Ticket #8458: trac_8458_independent_graphs.patch

sage/graphs/graph_generators.py

@@ -325 +325 @@
     - ``sparse`` -- (default: ``True``) ignored if implementation is not
       ``'c_graph'``.
 
+    - ``independent`` -- (default: ``True``) the graph generator can use parts
+      of already generated graphs to create new ones. That is fast, but any change
+      of an generated graph, like edge removal or label update, randomly propagates
+      to the others. Moreover, the generator can crash or produce incorrect results
+      if the changes to the resulted graphs are made before the end of generation.
+      Whenever ``independent`` is set to ``True``, the generator make sure that
+      resulting graphs are separated both from each other and from the inner state
+      of the generator.
+
     EXAMPLES:
 
     Print graphs on 3 or less vertices::
@@ -481 +490 @@
         sage: print 10, len([g for g in graphs(10,deg_seq=[3]*10) if g.is_connected()]) # not tested
         10 19
 
+    Make sure that the graphs are really independent and the generator
+    survives repeated vertex removal (trac 8458)::
+
+        sage: for G in graphs(3):
+        ...       G.delete_vertex(0)
+        ...       print(G.order())
+        2
+        2
+        2
+        2
+
     REFERENCE:
 
     - Brendan D. McKay, Isomorph-Free Exhaustive generation.  *Journal
@@ -6384 +6404 @@
     
     def __call__(self, vertices=None, property=lambda x: True, augment='edges',
         size=None, deg_seq=None, loops=False, implementation='c_graph',
-        sparse=True):
+        sparse=True, independent=True):
         """
         Accesses the generator of isomorphism class representatives.
         Iterates over distinct, exhaustive representatives. See the docstring
@@ -6430 +6450 @@
           pages 306-324.
         """
         from sage.graphs.all import Graph
+        from copy import copy
         if deg_seq is not None:
             if vertices is None:
                 raise NotImplementedError
@@ -6452 +6473 @@
             g = Graph(loops=loops, implementation=implementation, sparse=sparse)
             for gg in canaug_traverse_vert(g, [], vertices, property, loops=loops, implementation=implementation, sparse=sparse):
                 if extra_property(gg):
-                    yield gg
+                    yield copy(gg) if independent else gg
         elif augment == 'edges':
             if vertices is None:
                 from sage.rings.all import Integer
                 vertices = Integer(0)
                 while True:
                     for g in self(vertices, loops=loops, implementation=implementation, sparse=sparse):
-                        yield g
+                        yield copy(g) if independent else g
                     vertices += 1
             g = Graph(vertices, loops=loops, implementation=implementation, sparse=sparse)
             gens = []
@@ -6470 +6491 @@
                 gens.append(gen)
             for gg in canaug_traverse_edge(g, gens, property, loops=loops, implementation=implementation, sparse=sparse):
                 if extra_property(gg):
-                    yield gg
+                    yield copy(gg) if independent else gg
         else:
             raise NotImplementedError