Make nested _undo_sequence() join the active sequence

Author: StanFromIreland

Lib/test/test_turtle.py

@@ -706,6 +706,14 @@ def test_undo_sequence_resets_after_exception(self):
             self.assertRaises(ValueError, self.turtle.write, "spam")
         self.assertFalse(self.turtle.undobuffer.cumulate)
 
+    def test_nested_undo_sequence(self):
+        with self.turtle._undo_sequence():
+            self.turtle.teleport(10, 20)
+            self.turtle.forward(10)
+        self.assertEqual(self.turtle.undobufferentries(), 1)
+        self.turtle.undo()
+        self.assertEqual(self.turtle.pos(), (0, 0))
+
     def test_stamp_without_undobuffer(self):
         shape = turtle.Shape("polygon", ((0, 0), (5, 9), (-5, 9)))
         self.turtle.screen._shapes = {self.turtle.shape(): shape}

Lib/turtle.py

@@ -1656,14 +1656,15 @@ def _goto(self, end):
     def _undo_sequence(self):
         """Record the enclosed actions as a single undo step."""
         undobuffer = self.undobuffer
-        if undobuffer:
-            undobuffer.push(["seq"])
-            undobuffer.cumulate = True
+        if not undobuffer or undobuffer.cumulate:
+            yield
+            return
+        undobuffer.push(["seq"])
+        undobuffer.cumulate = True
         try:
             yield
         finally:
-            if undobuffer:
-                undobuffer.cumulate = False
+            undobuffer.cumulate = False
 
     def teleport(self, x=None, y=None, *, fill_gap: bool = False) -> None:
         """To be overwritten by child class RawTurtle.