NL writer: Add custom exception for no free variables

pyomo/common/errors.py

@@ -244,3 +244,12 @@ class TemplateExpressionError(ValueError):
     def __init__(self, template, *args, **kwds):
         self.template = template
         super(TemplateExpressionError, self).__init__(*args, **kwds)
+
+
+class EmptyModelError(PyomoException, ValueError):
+    """
+    Exception class used to throw an error for an empty
+    Pyomo model (i.e., no variables or constraints).
+    """
+
+    pass

pyomo/contrib/pynumero/interfaces/tests/test_external_grey_box_model.py

@@ -38,6 +38,7 @@
     check_sparse_matrix_specific_order,
 )
 import pyomo.contrib.pynumero.interfaces.tests.external_grey_box_models as ex_models
+from pyomo.common.errors import EmptyModelError
 
 
 class TestExternalGreyBoxModel(unittest.TestCase):
@@ -612,7 +613,7 @@ def test_error_no_variables(self):
         m.egb = ExternalGreyBoxBlock()
         m.egb.set_external_model(ex_models.PressureDropSingleOutput())
         m.obj = pyo.Objective(expr=1)
-        with self.assertRaises(ValueError):
+        with self.assertRaises(EmptyModelError):
             pyomo_nlp = PyomoGreyBoxNLP(m)
 
     def test_error_fixed_inputs_outputs(self):

pyomo/contrib/pynumero/interfaces/tests/test_pyomo_grey_box_nlp.py

@@ -41,6 +41,7 @@
     check_sparse_matrix_specific_order,
 )
 import pyomo.contrib.pynumero.interfaces.tests.external_grey_box_models as ex_models
+from pyomo.common.errors import EmptyModelError
 
 
 class TestExternalGreyBoxAsNLP(unittest.TestCase):
@@ -1033,7 +1034,7 @@ def test_error_no_variables(self):
         m.egb = ExternalGreyBoxBlock()
         m.egb.set_external_model(ex_models.PressureDropSingleOutput())
         m.obj = pyo.Objective(expr=1)
-        with self.assertRaises(ValueError):
+        with self.assertRaises(EmptyModelError):
             pyomo_nlp = PyomoNLPWithGreyBoxBlocks(m)
 
     def test_error_fixed_inputs_outputs(self):

pyomo/repn/plugins/ampl/ampl_.py

@@ -22,6 +22,7 @@
 
 from pyomo.common.fileutils import find_library
 from pyomo.common.gc_manager import PauseGC
+from pyomo.common.errors import EmptyModelError
 from pyomo.opt import ProblemFormat, AbstractProblemWriter, WriterFactory
 import pyomo.core.expr as EXPR
 from pyomo.core.expr.numvalue import (
@@ -1361,7 +1362,7 @@ def _print_model_NL(
             subsection_timer.reset()
 
         if len(full_var_list) < 1:
-            raise ValueError(
+            raise EmptyModelError(
                 "No variables appear in the Pyomo model constraints or"
                 " objective. This is not supported by the NL file interface"
             )

pyomo/repn/plugins/nl_writer.py

@@ -25,7 +25,7 @@
     document_kwargs_from_configdict,
 )
 from pyomo.common.deprecation import relocated_module_attribute
-from pyomo.common.errors import DeveloperError, InfeasibleConstraintException
+from pyomo.common.errors import InfeasibleConstraintException, EmptyModelError
 from pyomo.common.gc_manager import PauseGC
 from pyomo.common.timing import TicTocTimer
 
@@ -320,7 +320,7 @@ def __call__(self, model, filename, solver_capability, io_options):
             if config.symbolic_solver_labels:
                 os.remove(row_fname)
                 os.remove(col_fname)
-            raise ValueError(
+            raise EmptyModelError(
                 "No variables appear in the Pyomo model constraints or"
                 " objective. This is not supported by the NL file interface"
             )