From 5d226df04fe4a8942573c6625a3ed5c3c0b5d08d Mon Sep 17 00:00:00 2001
From: Christian Aurich Zanettini Martins <christian.aurichzm@gmail.com>
Date: Wed, 23 Sep 2026 00:40:22 -0300
Subject: [PATCH] gh-157976: Fix shelve.Shelf.sync() leaving writeback disabled
 on error

Shelf.sync() sets self.writeback to False while it writes the cached
entries back. If writing an entry raises, for example because a cached
object cannot be pickled, writeback is never set back to True. The shelf
keeps working, but later mutations of accessed objects are silently not
persisted.

Restore writeback in a finally block. The cache is kept on failure, so
the caller can fix the offending entry and call sync() again.
---
 Lib/shelve.py                                 |  8 +++++---
 Lib/test/test_shelve.py                       | 19 +++++++++++++++++++
 ...-09-23-03-52-29.gh-issue-157976.cf2fd0.rst |  2 ++
 3 files changed, 26 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-09-23-03-52-29.gh-issue-157976.cf2fd0.rst

diff --git a/Lib/shelve.py b/Lib/shelve.py
index 9f6296667fdb6bd..f791123f0457e35 100644
--- a/Lib/shelve.py
+++ b/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()
diff --git a/Lib/test/test_shelve.py b/Lib/test/test_shelve.py
index 5f6a030e018f96a..e0f7844ce900dd5 100644
--- a/Lib/test/test_shelve.py
+++ b/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:
diff --git a/Misc/NEWS.d/next/Library/2026-09-23-03-52-29.gh-issue-157976.cf2fd0.rst b/Misc/NEWS.d/next/Library/2026-09-23-03-52-29.gh-issue-157976.cf2fd0.rst
new file mode 100644
index 000000000000000..a67d59edb892665
--- /dev/null
+++ b/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.