Fix monkeypatch.setattr() teardown crash on failed setattr

changelog/14161.bugfix.rst

@@ -0,0 +1 @@
+Fixed :meth:`monkeypatch.setattr() <pytest.MonkeyPatch.setattr>` leaving a stale entry on the undo stack when the underlying ``setattr()`` call fails (e.g. on immutable targets), causing an ``AttributeError`` crash during teardown.

src/_pytest/monkeypatch.py

@@ -244,8 +244,8 @@ def setattr(
         # avoid class descriptors like staticmethod/classmethod
         if inspect.isclass(target):
             oldval = target.__dict__.get(name, NOTSET)
-        self._setattr.append((target, name, oldval))
         setattr(target, name, value)
+        self._setattr.append((target, name, oldval))
 
     def delattr(
         self,

testing/test_monkeypatch.py

@@ -89,6 +89,22 @@ def test_unknown_attr_non_raising(self, monkeypatch: MonkeyPatch) -> None:
             mp.setattr("os.path.qweqwe", 42, raising=False)
             assert os.path.qweqwe == 42  # type: ignore
 
+    def test_setattr_failure_does_not_corrupt_undo(
+        self, monkeypatch: MonkeyPatch
+    ) -> None:
+        """If setattr() raises (e.g. target doesn't support attribute
+        setting), the undo stack should not contain a stale entry that
+        crashes during teardown (#14161)."""
+
+        class Immutable:
+            __slots__ = ()
+
+        target = Immutable()
+        with pytest.raises(AttributeError):
+            monkeypatch.setattr(target, "x", 42, raising=False)
+        # undo() must not raise — no entry should be on the undo stack.
+        monkeypatch.undo()
+
     def test_delattr(self, monkeypatch: MonkeyPatch) -> None:
         with monkeypatch.context() as mp:
             mp.delattr("os.path.abspath")