CycleError: Report all nodes

pytools/graph.py

@@ -213,9 +213,17 @@ class CycleError(Exception):
     Raised when a topological ordering cannot be computed due to a cycle.
 
     :attr node: Node in a directed graph that is part of a cycle.
+
+    :attr nodes: List of nodes in a directed graph that are part of a cycle.
     """
-    def __init__(self, node: T) -> None:
-        self.node = node
+    def __init__(self, nodes: List[T]) -> None:
+        self.node = nodes[0]
+        self.nodes = nodes
+
+    def __str__(self) -> str:
+        le = len(self.nodes)
+        mlen = 10
+        return f"{[n for n in self.nodes[:mlen]] + (['...'] if le > mlen else [])}"
 
 
 class HeapEntry:
@@ -299,8 +307,8 @@ def compute_topological_order(graph: Mapping[T, Collection[T]],
 
     if len(order) != total_num_nodes:
         # any node which has a predecessor left is a part of a cycle
-        raise CycleError(next(iter(n for n, num_preds in
-            nodes_to_num_predecessors.items() if num_preds != 0)))
+        raise CycleError(list(n for n, num_preds in
+            nodes_to_num_predecessors.items() if num_preds != 0))
 
     return order
 

test/test_graph_tools.py

@@ -76,6 +76,10 @@ def test_compute_topological_order():
     with pytest.raises(CycleError):
         compute_topological_order(cycle)
 
+    cycle2 = {0: [2], 1: [2], 2: [3], 3: [1]}
+    with pytest.raises(CycleError, match=r"\[1, 2, 3\]"):
+        compute_topological_order(cycle2)
+
 
 def test_transitive_closure():
     from pytools.graph import compute_transitive_closure