Trac #23275: Bipartite graphs should not accept loops

Loops should not be allowed in a bipartite graph, but we can currently
do:
{{{
sage: B = BipartiteGraph(loops=True, multiedges=True)
sage: B.add_edge(0, 0)
sage: B.is_bipartite()
False
sage: B.add_edge(1, 1)
sage: B.add_edge(2, 2)
sage: B.edges(labels=0)
[(0, 0), (1, 1), (2, 2)]
sage: B.add_edge(0, 1, 'a')
------------------------------------------------------------------------
---
RuntimeError                              Traceback (most recent call
last)
...
RuntimeError: Edge vertices must lie in different partitions.
sage: B.add_edge(3, 3)
sage: B.edges()
[(0, 0, None), (1, 1, None), (2, 2, None), (3, 3, None)]
}}}
When adding edges in a different order, the behavior is more consistent.
{{{
sage: B = BipartiteGraph(loops=True, multiedges=True)
sage: B.add_edge(0, 1, 'a')
sage: B.add_edge(0, 1, 'b')
sage: B.is_bipartite()
True
sage: B.add_edge(0,0)
------------------------------------------------------------------------
---
RuntimeError                              Traceback (most recent call
last)
...
RuntimeError: Edge vertices must lie in different partitions.
}}}

URL: https://trac.sagemath.org/23275
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Zachary Gershkoff

src/sage/graphs/bipartite_graph.py

@@ -250,7 +250,20 @@ class BipartiteGraph(Graph):
         sage: sorted(g.left.union(g.right))
         [0, 1, 2, 3, 4, 5, 6, 7]
 
+    Make sure that loops are not allowed (:trac:`23275`)::
 
+        sage: B = BipartiteGraph(loops=True)
+        Traceback (most recent call last):
+        ...
+        ValueError: loops are not allowed in bipartite graphs
+        sage: B = BipartiteGraph(loops=None)
+        sage: B.allows_loops()
+        False
+        sage: B.add_edge(0,0)
+        Traceback (most recent call last):
+        ...
+        ValueError: cannot add edge from 0 to 0 in graph without loops
+        
     """
 
     def __init__(self, data=None, partition=None, check=True, *args, **kwds):
@@ -264,6 +277,13 @@ def __init__(self, data=None, partition=None, check=True, *args, **kwds):
             sage: partition = [list(range(5)), list(range(5,10))]
             sage: B = BipartiteGraph(P, partition, check=False)
         """
+        if kwds is None:
+            kwds = {'loops': False}
+        else:
+            if kwds.has_key('loops') and kwds['loops'] is True:
+                raise ValueError('loops are not allowed in bipartite graphs')
+            kwds['loops'] = False
+
         if data is None:
             if partition is not None and check:
                 if partition[0] or partition[1]:
@@ -798,6 +818,31 @@ def add_edge(self, u, v=None, label=None):
         Graph.add_edge(self, u, v, label)
         return
 
+    def allow_loops(self, new, check=True):
+        """
+        Change whether loops are permitted in the (di)graph
+
+        .. NOTE::
+
+            This method overwrite the
+            :meth:`~sage.graphs.generic_graph.GenericGraph.allow_loops` method
+            to ensure that loops are forbidden in :class:`~BipartiteGraph`.
+
+        INPUT:
+
+        - ``new`` - boolean.
+
+        EXAMPLES::
+
+            sage: B = BipartiteGraph()
+            sage: B.allow_loops(True)
+            Traceback (most recent call last):
+            ...
+            ValueError: loops are not allowed in bipartite graphs
+        """
+        if new is True:
+            raise ValueError("loops are not allowed in bipartite graphs")
+
     def complement(self):
         """
         Return a complement of this graph.