fix: from comments by @jmafoster1

causal_testing/specification/causal_dag.py

@@ -171,12 +171,15 @@ def check_iv_assumptions(self, treatment, outcome, instrument) -> bool:
         # (iii) Instrument and outcome do not share causes
 
         for cause in self.graph.nodes:
-            if cause not in (instrument, outcome):  # exclude self-cycles due to breaking changes in NetworkX > 3.2
-                instrument_paths = list(nx.all_simple_paths(self.graph, source=cause, target=instrument))
-                outcome_paths = list(nx.all_simple_paths(self.graph, source=cause, target=outcome))
-                if len(instrument_paths) > 0 and len(outcome_paths) > 0:
-                    print(cause, instrument, instrument_paths, outcome, outcome_paths)
-                    raise ValueError(f"Instrument {instrument} and outcome {outcome} share common causes")
+            # Exclude self-cycles due to breaking changes in NetworkX > 3.2
+            outcome_paths = (
+                list(nx.all_simple_paths(self.graph, source=cause, target=outcome)) if cause != outcome else []
+            )
+            instrument_paths = (
+                list(nx.all_simple_paths(self.graph, source=cause, target=instrument)) if cause != instrument else []
+            )
+            if len(instrument_paths) > 0 and len(outcome_paths) > 0:
+                raise ValueError(f"Instrument {instrument} and outcome {outcome} share common causes")
         return True
 
     def add_edge(self, u_of_edge: Node, v_of_edge: Node, **attr):