Fix cascading args and add a test

Author: erichlf

py_trees_parser/parser.py

@@ -403,7 +403,9 @@ def _sub_args(self, args, var):
         return None
 
     def _build_tree(
-        self, xml_node: ElementTree, args: dict | None = None
+        self,
+        xml_node: ElementTree,
+        args: dict = {},  # noqa
     ) -> py_trees.behaviour.Behaviour:
         """
         Build the behavior tree from an XML node.
@@ -443,7 +445,7 @@ def _build_tree(
                     )
                     self._process_args(child_xml, new_args)
                     args.update(new_args)
-                    return self._build_tree(child_xml, args)
+                    return self._build_tree(child_xml, {**args, **new_args})
 
         # we only need to find children if the node is a composite
         children = list()

test/data/test_cascade_args.xml

@@ -0,0 +1,8 @@
+<subtree xmlns:xi="http://www.w3.org/2001/XInclude">
+  <arg name="selector_name" value="Subtree Selector" />
+  <arg name="idle_name" value="Idle" />
+  <arg name="flip_name" value="Flip Eggs" />
+  <arg name="n" value="2" />
+  <xi:include href="test_subtree_cascade.xml" parse="xml" />
+</subtree>
+

test/data/test_subtree_cascade.xml

@@ -0,0 +1,7 @@
+<subtree xmlns:xi="http://www.w3.org/2001/XInclude">
+  <arg name="selector_name" value="${selector_name}" />
+  <arg name="idle_name" value="${idle_name}" />
+  <arg name="flip_name" value="${flip_name}" />
+  <arg name="n" value="${n}" />
+  <xi:include href="test_subtree_args.xml" parse="xml" />
+</subtree>

test/test_parser.py

@@ -89,3 +89,19 @@ def test_subtree_and_args(setup_parser):
             assert child.period == 2
         else:
             assert False, f"Unexpected child node type {type(child)}"  # noqa
+
+
+def test_subtree_cascaded_args(setup_parser):
+    """Test that cascaded args through subtrees is working as expected."""
+    tree_file = "test/data/test_cascade_args.xml"
+    root = setup_parser(tree_file)
+
+    assert root.name == "Subtree Selector"
+    for child in root.children:
+        if isinstance(child, py_trees.behaviours.Running):
+            assert child.name == "Idle"
+        elif isinstance(child, py_trees.behaviours.Periodic):
+            assert child.name == "Flip Eggs"
+            assert child.period == 2
+        else:
+            assert False, f"Unexpected child node type {type(child)}"  # noqa