Trac #27010: py3: avoid .vertices() in methods _ford_fulkerson, edge_…

…cut, bounded_outdegree_orientation and gomory_hu_tree

We fix all doctests errors in py3 of methods
`bounded_outdegree_orientation` and `gomory_hu_tree` of `graph.py` by
avoiding calls to `.vertices()` in these methods and in methods
`edge_cut` and `_ford_fulkerson` of `generic_graph.py`. This also fixes
some doctests in `generic_graph.py`.

URL: https://trac.sagemath.org/27010
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Travis Scrimshaw

src/sage/graphs/generic_graph.py

@@ -6174,7 +6174,7 @@ def edge_cut(self, s, t, value_only=True, use_edge_labels=False, vertices=False,
             return_value.append(self.edge_boundary(reachable_from_s))
 
             if vertices:
-                return_value.append([reachable_from_s, list(set(self.vertices()).difference(reachable_from_s))])
+                return_value.append([reachable_from_s, list(set(self).difference(reachable_from_s))])
 
             return return_value
 
@@ -8741,7 +8741,7 @@ def _ford_fulkerson(self, s, t, use_edge_labels=False, integer=False, value_only
         # edge is strictly less than its capacity, or when there exists a back
         # arc with non-null flow
         residual = DiGraph()
-        residual.add_vertices(self.vertices())
+        residual.add_vertices(self)
 
         # Initializing the variables
         if directed:

src/sage/graphs/graph.py

@@ -3692,7 +3692,7 @@ def bounded_outdegree_orientation(self, bound, solver=None, verbose=False):
         if not n:
             return DiGraph()
 
-        vertices = self.vertices()
+        vertices = list(self)
         vertices_id = {y: x for x,y in enumerate(vertices)}
 
         b = {}
@@ -7655,9 +7655,9 @@ def gomory_hu_tree(self, algorithm=None):
         if not self.is_connected():
             g = Graph()
             for cc in self.connected_components_subgraphs():
-                g = g.union(cc._gomory_hu_tree(frozenset(cc.vertices()), algorithm=algorithm))
+                g = g.union(cc._gomory_hu_tree(frozenset(cc.vertex_iterator()), algorithm=algorithm))
         else:
-            g = self._gomory_hu_tree(frozenset(self.vertices()), algorithm=algorithm)
+            g = self._gomory_hu_tree(frozenset(self.vertex_iterator()), algorithm=algorithm)
 
         if self.get_pos() is not None:
             g.set_pos(dict(self.get_pos()))