[BUG] Fixing networkx conversion with 0 edges (#94)

* fixing networkx conversion with 0 edges

* fixes as required

* remove obsolete comment

python/dgl/graph_index.py

@@ -523,6 +523,7 @@ class GraphIndex(object):
         """
         src, dst, eid = self.edges()
         ret = nx.MultiDiGraph() if self.is_multigraph() else nx.DiGraph()
+        ret.add_nodes_from(range(self.number_of_nodes()))
         for u, v, id in zip(src, dst, eid):
             ret.add_edge(u, v, id=id)
         return ret
@@ -548,16 +549,20 @@ class GraphIndex(object):
 
         num_nodes = nx_graph.number_of_nodes()
         self.add_nodes(num_nodes)
-        has_edge_id = 'id' in next(iter(nx_graph.edges))
+
+        if nx_graph.number_of_edges() == 0:
+            return
+
+        # nx_graph.edges(data=True) returns src, dst, attr_dict
+        has_edge_id = 'id' in next(iter(nx_graph.edges(data=True)))[-1]
         if has_edge_id:
             num_edges = nx_graph.number_of_edges()
             src = np.zeros((num_edges,), dtype=np.int64)
             dst = np.zeros((num_edges,), dtype=np.int64)
-            for e, attr in nx_graph.edges.items:
-                # MultiDiGraph returns a triplet in e while DiGraph returns a pair
+            for u, v, attr in nx_graph.edges(data=True):
                 eid = attr['id']
-                src[eid] = e[0]
-                dst[eid] = e[1]
+                src[eid] = u
+                dst[eid] = v
         else:
             src = []
             dst = []

tests/graph_index/test_basics.py

@@ -94,6 +94,27 @@ def test_nx():
     assert 0 in gi.edge_id(0, 1)
     assert 1 in gi.edge_id(0, 1)
 
+    nxg = nx.DiGraph()
+    nxg.add_nodes_from(range(3))
+    gi = create_graph_index(nxg)
+    assert gi.number_of_nodes() == 3
+    assert gi.number_of_edges() == 0
+
+    gi = create_graph_index()
+    gi.add_nodes(3)
+    nxg = gi.to_networkx()
+    assert len(nxg.nodes) == 3
+    assert len(nxg.edges) == 0
+
+    nxg = nx.DiGraph()
+    nxg.add_edge(0, 1, id=0)
+    nxg.add_edge(1, 2, id=1)
+    gi = create_graph_index(nxg)
+    assert 0 in gi.edge_id(0, 1)
+    assert 1 in gi.edge_id(1, 2)
+    assert gi.number_of_edges() == 2
+    assert gi.number_of_nodes() == 3
+
 def test_predsucc():
     gi = create_graph_index(multigraph=True)