Fix edgecase when converting to networkx w/o fallback (rapidsai#118)

This fixes a (probably rare) case that I encountered when calling a function with `nxcg.Graph` and `backend="networkx"` when `nx.config.fallback_to_nx is False` and the function calls another networkx function.

The `nxcg.Graph` is networkx-compatible, but it still has `__networkx_backend__ == "cugraph"`, so there are times when some behaviors may be different (but technically correct) b/c `__networkx_backend__` isn't `"networkx"`. So, we convert fully to networkx graph. An alternative would be to create a proxy graph that shares the data structures of the `nxcg.Graph` but has `G.__networkx_backend__ == "networkx"`, but the current approach is simplest and adequately covers what should be a very rare occurance.

Regression test added.

Authors:
  - Erik Welch (https://github.com/eriknw)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)

URL: rapidsai#118

Author: eriknw

nx_cugraph/convert.py

@@ -648,6 +648,10 @@ def to_networkx(
     """
     if isinstance(G, nxcg.Graph):
         # These graphs are already NetworkX graphs :)
+        if _nxver < (3, 4) or not nx.config.fallback_to_nx:
+            # Convert to nx graph (so G.__networkx_backend == "networkx") for safety
+            return G.to_networkx_class()(G)
+        # Should be fine to duck-type as networkx graph; will cleanly fall back to nx
         return G
     rv = G.to_networkx_class()()
     id_to_key = G.id_to_key

nx_cugraph/interface.py

@@ -34,8 +34,7 @@ def convert_from_nx(graph, *args, **kwargs):
 
     @staticmethod
     def convert_to_nx(obj, *, name: str | None = None):
-        if isinstance(obj, nxcg.CudaGraph):
-            # Observe that this does not try to convert Graph!
+        if isinstance(obj, nxcg.Graph | nxcg.CudaGraph):
             return nxcg.to_networkx(obj)
         return obj
 

nx_cugraph/tests/test_convert.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2023-2024, NVIDIA CORPORATION.
+# Copyright (c) 2023-2025, NVIDIA CORPORATION.
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
@@ -15,7 +15,7 @@
 import pytest
 
 import nx_cugraph as nxcg
-from nx_cugraph import interface
+from nx_cugraph import _nxver, interface
 
 
 @pytest.mark.parametrize(
@@ -267,3 +267,14 @@ def test_to_dict_of_lists():
     expected = nx.to_dict_of_lists(G, nodelist=[0, 3])
     result = nxcg.to_dict_of_lists(G, nodelist=[0, 3])
     assert expected == result
+
+
+@pytest.mark.skipif(_nxver < (3, 4), reason="Uses nx.config.fallback_to_nx")
+@pytest.mark.parametrize("fallback", [True, False])
+def test_convert_to_networkx_with_fallback(fallback):
+    G = nxcg.Graph()
+    G.add_edges_from([(0, 1), (1, 2)])
+    assert not hasattr(nxcg, "wiener_index")  # If fails, use a different function
+    with nx.config(fallback_to_nx=fallback):
+        # `closeness_vitality` calls `wiener_index`, which we don't yet implement
+        nx.closeness_vitality(G, backend="networkx")