Fix #531: Allow empty graphs in check_well_formed

graphix/flow/core.py

@@ -1378,13 +1378,18 @@ def _check_flow_general_properties(flow: PauliFlow[_AM_co]) -> None:
         - The nodes in the partial order are the nodes in the open graph.
         - The first layer of the partial order layers is :math:`O`, the output nodes of the open graph. This is guaranteed because open graphs without outputs do not have flow.
     """
+    if len(flow.og.graph.nodes) == 0:
+        return
+
     if not _check_correction_function_domain(flow.og, flow.correction_function):
         raise FlowGenericError(FlowGenericErrorReason.IncorrectCorrectionFunctionDomain)
 
     if not _check_correction_function_image(flow.og, flow.correction_function):
         raise FlowGenericError(FlowGenericErrorReason.IncorrectCorrectionFunctionImage)
 
-    if len(flow.partial_order_layers) == 0:
+    if len(flow.partial_order_layers) == 0 and len(flow.og.graph.nodes) > 0:
+        if not flow.og.graph:
+            return
         raise PartialOrderError(PartialOrderErrorReason.Empty)
 
     first_layer = flow.partial_order_layers[0]

tests/test_empty_graph_fix.py

@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import networkx as nx
+
+from graphix.opengraph import OpenGraph
+
+if TYPE_CHECKING:
+    from graphix.measurement import Measurement
+
+
+def test_empty_graph_well_formed() -> None:
+    """Test that empty graphs pass the well-formedness check.
+
+    This is a regression test for issue #531.
+    """
+    og: OpenGraph[Measurement] = OpenGraph(graph=nx.Graph(), input_nodes=[], output_nodes=[], measurements={})
+    pf = og.extract_causal_flow()
+    pf.check_well_formed()
+    assert True

tests/test_issue_531.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+import networkx as nx
+
+from graphix.opengraph import OpenGraph
+
+
+def test_empty_graph():
+    og = OpenGraph(graph=nx.Graph(), input_nodes=[], output_nodes=[], measurements={})
+    pf = og.extract_causal_flow()
+    pf.check_well_formed()  # This should NOT raise PartialOrderError anymore
+    print("✅ Test passed! Empty graph is now well-formed.")
+
+if __name__ == "__main__":
+    test_empty_graph()