fix: reassigning Node.edge no longer detaches the node from its parent

src/dendropy/datamodel/treemodel/_node.py

@@ -979,11 +979,6 @@ def _set_edge(self, new_edge):
         #     raise ValueError("A Node cannot have 'None' for an edge")
         if new_edge is self._edge:
             return
-        if self._parent_node is not None:
-            try:
-                self._parent_node._child_nodes.remove(self)
-            except ValueError:
-                pass
 
         ## Minimal management
         self._edge = new_edge

tests/unittests/test_datamodel_tree_node_fundamentals.py

@@ -322,6 +322,25 @@ def test_edge_head_node_setting(self):
         self.assertIs(node.edge, edge2)
         self.assertIs(node.edge.head_node, node)
 
+    def test_edge_setting_does_not_corrupt_parent_child_bookkeeping(self):
+        # Reassigning a node's ``edge`` is unrelated to the node's position
+        # in the tree, and should not affect the parent's list of children
+        # or the node's own parent reference.
+        parent = dendropy.Node(label="parent")
+        child1 = dendropy.Node(label="child1")
+        child2 = dendropy.Node(label="child2")
+        parent.set_child_nodes([child1, child2])
+        self.assertEqual(len(parent.child_nodes()), 2)
+        self.assertIn(child1, parent.child_nodes())
+        self.assertIs(child1.parent_node, parent)
+
+        child1.edge = dendropy.Edge(length=5.0)
+
+        self.assertEqual(len(parent.child_nodes()), 2)
+        self.assertIn(child1, parent.child_nodes())
+        self.assertIs(child1.parent_node, parent)
+
+
 class TestNodeDistanceFromRoot(unittest.TestCase):
 
     def test_none_edge_length_with_parent_also_none(self):