PYOMO: How to use abstract models with internal data - pyomo

Question

Hei all,
I am trying to set up an abstract model for a very simple QP of the form
min (x-x0)^2
s.t.
A x = b
C x <= d
I would like to use an abstract model, as I need to resolve with changing parameters (mainly x0, but potentially also A, b, C, d). I am right now struggeling with simply setting the parameters in the model instance. I do not want to use an external data file, but rather internal python variables. All examples I find online use AMPL formatted data files.
This is the code I have right now
import pyomo.environ as pe
model = pe.AbstractModel()
# the sets
model.n = pe.Param(within=pe.NonNegativeIntegers)
model.m = pe.Param(initialize = 1)
model.ss = pe.RangeSet(1, model.n)
model.os = pe.RangeSet(1, model.m)
# the starting point and the constraint parameters
model.x_hat = pe.Param(model.ss)
model.A = pe.Param(model.os, model.ss)
model.b = pe.Param(model.os)
model.C = pe.Param(model.os, model.os)
model.d = pe.Param(model.ss, model.os)
# the decision variables
model.x_projected = pe.Var(model.ss)
# the cosntraints
# A x = b
def sum_of_elements_rule(model):
value = model.A * model.x_projected
return value == model.d
model.sumelem = pe.Constraint(model.os, rule=sum_of_elements_rule)
# C x <= d
def positivity_constraint(model):
return model.C*model.x_projected <= model.d
model.bounds = pe.Constraint(model.ss, rule=positivity_constraint)
# the cost
def cost_rule(model):
return sum((model.x_projected[i] - model.x[i])**2 for i in model.ss)
model.cost = pe.Objective(rule=cost_rule)
instance = model.create_instance()
And somehow here I am stuck. How do I set the parameters now?
Thanks and best, Theo

Answer 1

I know this is an old post but a solution to this could have helped me so here is the solution to this problem:
## TEST
data_init= {None: dict(
n = {None : 3},
d = {0:0, 1:1, 2:2},
x_hat = {0:10, 1:-1, 2:-100},
b = {None: 10}
)}
# create instance
instance = model.create_instance(data_init)
This creates the instance in an equivalent way than what you did but in a more formal way.

Answer 2

Ok, I seemed to have figured out what the problem is. If I want to set a parameter after I create an instance, I need the
mutable=True
flag. Then, I can set the parameter with something like
for i in range(model_dimension):
getattr(instance, 'd')[i] = i
The model dimension I need to choose before i create an instance (which is ok for my case). The instance can be reused with different parameters for the constraints.
The code below should work for the problem
min (x-x_hat)' * (x-x_hat)
s.t.
sum(x) = b
x[i] >= d[i]
with x_hat, b, d as parameters.
import pyomo.environ as pe
model = pe.AbstractModel()
# model dimension
model.n = pe.Param(default=2)
# state space set
model.ss = pe.RangeSet(0, model.n-1)
# equality
model.b = pe.Param(default=5, mutable=True)
# inequality
model.d = pe.Param(model.ss, default=0.0, mutable=True)
# decision var
model.x = pe.Var(model.ss)
model.x_hat = pe.Param(model.ss, default=0.0, mutable=True)
# the cost
def cost_rule(model):
return sum((model.x[i] - model.x_hat[i])**2 for i in model.ss)
model.cost = pe.Objective(rule=cost_rule)
# CONSTRAINTS
# each x_i bigger than d_i
def lb_rule(model, i):
return (model.x[i] >= model.d[i])
model.state_bound = pe.Constraint(model.ss, rule=lb_rule)
# sum of x == P_tot
def sum_rule(model):
return (sum(model.x[i] for i in model.ss) == model.b)
model.state_sum = pe.Constraint(rule=sum_rule)
## TEST
# define model dimension
model_dimension = 3
model.n = model_dimension
# create instance
instance = model.create_instance()
# set d
for i in range(model_dimension):
getattr(instance, 'd')[i] = i
# set x_hat
xh = (10,1,-100)
for i in range(model_dimension):
getattr(instance, 'x_hat')[i] = xh[i]
# set b
instance.b = 10
# solve
solver = pe.SolverFactory('ipopt')
result = solver.solve(instance)
instance.display()