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Fix G.subgraph(edges=generator) deleting all edges #41130

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Nov 11, 2025
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Fix G.subgraph(edges=generator) deleting all edges #41130

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26 changes: 22 additions & 4 deletions src/sage/graphs/generic_graph.py
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Original file line number Diff line number Diff line change
Expand Up @@ -14367,6 +14367,14 @@ def subgraph(self, vertices=None, edges=None, inplace=False,
sage: g.subgraph(list(range(10))) # uses the 'add' algorithm
Subgraph of (Path graph): Graph on 10 vertices

The vertices and edges can be specified using generator expressions
(see :issue:`41130`)::

sage: g = graphs.CompleteGraph(5)
sage: h = g.subgraph(vertices=(v for v in range(4)), edges=((0, v) for v in range(5)))
sage: h.edges(labels=False)
[(0, 1), (0, 2), (0, 3)]

TESTS:

The appropriate properties are preserved::
Expand Down Expand Up @@ -14519,8 +14527,13 @@ def _subgraph_by_adding(self, vertices=None, edges=None, edge_property=None, imm
G.add_vertices(self if vertices is None else vertices)

if edges is not None:
edges_to_keep_labeled = frozenset(e for e in edges if len(e) == 3)
edges_to_keep_unlabeled = frozenset(e for e in edges if len(e) == 2)
edges_to_keep_labeled = set()
edges_to_keep_unlabeled = set()
for e in edges:
if len(e) == 3:
edges_to_keep_labeled.add(e)
elif len(e) == 2:
edges_to_keep_unlabeled.add(e)

edges_to_keep = []
if self._directed:
Expand Down Expand Up @@ -14701,8 +14714,13 @@ def _subgraph_by_deleting(self, vertices=None, edges=None, inplace=False,

edges_to_delete = []
if edges is not None:
edges_to_keep_labeled = frozenset(e for e in edges if len(e) == 3)
edges_to_keep_unlabeled = frozenset(e for e in edges if len(e) == 2)
edges_to_keep_labeled = set()
edges_to_keep_unlabeled = set()
for e in edges:
if len(e) == 3:
edges_to_keep_labeled.add(e)
elif len(e) == 2:
edges_to_keep_unlabeled.add(e)
edges_to_delete = []
if G._directed:
for e in G.edge_iterator():
Expand Down
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