Fix G.subgraph(edges=generator) deleting all edges #41130
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Good catch. |
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I promoted the doctest from TESTS to EXAMPLES so that the issue number (PR number to be precise) is part of the rendered documentation. |
dcoudert
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Nov 4, 2025
The edges generator was looped over twice. If the generator is a generator expression like `((0, v) for v in range(5))` it becomes empty after the first loop, causing `edges_to_keep_unlabeled` to be empty.
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sagemathgh-41130: Fix G.subgraph(edges=generator) deleting all edges This is a simple fix for a nasty issue: I would expect that the Sage expression `graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))` returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because the `subgraph` function expects the `edges` parameter to be a list that can be iterated over twice but the [generator expression](https://peps.python.org/pep-0289/) `((0, v) for v in range(5))` becomes empty after looping over it once, which caused `edges_to_keep_unlabeled` to be empty. While the [documentation for `subgraph()`](https://doc-develop--sagemath .netlify.app/html/en/reference/graphs/sage/graphs/generic_graph.html#sag e.graphs.generic_graph.GenericGraph.subgraph) does not mention that `edges` is allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example, `subdivide_edges()`, `delete_edges()` and `G.subgraph(v for v in range(4))` work as expected. There are multiple ways to fix this issue. The simplest and most reliable would be to write `edges = list(edges)`, which would go well with [`vertices = list(vertices)`](https://github.com/sagemath/sage/blob /aa27703384a568d85f1751197efe06ff135be352/src/sage/graphs/generic_graph. py#L14400). Instead I opted to fix the symptom by only iterating over `edges` once, which I believe is faster and uses less memory. URL: sagemath#41130 Reported by: Lennard Hofmann Reviewer(s): David Coudert
vbraun
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Nov 8, 2025
sagemathgh-41130: Fix G.subgraph(edges=generator) deleting all edges This is a simple fix for a nasty issue: I would expect that the Sage expression `graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))` returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because the `subgraph` function expects the `edges` parameter to be a list that can be iterated over twice but the [generator expression](https://peps.python.org/pep-0289/) `((0, v) for v in range(5))` becomes empty after looping over it once, which caused `edges_to_keep_unlabeled` to be empty. While the [documentation for `subgraph()`](https://doc-develop--sagemath .netlify.app/html/en/reference/graphs/sage/graphs/generic_graph.html#sag e.graphs.generic_graph.GenericGraph.subgraph) does not mention that `edges` is allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example, `subdivide_edges()`, `delete_edges()` and `G.subgraph(v for v in range(4))` work as expected. There are multiple ways to fix this issue. The simplest and most reliable would be to write `edges = list(edges)`, which would go well with [`vertices = list(vertices)`](https://github.com/sagemath/sage/blob /aa27703384a568d85f1751197efe06ff135be352/src/sage/graphs/generic_graph. py#L14400). Instead I opted to fix the symptom by only iterating over `edges` once, which I believe is faster and uses less memory. URL: sagemath#41130 Reported by: Lennard Hofmann Reviewer(s): David Coudert
vbraun
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Nov 9, 2025
sagemathgh-41130: Fix G.subgraph(edges=generator) deleting all edges This is a simple fix for a nasty issue: I would expect that the Sage expression `graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))` returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because the `subgraph` function expects the `edges` parameter to be a list that can be iterated over twice but the [generator expression](https://peps.python.org/pep-0289/) `((0, v) for v in range(5))` becomes empty after looping over it once, which caused `edges_to_keep_unlabeled` to be empty. While the [documentation for `subgraph()`](https://doc-develop--sagemath .netlify.app/html/en/reference/graphs/sage/graphs/generic_graph.html#sag e.graphs.generic_graph.GenericGraph.subgraph) does not mention that `edges` is allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example, `subdivide_edges()`, `delete_edges()` and `G.subgraph(v for v in range(4))` work as expected. There are multiple ways to fix this issue. The simplest and most reliable would be to write `edges = list(edges)`, which would go well with [`vertices = list(vertices)`](https://github.com/sagemath/sage/blob /aa27703384a568d85f1751197efe06ff135be352/src/sage/graphs/generic_graph. py#L14400). Instead I opted to fix the symptom by only iterating over `edges` once, which I believe is faster and uses less memory. URL: sagemath#41130 Reported by: Lennard Hofmann Reviewer(s): David Coudert
vbraun
pushed a commit
to vbraun/sage
that referenced
this pull request
Nov 9, 2025
sagemathgh-41130: Fix G.subgraph(edges=generator) deleting all edges This is a simple fix for a nasty issue: I would expect that the Sage expression `graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))` returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because the `subgraph` function expects the `edges` parameter to be a list that can be iterated over twice but the [generator expression](https://peps.python.org/pep-0289/) `((0, v) for v in range(5))` becomes empty after looping over it once, which caused `edges_to_keep_unlabeled` to be empty. While the [documentation for `subgraph()`](https://doc-develop--sagemath .netlify.app/html/en/reference/graphs/sage/graphs/generic_graph.html#sag e.graphs.generic_graph.GenericGraph.subgraph) does not mention that `edges` is allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example, `subdivide_edges()`, `delete_edges()` and `G.subgraph(v for v in range(4))` work as expected. There are multiple ways to fix this issue. The simplest and most reliable would be to write `edges = list(edges)`, which would go well with [`vertices = list(vertices)`](https://github.com/sagemath/sage/blob /aa27703384a568d85f1751197efe06ff135be352/src/sage/graphs/generic_graph. py#L14400). Instead I opted to fix the symptom by only iterating over `edges` once, which I believe is faster and uses less memory. URL: sagemath#41130 Reported by: Lennard Hofmann Reviewer(s): David Coudert
vbraun
pushed a commit
to vbraun/sage
that referenced
this pull request
Nov 10, 2025
sagemathgh-41130: Fix G.subgraph(edges=generator) deleting all edges This is a simple fix for a nasty issue: I would expect that the Sage expression `graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))` returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because the `subgraph` function expects the `edges` parameter to be a list that can be iterated over twice but the [generator expression](https://peps.python.org/pep-0289/) `((0, v) for v in range(5))` becomes empty after looping over it once, which caused `edges_to_keep_unlabeled` to be empty. While the [documentation for `subgraph()`](https://doc-develop--sagemath .netlify.app/html/en/reference/graphs/sage/graphs/generic_graph.html#sag e.graphs.generic_graph.GenericGraph.subgraph) does not mention that `edges` is allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example, `subdivide_edges()`, `delete_edges()` and `G.subgraph(v for v in range(4))` work as expected. There are multiple ways to fix this issue. The simplest and most reliable would be to write `edges = list(edges)`, which would go well with [`vertices = list(vertices)`](https://github.com/sagemath/sage/blob /aa27703384a568d85f1751197efe06ff135be352/src/sage/graphs/generic_graph. py#L14400). Instead I opted to fix the symptom by only iterating over `edges` once, which I believe is faster and uses less memory. URL: sagemath#41130 Reported by: Lennard Hofmann Reviewer(s): David Coudert
vbraun
pushed a commit
to vbraun/sage
that referenced
this pull request
Nov 11, 2025
sagemathgh-41130: Fix G.subgraph(edges=generator) deleting all edges This is a simple fix for a nasty issue: I would expect that the Sage expression `graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))` returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because the `subgraph` function expects the `edges` parameter to be a list that can be iterated over twice but the [generator expression](https://peps.python.org/pep-0289/) `((0, v) for v in range(5))` becomes empty after looping over it once, which caused `edges_to_keep_unlabeled` to be empty. While the [documentation for `subgraph()`](https://doc-develop--sagemath .netlify.app/html/en/reference/graphs/sage/graphs/generic_graph.html#sag e.graphs.generic_graph.GenericGraph.subgraph) does not mention that `edges` is allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example, `subdivide_edges()`, `delete_edges()` and `G.subgraph(v for v in range(4))` work as expected. There are multiple ways to fix this issue. The simplest and most reliable would be to write `edges = list(edges)`, which would go well with [`vertices = list(vertices)`](https://github.com/sagemath/sage/blob /aa27703384a568d85f1751197efe06ff135be352/src/sage/graphs/generic_graph. py#L14400). Instead I opted to fix the symptom by only iterating over `edges` once, which I believe is faster and uses less memory. URL: sagemath#41130 Reported by: Lennard Hofmann Reviewer(s): David Coudert
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This is a simple fix for a nasty issue:
I would expect that the Sage expression
graphs.CompleteGraph(5).subgraph(edges=((0, v) for v in range(5)))returns a star graph on 4 edges. But it actually returned an edgeless graph! This is because thesubgraphfunction expects theedgesparameter to be a list that can be iterated over twice but the generator expression((0, v) for v in range(5))becomes empty after looping over it once, which causededges_to_keep_unlabeledto be empty.While the documentation for
subgraph()does not mention thatedgesis allowed to be a generator expression, many other functions that expect an "iterable container" as parameter work just fine if a generator expression is passed instead. For example,subdivide_edges(),delete_edges()andG.subgraph(v for v in range(4))work as expected.There are multiple ways to fix this issue. The simplest and most reliable would be to write
edges = list(edges), which would go well withvertices = list(vertices). Instead I opted to fix the symptom by only iterating overedgesonce, which I believe is faster and uses less memory.