Raise ValueError if None is added as a node. (#4892)

* Raise ValueError if None is added as a node.

Removed some tests that checked that errors raised when None was a node.

* update tutorial to make a stronger statement about None

8 files changed, 55 insertions, 22 deletions

diff --git a/doc/tutorial.rst b/doc/tutorial.rst
index 4e28dafa..5f8ada5c 100644
--- a/doc/tutorial.rst
+++ b/doc/tutorial.rst
@@ -20,8 +20,9 @@ identified pairs of nodes (called edges, links, etc).  In NetworkX, nodes can
 be any :py:term:`hashable` object e.g., a text string, an image, an XML object,
 another Graph, a customized node object, etc.
 
-.. note:: Python's ``None`` object should not be used as a node as it determines
-   whether optional function arguments have been assigned in many functions.
+.. note:: Python's ``None`` object is not allowed to be used as a node. It
+    determines whether optional function arguments have been assigned in many
+    functions. And it can be used as a sentinel object meaning "not a node".
 
 Nodes
 -----
diff --git a/networkx/algorithms/tests/test_lowest_common_ancestors.py b/networkx/algorithms/tests/test_lowest_common_ancestors.py
index fb09e1d6..4724f383 100644
--- a/networkx/algorithms/tests/test_lowest_common_ancestors.py
+++ b/networkx/algorithms/tests/test_lowest_common_ancestors.py
@@ -123,14 +123,9 @@ class TestTreeLCA:
         """Test that pairs not in the graph raises error."""
         lca = tree_all_pairs_lca(self.DG, 0, [(-1, -1)])
         pytest.raises(nx.NodeNotFound, list, lca)
-
-    def test_tree_all_pairs_lowest_common_ancestor11(self):
-        """Test that None as a node in the graph raises an error."""
-        G = nx.DiGraph([(None, 3)])
-        pytest.raises(nx.NetworkXError, list, tree_all_pairs_lca(G))
-        pytest.raises(
-            nx.NodeNotFound, list, tree_all_pairs_lca(self.DG, pairs=G.edges())
-        )
+        # check if node is None
+        lca = tree_all_pairs_lca(self.DG, None, [(-1, -1)])
+        pytest.raises(nx.NodeNotFound, list, lca)
 
     def test_tree_all_pairs_lowest_common_ancestor12(self):
         """Test that tree routine bails on DAGs."""
@@ -292,12 +287,6 @@ class TestDAGLCA:
         ans = list(all_pairs_lca(G))
         assert ans == [((3, 3), 3)]
 
-    def test_all_pairs_lowest_common_ancestor10(self):
-        """Test that it bails on None as a node."""
-        G = nx.DiGraph([(None, 3)])
-        pytest.raises(nx.NetworkXError, all_pairs_lca, G)
-        pytest.raises(nx.NodeNotFound, all_pairs_lca, self.DG, pairs=G.edges())
-
     def test_lowest_common_ancestor1(self):
         """Test that the one-pair function works on default."""
         G = nx.DiGraph([(0, 1), (2, 1)])
diff --git a/networkx/classes/digraph.py b/networkx/classes/digraph.py
index 14da0b3f..30b6a393 100644
--- a/networkx/classes/digraph.py
+++ b/networkx/classes/digraph.py
@@ -416,6 +416,8 @@ class DiGraph(Graph):
         doesn't change on mutables.
         """
         if node_for_adding not in self._succ:
+            if node_for_adding is None:
+                raise ValueError("None cannot be a node")
             self._succ[node_for_adding] = self.adjlist_inner_dict_factory()
             self._pred[node_for_adding] = self.adjlist_inner_dict_factory()
             attr_dict = self._node[node_for_adding] = self.node_attr_dict_factory()
@@ -477,6 +479,8 @@ class DiGraph(Graph):
                 newdict = attr.copy()
                 newdict.update(ndict)
             if newnode:
+                if n is None:
+                    raise ValueError("None cannot be a node")
                 self._succ[n] = self.adjlist_inner_dict_factory()
                 self._pred[n] = self.adjlist_inner_dict_factory()
                 self._node[n] = self.node_attr_dict_factory()
@@ -614,10 +618,14 @@ class DiGraph(Graph):
         u, v = u_of_edge, v_of_edge
         # add nodes
         if u not in self._succ:
+            if u is None:
+                raise ValueError("None cannot be a node")
             self._succ[u] = self.adjlist_inner_dict_factory()
             self._pred[u] = self.adjlist_inner_dict_factory()
             self._node[u] = self.node_attr_dict_factory()
         if v not in self._succ:
+            if v is None:
+                raise ValueError("None cannot be a node")
             self._succ[v] = self.adjlist_inner_dict_factory()
             self._pred[v] = self.adjlist_inner_dict_factory()
             self._node[v] = self.node_attr_dict_factory()
@@ -675,10 +683,14 @@ class DiGraph(Graph):
             else:
                 raise NetworkXError(f"Edge tuple {e} must be a 2-tuple or 3-tuple.")
             if u not in self._succ:
+                if u is None:
+                    raise ValueError("None cannot be a node")
                 self._succ[u] = self.adjlist_inner_dict_factory()
                 self._pred[u] = self.adjlist_inner_dict_factory()
                 self._node[u] = self.node_attr_dict_factory()
             if v not in self._succ:
+                if v is None:
+                    raise ValueError("None cannot be a node")
                 self._succ[v] = self.adjlist_inner_dict_factory()
                 self._pred[v] = self.adjlist_inner_dict_factory()
                 self._node[v] = self.node_attr_dict_factory()
diff --git a/networkx/classes/graph.py b/networkx/classes/graph.py
index 6b0fb748..26586779 100644
--- a/networkx/classes/graph.py
+++ b/networkx/classes/graph.py
@@ -28,7 +28,7 @@ class Graph:
     (parallel) edges are not.
 
     Nodes can be arbitrary (hashable) Python objects with optional
-    key/value attributes. By convention `None` is not used as a node.
+    key/value attributes, except that `None` is not allowed as a node.
 
     Edges are represented as links between nodes with optional
     key/value attributes.
@@ -515,6 +515,8 @@ class Graph:
         doesn't change on mutables.
         """
         if node_for_adding not in self._node:
+            if node_for_adding is None:
+                raise ValueError("None cannot be a node")
             self._adj[node_for_adding] = self.adjlist_inner_dict_factory()
             attr_dict = self._node[node_for_adding] = self.node_attr_dict_factory()
             attr_dict.update(attr)
@@ -575,6 +577,8 @@ class Graph:
                 newdict = attr.copy()
                 newdict.update(ndict)
             if newnode:
+                if n is None:
+                    raise ValueError("None cannot be a node")
                 self._adj[n] = self.adjlist_inner_dict_factory()
                 self._node[n] = self.node_attr_dict_factory()
             self._node[n].update(newdict)
@@ -873,9 +877,13 @@ class Graph:
         u, v = u_of_edge, v_of_edge
         # add nodes
         if u not in self._node:
+            if u is None:
+                raise ValueError("None cannot be a node")
             self._adj[u] = self.adjlist_inner_dict_factory()
             self._node[u] = self.node_attr_dict_factory()
         if v not in self._node:
+            if v is None:
+                raise ValueError("None cannot be a node")
             self._adj[v] = self.adjlist_inner_dict_factory()
             self._node[v] = self.node_attr_dict_factory()
         # add the edge
@@ -932,9 +940,13 @@ class Graph:
             else:
                 raise NetworkXError(f"Edge tuple {e} must be a 2-tuple or 3-tuple.")
             if u not in self._node:
+                if u is None:
+                    raise ValueError("None cannot be a node")
                 self._adj[u] = self.adjlist_inner_dict_factory()
                 self._node[u] = self.node_attr_dict_factory()
             if v not in self._node:
+                if v is None:
+                    raise ValueError("None cannot be a node")
                 self._adj[v] = self.adjlist_inner_dict_factory()
                 self._node[v] = self.node_attr_dict_factory()
             datadict = self._adj[u].get(v, self.edge_attr_dict_factory())
diff --git a/networkx/classes/multidigraph.py b/networkx/classes/multidigraph.py
index 51e590fe..6da147f9 100644
--- a/networkx/classes/multidigraph.py
+++ b/networkx/classes/multidigraph.py
@@ -464,10 +464,14 @@ class MultiDiGraph(MultiGraph, DiGraph):
         u, v = u_for_edge, v_for_edge
         # add nodes
         if u not in self._succ:
+            if u is None:
+                raise ValueError("None cannot be a node")
             self._succ[u] = self.adjlist_inner_dict_factory()
             self._pred[u] = self.adjlist_inner_dict_factory()
             self._node[u] = self.node_attr_dict_factory()
         if v not in self._succ:
+            if v is None:
+                raise ValueError("None cannot be a node")
             self._succ[v] = self.adjlist_inner_dict_factory()
             self._pred[v] = self.adjlist_inner_dict_factory()
             self._node[v] = self.node_attr_dict_factory()
diff --git a/networkx/classes/multigraph.py b/networkx/classes/multigraph.py
index b2e69561..90c50620 100644
--- a/networkx/classes/multigraph.py
+++ b/networkx/classes/multigraph.py
@@ -469,9 +469,13 @@ class MultiGraph(Graph):
         u, v = u_for_edge, v_for_edge
         # add nodes
         if u not in self._adj:
+            if u is None:
+                raise ValueError("None cannot be a node")
             self._adj[u] = self.adjlist_inner_dict_factory()
             self._node[u] = self.node_attr_dict_factory()
         if v not in self._adj:
+            if v is None:
+                raise ValueError("None cannot be a node")
             self._adj[v] = self.adjlist_inner_dict_factory()
             self._node[v] = self.node_attr_dict_factory()
         if key is None:
diff --git a/networkx/classes/tests/test_function.py b/networkx/classes/tests/test_function.py
index 5c3fbe95..0eeb6c20 100644
--- a/networkx/classes/tests/test_function.py
+++ b/networkx/classes/tests/test_function.py
@@ -107,16 +107,16 @@ class TestFunction:
         )
 
         G = self.G.copy()
-        nlist = [None]
+        nlist = ["node"]
         nx.add_path(G, nlist)
         assert edges_equal(G.edges(nlist), [])
-        assert nodes_equal(G, list(self.G) + [None])
+        assert nodes_equal(G, list(self.G) + ["node"])
 
         G = self.G.copy()
-        nlist = iter([None])
+        nlist = iter(["node"])
         nx.add_path(G, nlist)
-        assert edges_equal(G.edges([None]), [])
-        assert nodes_equal(G, list(self.G) + [None])
+        assert edges_equal(G.edges(["node"]), [])
+        assert nodes_equal(G, list(self.G) + ["node"])
 
         G = self.G.copy()
         nlist = [12]
diff --git a/networkx/classes/tests/test_graph.py b/networkx/classes/tests/test_graph.py
index 86744403..288a7efb 100644
--- a/networkx/classes/tests/test_graph.py
+++ b/networkx/classes/tests/test_graph.py
@@ -29,6 +29,17 @@ class BaseGraphTester:
         assert sorted(G.nodes()) == self.k3nodes
         assert sorted(G.nodes(data=True)) == [(0, {}), (1, {}), (2, {})]
 
+    def test_none_node(self):
+        G = self.Graph()
+        with pytest.raises(ValueError):
+            G.add_node(None)
+        with pytest.raises(ValueError):
+            G.add_nodes_from([None])
+        with pytest.raises(ValueError):
+            G.add_edge(0, None)
+        with pytest.raises(ValueError):
+            G.add_edges_from([(0, None)])
+
     def test_has_node(self):
         G = self.K3
         assert G.has_node(1)