Raise CycleError each time prepare() is called

Author: lordmauve

Lib/graphlib.py

@@ -90,15 +90,17 @@ def prepare(self):
         still be used to obtain as many nodes as possible until cycles block more
         progress. After a call to this function, the graph cannot be modified and
         therefore no more nodes can be added using "add".
+
+        Raise ValueError if nodes have already been passed out of the sorter.
+
         """
-        if self._ready_nodes is not None:
-            if self._npassedout > 0:
-                raise ValueError("cannot prepare() after starting sort")
-            return
+        if self._npassedout > 0:
+            raise ValueError("cannot prepare() after starting sort")
 
-        self._ready_nodes = [
-            i.node for i in self._node2info.values() if i.npredecessors == 0
-        ]
+        if self._ready_nodes is None:
+            self._ready_nodes = [
+                i.node for i in self._node2info.values() if i.npredecessors == 0
+            ]
         # ready_nodes is set before we look for cycles on purpose:
         # if the user wants to catch the CycleError, that's fine,
         # they can continue using the instance to grab as many

Lib/test/test_graphlib.py

@@ -149,6 +149,12 @@ def test_prepare_after_pass_out(self):
         with self.assertRaisesRegex(ValueError, r"cannot prepare\(\) after starting sort"):
             ts.prepare()
 
+    def test_prepare_cycleerror_each_time(self):
+        ts = graphlib.TopologicalSorter({'a': 'b', 'b': 'a'})
+        for attempt in range(1, 4):
+            with self.assertRaises(graphlib.CycleError, msg=f"{attempt=}"):
+                ts.prepare()
+
     def test_invalid_nodes_in_done(self):
         ts = graphlib.TopologicalSorter()
         ts.add(1, 2, 3, 4)