Fixing bug related to external functions with string arguments

idaes/core/util/scaling.py

@@ -1472,10 +1472,14 @@ def _get_nominal_value_expr_if(self, node, child_nominal_values):
 
     def _get_nominal_value_external_function(self, node, child_nominal_values):
         # First, need to get expected magnitudes of input terms, which may be sub-expressions
-        input_mag = [
-            self._get_nominal_value_for_sum_subexpression(i)
-            for i in child_nominal_values
-        ]
+        input_mag = []
+        for i in child_nominal_values:
+            if isinstance(i[0], str):
+                # Sometimes external functions might have string arguments
+                # Check here, and return the string if true
+                input_mag.append(i[0])
+            else:
+                input_mag.append(self._get_nominal_value_for_sum_subexpression(i))
 
         # Next, create a copy of the external function with expected magnitudes as inputs
         newfunc = node.create_node_with_local_data(input_mag)

idaes/core/util/tests/test_scaling.py

@@ -39,6 +39,7 @@
     CubicThermoExpressions,
     CubicType as CubicEoS,
 )
+from idaes.models.properties import iapws95
 
 __author__ = "John Eslick, Tim Bartholomew"
 
@@ -2230,6 +2231,16 @@ def test_constraint(self, m):
             expr=m.constraint.expr
         ) == [21, 0.5 ** (22 + 23 + 24)]
 
+    @pytest.mark.component
+    def test_external_function_w_string_argument(self):
+        m = pyo.ConcreteModel()
+        m.properties = iapws95.Iapws95ParameterBlock()
+        m.state = m.properties.build_state_block([0])
+
+        assert sc.NominalValueExtractionVisitor().walk_expression(
+            expr=m.state[0].temperature
+        ) == [pytest.approx(235.0, rel=1e-8)]
+
 
 @pytest.fixture(scope="function")
 def m():