From fa924997967d9fa3908f59226a15e8cb60c48b23 Mon Sep 17 00:00:00 2001
From: Fabio Zanini <fabio.zanini@fastmail.fm>
Date: Wed, 25 Feb 2026 13:59:09 +1100
Subject: [PATCH] Export to pytorch geometric

---
 setup.py                   |  4 ++++
 src/igraph/__init__.py     | 12 ++++++----
 src/igraph/io/libraries.py | 45 +++++++++++++++++++++++++++++++++++++
 tests/test_foreign.py      | 46 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 103 insertions(+), 4 deletions(-)

diff --git a/setup.py b/setup.py
index c919d3507..9c5b7c7f9 100644
--- a/setup.py
+++ b/setup.py
@@ -1018,6 +1018,10 @@ def get_tag(self):
             "plotly>=5.3.0",
             "Pillow>=9; platform_python_implementation != 'PyPy'",
         ],
+        "test-pyg": [
+            "torch>=2.0.0; platform_python_implementation != 'PyPy'",
+            "torch-geometric>=2.0.0; platform_python_implementation != 'PyPy'",
+        ],
         # Dependencies needed for testing on Windows ARM64; only those that are either
         # pure Python or have Windows ARM64 wheels as we don't want to compile wheels
         # in CI
diff --git a/src/igraph/__init__.py b/src/igraph/__init__.py
index 6a4e189b9..85ad1a47a 100644
--- a/src/igraph/__init__.py
+++ b/src/igraph/__init__.py
@@ -211,6 +211,7 @@
     _export_graph_to_networkx,
     _construct_graph_from_graph_tool,
     _export_graph_to_graph_tool,
+    _export_graph_to_torch_geometric,
 )
 from igraph.io.random import (
     _construct_random_geometric_graph,
@@ -463,6 +464,8 @@ def __init__(self, *args, **kwds):
     from_graph_tool = classmethod(_construct_graph_from_graph_tool)
     to_graph_tool = _export_graph_to_graph_tool
 
+    to_torch_geometric = _export_graph_to_torch_geometric
+
     # Files
     Read_DIMACS = classmethod(_construct_graph_from_dimacs_file)
     write_dimacs = _write_graph_to_dimacs_file
@@ -708,7 +711,9 @@ def es(self):
 
     ###########################
     # Paths/traversals
-    def get_all_simple_paths(self, v, to=None, minlen=0, maxlen=-1, mode="out", max_results=None):
+    def get_all_simple_paths(
+        self, v, to=None, minlen=0, maxlen=-1, mode="out", max_results=None
+    ):
         """Calculates all the simple paths from a given node to some other nodes
         (or all of them) in a graph.
 
@@ -973,15 +978,14 @@ def Incidence(cls, *args, **kwds):
     def are_connected(self, *args, **kwds):
         """Deprecated alias to L{Graph.are_adjacent()}."""
         deprecated(
-            "Graph.are_connected() is deprecated; use Graph.are_adjacent() " "instead"
+            "Graph.are_connected() is deprecated; use Graph.are_adjacent() instead"
         )
         return self.are_adjacent(*args, **kwds)
 
     def get_incidence(self, *args, **kwds):
         """Deprecated alias to L{Graph.get_biadjacency()}."""
         deprecated(
-            "Graph.get_incidence() is deprecated; use Graph.get_biadjacency() "
-            "instead"
+            "Graph.get_incidence() is deprecated; use Graph.get_biadjacency() instead"
         )
         return self.get_biadjacency(*args, **kwds)
 
diff --git a/src/igraph/io/libraries.py b/src/igraph/io/libraries.py
index f35cc9545..9b06f41f1 100644
--- a/src/igraph/io/libraries.py
+++ b/src/igraph/io/libraries.py
@@ -270,3 +270,48 @@ def _construct_graph_from_graph_tool(cls, g):
     graph.add_edges(edges, eattr)
 
     return graph
+
+
+def _export_graph_to_torch_geometric(
+    graph, vertex_attributes=None, edge_attributes=None
+):
+    """Converts the graph to torch geometric
+
+    Data types: graph-tool only accepts specific data types. See the
+    following web page for a list:
+
+    https://pytorch-geometric.readthedocs.io/en/latest/generated/torch_geometric.data.Data.html#torch_geometric.data.Data
+
+    @param g: graph-tool Graph
+    @param vertex_attributes: dictionary of vertex attributes to transfer.
+      Keys are attributes from the vertices, values are data types (see
+      below). C{None} means no vertex attributes are transferred.
+    @param edge_attributes: dictionary of edge attributes to transfer.
+      Keys are attributes from the edges, values are data types (see
+      below). C{None} means no vertex attributes are transferred.
+    """
+    import torch
+    from torch_geometric.data import Data
+
+    if vertex_attributes is None:
+        vertex_attributes = graph.vertex_attributes()
+    if edge_attributes is None:
+        edge_attributes = graph.edge_attributes()
+
+    # Edge index
+    edge_index = torch.tensor(graph.get_edgelist(), dtype=torch.long)
+
+    # Node attributes
+    x = torch.tensor([graph.vs[attr] for attr in vertex_attributes])
+    if x.ndim > 1:
+        x = x.permute(*torch.arange(x.ndim - 1, -1, -1))
+
+    # Edge attributes
+    edge_attr = torch.tensor([graph.es[attr] for attr in edge_attributes])
+    if edge_attr.ndim > 1:
+        edge_attr = edge_attr.permute(*torch.arange(edge_attr.ndim - 1, -1, -1))
+
+    # Wrap into correct data structure
+    data = Data(x=x, edge_index=edge_index, edge_attr=edge_attr)
+
+    return data
diff --git a/tests/test_foreign.py b/tests/test_foreign.py
index 83664e5f5..b9caf6b52 100644
--- a/tests/test_foreign.py
+++ b/tests/test_foreign.py
@@ -23,6 +23,14 @@
     pd = None
 
 
+try:
+    import torch
+    from torch_geometric.data import Data as PyGData
+except ImportError:
+    torch = None
+    PyGData = None
+
+
 GRAPHML_EXAMPLE_FILE = """\
 <?xml version="1.0" encoding="UTF-8"?>
 <graphml xmlns="http://graphml.graphdrawing.org/xmlns"
@@ -821,6 +829,44 @@ def testGraphGraphTool(self):
         self.assertEqual(g.vcount(), g2.vcount())
         self.assertEqual(sorted(g.get_edgelist()), sorted(g2.get_edgelist()))
 
+    @unittest.skipIf(PyGData is None, "test case depends on torch_geometric")
+    def testGraphTorchGeometric(self):
+        # Undirected
+        g = Graph.Ring(10)
+        g.vs["vattr"] = list(range(g.vcount()))
+        g.es["eattr"] = list(range(len(g.es)))
+
+        # Go to torch geometric
+        data_pyg = g.to_torch_geometric()
+
+        self.assertEqual(g.vcount(), data_pyg.num_nodes)
+        self.assertEqual(
+            sorted([list(x) for x in g.get_edgelist()]),
+            sorted(data_pyg.edge_index.tolist()),
+        )
+
+        # Test attributes
+        self.assertEqual(g.vs["vattr"], data_pyg.x[:, 0].tolist())
+        self.assertEqual(g.es["eattr"], data_pyg.edge_attr[:, 0].tolist())
+
+        # Directed
+        g = Graph.Ring(10, directed=True)
+        g.vs["vattr"] = list(range(g.vcount()))
+        g.es["eattr"] = list(range(len(g.es)))
+
+        # Go to torch geometric
+        data_pyg = g.to_torch_geometric()
+
+        self.assertEqual(g.vcount(), data_pyg.num_nodes)
+        self.assertEqual(
+            sorted([list(x) for x in g.get_edgelist()]),
+            sorted(data_pyg.edge_index.tolist()),
+        )
+
+        # Test attributes
+        self.assertEqual(g.vs["vattr"], data_pyg.x[:, 0].tolist())
+        self.assertEqual(g.es["eattr"], data_pyg.edge_attr[:, 0].tolist())
+
     @unittest.skipIf(gt is None, "test case depends on graph-tool")
     def testMultigraphGraphTool(self):
         # Undirected