sdk: Fix walk_submodel() skipping Entity and Operation children

Fixes #423. The traversal now recurses into Entity.statement and
Operation.input_variable/output_variable/in_output_variable, so
File SubmodelElements nested inside these containers are no longer
silently dropped when reading AASX files.

Adds test_traversal.py with 13 unit tests covering the fix and
regression cases.

Author: zrgt

sdk/basyx/aas/util/traversal.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2025 the Eclipse BaSyx Authors
+# Copyright (c) 2026 the Eclipse BaSyx Authors
 #
 # This program and the accompanying materials are made available under the terms of the MIT License, available in
 # the LICENSE file of this project.
@@ -8,27 +8,38 @@
 A module with helper functions for traversing AAS object structures.
 """
 
-from typing import Union, Iterator
+from typing import Iterable, Iterator, Union
 
 from .. import model
 
 
+def _walk_submodel_helper(elements: Iterable[model.SubmodelElement]) -> Iterator[model.SubmodelElement]:
+    for element in elements:
+        if isinstance(element, (model.SubmodelElementCollection, model.SubmodelElementList)):
+            yield from _walk_submodel_helper(element.value)
+        elif isinstance(element, model.Operation):
+            for var_list in (element.input_variable, element.output_variable, element.in_output_variable):
+                yield from _walk_submodel_helper(var_list)
+        elif isinstance(element, model.Entity):
+            yield from _walk_submodel_helper(element.statement)
+        yield element
+
+
 def walk_submodel(collection: Union[model.Submodel, model.SubmodelElementCollection, model.SubmodelElementList]) \
         -> Iterator[model.SubmodelElement]:
     """
     Traverse the :class:`SubmodelElements <basyx.aas.model.submodel.SubmodelElement>` in a
-    :class:`~basyx.aas.model.submodel.Submodel`, :class:`~basyx.aas.model.submodel.SubmodelElementCollection` or a
-    :class:`~basyx.aas.model.submodel.SubmodelElementList` recursively in post-order tree-traversal.
+    :class:`~basyx.aas.model.submodel.Submodel`, :class:`~basyx.aas.model.submodel.SubmodelElementCollection`,
+    :class:`~basyx.aas.model.submodel.SubmodelElementList`, :class:`~basyx.aas.model.submodel.Entity`
+    (via ``statement``) or :class:`~basyx.aas.model.submodel.Operation` (via ``input_variable``, ``output_variable``,
+    ``in_output_variable``) recursively in post-order tree-traversal.
 
     This is a generator function, yielding all the :class:`SubmodelElements <basyx.aas.model.submodel.SubmodelElement>`.
     No :class:`SubmodelElements <basyx.aas.model.submodel.SubmodelElement>` should be added, removed or
     moved while iterating, as this could result in undefined behaviour.
     """
     elements = collection.submodel_element if isinstance(collection, model.Submodel) else collection.value
-    for element in elements:
-        if isinstance(element, (model.SubmodelElementCollection, model.SubmodelElementList)):
-            yield from walk_submodel(element)
-        yield element
+    yield from _walk_submodel_helper(elements)
 
 
 def walk_semantic_ids_recursive(root: model.Referable) -> Iterator[model.Reference]:

sdk/test/util/test_traversal.py

@@ -0,0 +1,120 @@
+# Copyright (c) 2026 the Eclipse BaSyx Authors
+#
+# This program and the accompanying materials are made available under the terms of the MIT License, available in
+# the LICENSE file of this project.
+#
+# SPDX-License-Identifier: MIT
+
+import unittest
+from typing import List
+
+from basyx.aas import model
+from basyx.aas.util.traversal import walk_submodel
+
+
+class TestWalkSubmodel(unittest.TestCase):
+    def _submodel(self, *elements: model.SubmodelElement) -> model.Submodel:
+        return model.Submodel("test-submodel", submodel_element=list(elements))
+
+    def test_flat_submodel(self):
+        prop = model.Property("prop", model.datatypes.String)
+        sm = self._submodel(prop)
+        result = list(walk_submodel(sm))
+        self.assertEqual([prop], result)
+
+    def test_collection_post_order(self):
+        child1 = model.Property("child1", model.datatypes.String)
+        child2 = model.Property("child2", model.datatypes.String)
+        coll = model.SubmodelElementCollection("coll", value=[child1, child2])
+        sm = self._submodel(coll)
+        result = list(walk_submodel(sm))
+        # post-order: children before parent
+        self.assertEqual([child1, child2, coll], result)
+
+    def test_list_post_order(self):
+        child = model.Property(None, model.datatypes.String)
+        sml = model.SubmodelElementList("sml", type_value_list_element=model.Property,
+                                        value_type_list_element=model.datatypes.String, value=[child])
+        sm = self._submodel(sml)
+        result = list(walk_submodel(sm))
+        self.assertEqual([child, sml], result)
+
+    def test_entity_statement_post_order(self):
+        stmt = model.Property("stmt", model.datatypes.String)
+        entity = model.Entity("entity", model.EntityType.CO_MANAGED_ENTITY, statement=[stmt])
+        sm = self._submodel(entity)
+        result = list(walk_submodel(sm))
+        # post-order: statement element before Entity
+        self.assertEqual([stmt, entity], result)
+
+    def test_entity_empty_statement(self):
+        entity = model.Entity("entity", model.EntityType.CO_MANAGED_ENTITY)
+        sm = self._submodel(entity)
+        result = list(walk_submodel(sm))
+        self.assertEqual([entity], result)
+
+    def test_operation_variables_post_order(self):
+        in_var = model.Property("in_var", model.datatypes.String)
+        out_var = model.Property("out_var", model.datatypes.String)
+        inout_var = model.Property("inout_var", model.datatypes.String)
+        op = model.Operation("op", input_variable=[in_var], output_variable=[out_var],
+                             in_output_variable=[inout_var])
+        sm = self._submodel(op)
+        result = list(walk_submodel(sm))
+        # post-order: all variable elements before Operation
+        self.assertEqual([in_var, out_var, inout_var, op], result)
+
+    def test_operation_empty_variables(self):
+        op = model.Operation("op")
+        sm = self._submodel(op)
+        result = list(walk_submodel(sm))
+        self.assertEqual([op], result)
+
+    def test_collection_inside_entity_statement(self):
+        inner = model.Property("inner", model.datatypes.String)
+        coll = model.SubmodelElementCollection("coll", value=[inner])
+        entity = model.Entity("entity", model.EntityType.CO_MANAGED_ENTITY, statement=[coll])
+        sm = self._submodel(entity)
+        result = list(walk_submodel(sm))
+        # post-order: inner → coll → entity
+        self.assertEqual([inner, coll, entity], result)
+
+    def test_entity_inside_operation_input_variable(self):
+        stmt = model.Property("stmt", model.datatypes.String)
+        entity = model.Entity("entity", model.EntityType.CO_MANAGED_ENTITY, statement=[stmt])
+        op = model.Operation("op", input_variable=[entity])
+        sm = self._submodel(op)
+        result = list(walk_submodel(sm))
+        # post-order: stmt → entity → op
+        self.assertEqual([stmt, entity, op], result)
+
+    def test_walk_from_collection(self):
+        prop = model.Property("prop", model.datatypes.String)
+        entity = model.Entity("entity", model.EntityType.CO_MANAGED_ENTITY, statement=[prop])
+        coll = model.SubmodelElementCollection("coll", value=[entity])
+        # walk_submodel(coll) yields the contents of coll, not coll itself
+        result = list(walk_submodel(coll))
+        self.assertEqual([prop, entity], result)
+
+    def test_walk_from_list(self):
+        op = model.Operation(None)
+        sml = model.SubmodelElementList("sml", type_value_list_element=model.Operation, value=[op])
+        # walk_submodel(sml) yields the contents of sml, not sml itself
+        result = list(walk_submodel(sml))
+        self.assertEqual([op], result)
+
+    def test_file_inside_entity_is_found(self):
+        """Regression test for issue #423: File inside Entity.statement must be yielded."""
+        f = model.File("file", content_type="application/pdf", value="/some/file.pdf")
+        entity = model.Entity("entity", model.EntityType.CO_MANAGED_ENTITY, statement=[f])
+        sm = self._submodel(entity)
+        files: List[model.File] = [e for e in walk_submodel(sm) if isinstance(e, model.File)]
+        self.assertEqual([f], files)
+
+    def test_file_inside_operation_variable_is_found(self):
+        """Regression test for issue #423: File inside Operation variable must be yielded."""
+        f = model.File("file", content_type="application/pdf", value="/some/file.pdf")
+        op = model.Operation("op", input_variable=[f])
+        sm = self._submodel(op)
+        files: List[model.File] = [e for e in walk_submodel(sm) if isinstance(e, model.File)]
+        self.assertEqual([f], files)