use get_vars_from_components in create_subsystem_block

pyomo/util/subsystems.py

@@ -14,7 +14,7 @@
 from pyomo.core.expr.visitor import identify_variables
 from pyomo.common.collections import ComponentSet, ComponentMap
 from pyomo.common.modeling import unique_component_name
-
+from pyomo.util.vars_from_expressions import get_vars_from_components
 from pyomo.core.base.constraint import Constraint
 from pyomo.core.base.expression import Expression
 from pyomo.core.base.objective import Objective
@@ -131,12 +131,12 @@ def create_subsystem_block(constraints, variables=None, include_fixed=False):
     block.vars = Reference(variables)
     block.cons = Reference(constraints)
     var_set = ComponentSet(variables)
-    input_vars = []
-    for con in constraints:
-        for var in identify_variables(con.expr, include_fixed=include_fixed):
-            if var not in var_set:
-                input_vars.append(var)
-                var_set.add(var)
+    input_vars = [
+        var for var in get_vars_from_components(
+            block, Constraint, include_fixed=include_fixed
+        )
+        if var not in var_set
+    ]
     block.input_vars = Reference(input_vars)
     add_local_external_functions(block)
     return block