gh-157976: Fix shelve.Shelf.sync() leaving writeback disabled on error

Lib/shelve.py

@@ -176,9 +176,11 @@ def __del__(self):
     def sync(self):
         if self.writeback and self.cache:
             self.writeback = False
-            for key, entry in self.cache.items():
-                self[key] = entry
-            self.writeback = True
+            try:
+                for key, entry in self.cache.items():
+                    self[key] = entry
+            finally:
+                self.writeback = True
             self.cache = {}
         if hasattr(self.dict, 'sync'):
             self.dict.sync()

Lib/test/test_shelve.py

@@ -148,6 +148,25 @@ def test_writeback_also_writes_immediately(self):
         p2 = d[encodedkey]
         self.assertNotEqual(p1, p2)  # Write creates new object in store
 
+    def test_writeback_restored_after_failed_sync(self):
+        class Unpicklable:
+            def __reduce__(self):
+                raise RuntimeError('cannot pickle')
+
+        d = {}
+        with shelve.Shelf(d, writeback=True) as s:
+            s['key'] = []
+            s['key'].append(Unpicklable())
+            with self.assertRaises(RuntimeError):
+                s.sync()
+            self.assertTrue(s.writeback)
+            self.assertIn('key', s.cache)
+            s['key'].clear()
+            s['other'] = [1]
+            s['other'].append(2)
+        self.assertEqual(pickle.loads(d[b'key']), [])
+        self.assertEqual(pickle.loads(d[b'other']), [1, 2])
+
     def test_with(self):
         d1 = {}
         with shelve.Shelf(d1, protocol=2, writeback=False) as s:

Misc/NEWS.d/next/Library/2026-09-23-03-52-29.gh-issue-157976.cf2fd0.rst

@@ -0,0 +1,2 @@
+Fix :meth:`shelve.Shelf.sync` leaving *writeback* disabled when writing back
+a cached entry raises an exception. Patch by Christian Aurich Zanettini Martins.