Make graphlib.TopologicalSorter.prepare() idempotent

Fixes #130914

Author: lordmauve

Lib/graphlib.py

@@ -92,7 +92,9 @@ def prepare(self):
         therefore no more nodes can be added using "add".
         """
         if self._ready_nodes is not None:
-            raise ValueError("cannot prepare() more than once")
+            if self._npassedout > 0:
+                raise ValueError("cannot prepare() after starting sort")
+            return
 
         self._ready_nodes = [
             i.node for i in self._node2info.values() if i.npredecessors == 0

Lib/test/test_graphlib.py

@@ -140,7 +140,13 @@ def test_calls_before_prepare(self):
     def test_prepare_multiple_times(self):
         ts = graphlib.TopologicalSorter()
         ts.prepare()
-        with self.assertRaisesRegex(ValueError, r"cannot prepare\(\) more than once"):
+        ts.prepare()
+
+    def test_prepare_after_pass_out(self):
+        ts = graphlib.TopologicalSorter({'a': 'bc'})
+        ts.prepare()
+        self.assertEqual(set(ts.get_ready()), {'b', 'c'})
+        with self.assertRaisesRegex(ValueError, r"cannot prepare\(\) after starting sort"):
             ts.prepare()
 
     def test_invalid_nodes_in_done(self):