Ensure node 'remove' and 'disassociate' can not be called when frozen

When a tree node is frozen it should not be possible to remove nodes
from it or cause that tree node to disassociate itself from its parent,
so make sure that the appropriate decorator is used to ensure this
can not happen (and add tests that verify an exception is raised when
mutations are attempted).

Change-Id: I27e4af7b891ea3fd1826d5a5e71e808ad5af7647

taskflow/tests/unit/test_types.py

@@ -390,6 +390,18 @@ CEO
         root = tree.Node("josh")
         self.assertTrue(root.empty())
 
+    def test_after_frozen(self):
+        root = tree.Node("josh")
+        root.add(tree.Node("josh.1"))
+        root.freeze()
+        self.assertTrue(
+            all(n.frozen for n in root.dfs_iter(include_self=True)))
+        self.assertRaises(tree.FrozenNode,
+                          root.remove, "josh.1")
+        self.assertRaises(tree.FrozenNode, root.disassociate)
+        self.assertRaises(tree.FrozenNode, root.add,
+                          tree.Node("josh.2"))
+
     def test_removal(self):
         root = self._make_species()
         self.assertIsNotNone(root.remove('reptile'))

taskflow/types/tree.py

@@ -200,6 +200,7 @@ class Node(object):
                                      only_direct=only_direct,
                                      include_self=include_self)
 
+    @misc.disallow_when_frozen(FrozenNode)
     def disassociate(self):
         """Removes this node from its parent (if any).
 
@@ -219,6 +220,7 @@ class Node(object):
                     occurrences += 1
         return occurrences
 
+    @misc.disallow_when_frozen(FrozenNode)
     def remove(self, item, only_direct=False, include_self=True):
         """Removes a item from this nodes children.