How to find out which constraint causes problems in Pyomo

Question

I have a Pyomo model with many constraints and variables. When solving it with Gurobi, I get the message "Model is infeasible or unbounded". Now I would like to find out which constraints cause the problem. Here in this question Finding out reason of Pyomo model infeasibility there is an advice to use the following command

log_infeasible_constraints(model)

The problem with this approach is that it just prints out every single value of a variable that causes problems. As I have quite many variables the printed out information is too large for the console of Spyder. So basically I have 2 questions:

1. Can I print out the information from the log into a txt file? I tried the following but I get an error message:

file = open("Log.txt", "w") 
file.write(log_infeasible_constraints(model)) 
file.close()

--> TypeError: write() argument must be str, not None

2. Is there another approach for finding out what is causing the problem in Pyomo? Can I somehow see the values of all variables at the time the infeasibility or unboundedness is found out?

Reminder: Does anyone have an idea how I can get more precise information about the error? Especially seeing the values of the variables would be quite helpful. Any idea?

Answer 1

You can find out which constraints cause the infeasibility by the following code. For details look at here:

log_infeasible_constraints(m)

$m$ is your model's name. In the provided link, you can find details of how the infeasible constraints log in Pyomo. The output of the provided code is a dictionary with constraint name, constraint's body value, constraint's lower bound, etc. These values can be obtained individually as well. Instead of write, I suggest using print to see what the output is.

UPDATE: The following is an MWE for your question:

from pyomo.environ import Param, ConcreteModel, Var, Objective, ConstraintList, value, minimize
from pyomo.opt import SolverFactory
from pyomo.util.infeasible import log_infeasible_constraints
import logging

m = ConcreteModel()
m.LE = set([1, 2, 3])
m.x = Var(m.LE, initialize=0)
m.M = Param(initialize=1000000)

def obj_rule(m):
        return sum(m.x[i] * 1 for i in m.LE)

m.z = Objective(rule=obj_rule, sense=minimize)
m.cons1 = ConstraintList()

for i in m.LE:
    m.cons1.add(10**2 * m.x[i] >= m.M)
    m.cons1.add(10**2 * m.x[i] <= -3)

solver = SolverFactory('glpk')
solution = solver.solve(m, tee=False)
log_infeasible_constraints(m, log_expression=True, log_variables=True)
logging.basicConfig(filename='example.log', encoding='utf-8', level=logging.INFO)
print(value(m.z))

And the example.log is the generated .txt file which includes the following data:

INFO:pyomo.util.infeasible:CONSTR cons1[1]: 1000000.0 </= 0
  - EXPR: 1000000.0 </= 100*x[1]
  - VAR x[1]: 0
INFO:pyomo.util.infeasible:CONSTR cons1[2]: 0 </= -3.0
  - EXPR: 100*x[1] </= -3.0
  - VAR x[1]: 0
INFO:pyomo.util.infeasible:CONSTR cons1[3]: 1000000.0 </= 0
  - EXPR: 1000000.0 </= 100*x[2]
  - VAR x[2]: 0
INFO:pyomo.util.infeasible:CONSTR cons1[4]: 0 </= -3.0
  - EXPR: 100*x[2] </= -3.0
  - VAR x[2]: 0
INFO:pyomo.util.infeasible:CONSTR cons1[5]: 1000000.0 </= 0
  - EXPR: 1000000.0 </= 100*x[3]
  - VAR x[3]: 0
INFO:pyomo.util.infeasible:CONSTR cons1[6]: 0 </= -3.0
  - EXPR: 100*x[3] </= -3.0
  - VAR x[3]: 0

example.log is generated in the same location that you run your code (same folder).