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.measurements 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 +1,18 @@
+from __future__ import annotations
+
+from typing import Any
+
+import networkx as nx
+
+from graphix.opengraph import OpenGraph
+
+
+def test_empty_graph() -> None:
+    og: OpenGraph[Any] = 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()

tests/test_tnsim.py

@@ -356,7 +356,7 @@ def test_with_graphtrans_sequential(self, fx_bg: PCG64, jumps: int, fx_rng: Gene
         for qargs in itertools.permutations(input_list):
             value1 = state.expectation_value(random_op3, list(qargs))
             value2 = tn_mbqc.expectation_value(random_op3, list(qargs))
-            assert value1 == pytest.approx(value2)
+            assert value1 == pytest.approx(value2)  # pyright: ignore[reportUnknownMemberType]
 
     @pytest.mark.parametrize("jumps", range(1, 11))
     def test_coef_state(self, fx_bg: PCG64, jumps: int, fx_rng: Generator) -> None:
@@ -373,7 +373,7 @@ def test_coef_state(self, fx_bg: PCG64, jumps: int, fx_rng: Generator) -> None:
             coef_tn = tn.basis_coefficient(number)
             coef_sv = statevec_ref.flatten()[number]
 
-            assert abs(coef_tn) == pytest.approx(abs(coef_sv))
+            assert abs(coef_tn) == pytest.approx(abs(coef_sv))  # pyright: ignore[reportUnknownMemberType]
 
     @pytest.mark.parametrize(("nqubits", "jumps"), itertools.product(range(2, 6), range(1, 6)))
     def test_to_statevector(self, fx_bg: PCG64, nqubits: int, jumps: int, fx_rng: Generator) -> None:
@@ -408,4 +408,4 @@ def test_evolve(self, fx_bg: PCG64, jumps: int, fx_rng: Generator) -> None:
         expval_tn = tn_mbqc.expectation_value(random_op3_exp, [0, 1, 2])
         expval_ref = state.expectation_value(random_op3_exp, [0, 1, 2])
 
-        assert expval_tn == pytest.approx(expval_ref)
+        assert expval_tn == pytest.approx(expval_ref)  # pyright: ignore[reportUnknownMemberType]