Improve performance of Graph.edge(s,t) when the degrees of the vertices are unbalanced

src/graph/graph_python_interface.cc

@@ -250,13 +250,34 @@ struct get_edge_dispatch
                     bool all_edges, boost::python::list& es) const
     {
         std::shared_ptr<Graph> gp = retrieve_graph_view<Graph>(gi, g);
-        for (auto e : out_edges_range(vertex(s, g), g))
+        size_t k_t = is_directed::apply<Graph>::type::value ?
+            in_degreeS()(t, g) : out_degree(t, g);
+        if (out_degree(s, g) <= k_t)
         {
-            if (target(e, g) == vertex(t, g))
+            for (auto e : out_edges_range(vertex(s, g), g))
             {
-                es.append(PythonEdge<Graph>(gp, e));
-                if (!all_edges)
-                    break;
+                if (target(e, g) == vertex(t, g))
+                {
+                    es.append(PythonEdge<Graph>(gp, e));
+                    if (!all_edges)
+                        break;
+                }
+            }
+        }
+        else
+        {
+            for (auto e : in_or_out_edges_range(vertex(t, g), g))
+            {
+                auto w = is_directed::apply<Graph>::type::value ?
+                    source(e, g) : target(e, g);
+                if (w == vertex(s, g))
+                {
+                    if (!is_directed::apply<Graph>::type::value)
+                        e.inv ^= true;
+                    es.append(PythonEdge<Graph>(gp, e));
+                    if (!all_edges)
+                        break;
+                }
             }
         }
     }

src/graph_tool/__init__.py

@@ -1763,8 +1763,9 @@ class Graph(object):
         If ``add_missing == True``, a new edge is created and returned, if none
         currently exists.
 
-        This operation will take :math:`O(k(s))` time, where :math:`k(s)` is the
-        out-degree of vertex :math:`s`.
+        This operation will take :math:`O(min(k(s), k(t)))` time, where
+        :math:`k(s)` and :math:`k(t)` are the out-degree and in-degree (or
+        out-degree if undirected) of vertices :math:`s` and :math:`t`.
 
         """
         s = self.vertex(int(s))