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Description
I'm on Windows 11. I have HiGHS version 1.7.2 Githash 5ce7a2753. Copyright (c) 2024 HiGHS under MIT licence terms in a given directory (EDIT: same problem with AMPL/HiGHS Optimizer [1.7.1] (Windows AMD64), driver(20240724), MP(20240724)).
This program (standard solver factory doesn't accept 'highs' argument which is another bug):
import os
from pyomo.environ import ConcreteModel, Var, Objective, NonNegativeReals, Constraint
from pyomo.contrib.appsi.solvers import Highs
# Set the environment variable PATH to include the directory of HiGHS executable
os.environ['PATH'] += os.pathsep + r"C:\bin\Highs\bin"
# Define the model
model = ConcreteModel("MyModel")
# Variables (Renamed to x1 and x2)
model.x1 = Var(name='x1', domain=NonNegativeReals)
model.x2 = Var(name='x2', domain=NonNegativeReals)
# Quadratic Objective function
model.obj = Objective(expr=model.x1 + model.x2 , sense=1) # Minimize
# Constraints
model.con1 = Constraint(expr=model.x1 + 2 * model.x2 >= 1)
model.con2 = Constraint(expr=3 * model.x1 + 4 * model.x2 <= 2)
# Instantiate the HiGHS solver directly via APPSI
solver = Highs()
# Solve the model
results = solver.solve(model)
# Display the results
print("Termination Condition:", results.termination_condition)
# Print the values of the variables
print(f"x1 = {model.x1.value}")
print(f"x2 = {model.x2.value}")
# Display full model details
print()
model.display()
works well but changing objective to
model.obj = Objective(expr=model.x1*model.x1 + model.x2*model.x2, sense=1)
results in
Traceback (most recent call last):
File "C:\Micha\Programming\Python\Optimierung\highs_pyomo.py", line 26, in <module>
results = solver.solve(model)
^^^^^^^^^^^^^^^^^^^
File "C:\Python312\Lib\site-packages\pyomo\contrib\appsi\solvers\highs.py", line 263, in solve
self.set_instance(model)
File "C:\Python312\Lib\site-packages\pyomo\contrib\appsi\solvers\highs.py", line 383, in set_instance
self.add_block(model)
File "C:\Python312\Lib\site-packages\pyomo\contrib\appsi\base.py", line 1128, in add_block
self.set_objective(obj)
File "C:\Python312\Lib\site-packages\pyomo\contrib\appsi\base.py", line 1083, in set_objective
self._set_objective(obj)
File "C:\Python312\Lib\site-packages\pyomo\contrib\appsi\solvers\highs.py", line 595, in _set_objective
raise DegreeError(
pyomo.contrib.appsi.solvers.highs.DegreeError: Highs interface does not support expressions of degree None
Solving the corresponding lp file model.write('model_output.lp', format='lp') on commandline using highs.exe works (after some syntactic corrections).
EDIT: Changing the line 591ff in highs.py
repn = generate_standard_repn(
obj.expr, quadratic=False, compute_values=False
)
from quadratic=False to quadratic=True removes the error and solution is produced.