m.e.pprint()
e : Size=1, Index=None
    Key  : Expression
    None : z[0] + z[1] + z[2]
>>> m.e.expr += m.z[3]
>>> m.c.pprint()
c : Size=1, Index=None, Active=True
    Key  : Lower : Body : Upper : Active
    None :   1.0 :    e :   1.0 :   True
>>> m.e.pprint()
e : Size=1, Index=None
    Key  : Expression
    None : z[0] + z[1] + z[2] + z[3]