prevent adding the same child, parent, view twice in graph

src/plopp/core/node.py

@@ -5,11 +5,21 @@
 
 import uuid
 from itertools import chain
-from typing import Any, Union
+from typing import Any, List, Union
 
 from .view import View
 
 
+def _no_replace_append(container: List[Node], item: Node, kind: str):
+    """
+    Append ``item`` to ``container`` if it is not already in it.
+    """
+    if item in container:
+        tpe = 'View' if kind == 'view' else 'Node'
+        raise ValueError(f"{tpe} {item} is already a {kind} in {container}.")
+    container.append(item)
+
+
 class Node:
     """
     A node that can have parent and children nodes, to create a graph.
@@ -43,8 +53,6 @@ def __init__(self, func: Any, *parents, **kwparents):
         self.kwparents = {
             key: p if isinstance(p, Node) else Node(p) for key, p in kwparents.items()
         }
-        for parent in chain(self.parents, self.kwparents.values()):
-            parent.children.append(self)
         self._data = None
 
         if func_is_callable:
@@ -61,6 +69,10 @@ def __init__(self, func: Any, *parents, **kwparents):
             val_str = f'={repr(func)}' if isinstance(func, (int, float, str)) else ""
             self.name = f'Input <{type(func).__name__}{val_str}>'
 
+        # Attempt to set children after setting name in case error message is needed
+        for parent in chain(self.parents, self.kwparents.values()):
+            _no_replace_append(parent.children, self, 'child')
+
     def __call__(self):
         return self.request_data()
 
@@ -125,22 +137,22 @@ def add_parents(self, *parents: Node):
         Add one or more parents to the node.
         """
         for parent in parents:
-            self.parents.append(parent)
-            parent.children.append(self)
+            _no_replace_append(self.parents, parent, 'parent')
+            _no_replace_append(parent.children, self, 'child')
 
     def add_kwparents(self, **parents: Node):
         """
         Add one or more keyword parents to the node.
         """
         for key, parent in parents.items():
             self.kwparents[key] = parent
-            parent.children.append(self)
+            _no_replace_append(parent.children, self, 'child')
 
     def add_view(self, view: View):
         """
         Add a view to the node.
         """
-        self.views.append(view)
+        _no_replace_append(self.views, view, 'view')
         view.graph_nodes[self.id] = self
 
     def notify_children(self, message: Any):

tests/core/node_test.py

@@ -309,3 +309,40 @@ def test_add_multiple_kwparents():
     assert d in a.children
     assert d in b.children
     assert d in c.children
+
+
+def test_adding_same_child_twice_raises():
+    a = Node(lambda: 5)
+    with pytest.raises(ValueError, match="Node .* is already a child in"):
+        Node(lambda x, y: x * y - 2, a, a)
+    with pytest.raises(ValueError, match="Node .* is already a child in"):
+        Node(lambda x, y: x * y - 2, x=a, y=a)
+
+
+def test_adding_same_parent_twice_raises():
+    a = Node(lambda: 5)
+    b = Node(lambda x, y: x * y - 2)
+    b.add_parents(a)
+    with pytest.raises(ValueError, match="Node .* is already a parent in"):
+        b.add_parents(a)
+
+
+def test_adding_same_parent_twice_at_once_raises():
+    a = Node(lambda: 5)
+    b = Node(lambda x, y: x * y - 2)
+    with pytest.raises(ValueError, match="Node .* is already a parent in"):
+        b.add_parents(a, a)
+
+
+def test_adding_same_kwparent_twice_raises():
+    a = Node(lambda: 5)
+    b = Node(lambda x, y: x * y - 2)
+    with pytest.raises(ValueError, match="Node .* is already a child in"):
+        b.add_kwparents(x=a, y=a)
+
+
+def test_adding_same_view_twice_raises():
+    a = Node(lambda: 15.0)
+    av = SimpleView(a)
+    with pytest.raises(ValueError, match="View .* is already a view in"):
+        a.add_view(av)