Fix monkeypatch recording failed mutations

changelog/14927.bugfix.rst

@@ -0,0 +1 @@
+Fixes MonkeyPatch so failed mutations are not recorded for undo.

src/_pytest/monkeypatch.py

@@ -282,14 +282,15 @@ def delattr(
             # Avoid class descriptors like staticmethod/classmethod.
             if inspect.isclass(target):
                 oldval = target.__dict__.get(name, NOTSET)
-            self._setattr.append((target, name, oldval))
             delattr(target, name)
+            self._setattr.append((target, name, oldval))
 
     def setitem(self, dic: Mapping[K, V], name: K, value: V) -> None:
         """Set dictionary entry ``name`` to value."""
-        self._setitem.append((dic, name, dic.get(name, NOTSET)))
+        oldval = dic.get(name, NOTSET)
         # Not all Mapping types support indexing, but MutableMapping doesn't support TypedDict
         dic[name] = value  # type: ignore[index]
+        self._setitem.append((dic, name, oldval))
 
     def delitem(self, dic: Mapping[K, V], name: K, raising: bool = True) -> None:
         """Delete ``name`` from dict.
@@ -301,9 +302,10 @@ def delitem(self, dic: Mapping[K, V], name: K, raising: bool = True) -> None:
             if raising:
                 raise KeyError(name)
         else:
-            self._setitem.append((dic, name, dic.get(name, NOTSET)))
+            oldval = dic.get(name, NOTSET)
             # Not all Mapping types support indexing, but MutableMapping doesn't support TypedDict
             del dic[name]  # type: ignore[attr-defined]
+            self._setitem.append((dic, name, oldval))
 
     def setenv(self, name: str, value: str, prepend: str | None = None) -> None:
         """Set environment variable ``name`` to ``value``.

testing/test_monkeypatch.py

@@ -135,6 +135,22 @@ class A:
     assert A.x == 1
 
 
+def test_delattr_does_not_record_failed_mutation() -> None:
+    class BrokenDelete:
+        value = 1
+
+        def __delattr__(self, name: str) -> None:
+            raise RuntimeError("delete failed")
+
+    monkeypatch = MonkeyPatch()
+    obj = BrokenDelete()
+
+    with pytest.raises(RuntimeError, match="delete failed"):
+        monkeypatch.delattr(obj, "value")
+
+    monkeypatch.undo()
+
+
 def test_setitem() -> None:
     d = {"x": 1}
     monkeypatch = MonkeyPatch()
@@ -162,6 +178,23 @@ def test_setitem_deleted_meanwhile() -> None:
     assert not d
 
 
+def test_setitem_does_not_record_failed_mutation() -> None:
+    class BrokenDict(dict[str, object]):
+        def __setitem__(self, key: str, value: object) -> None:
+            raise RuntimeError("set failed")
+
+        def __delitem__(self, key: str) -> None:
+            raise RuntimeError("delete failed")
+
+    monkeypatch = MonkeyPatch()
+    d = BrokenDict()
+
+    with pytest.raises(RuntimeError, match="set failed"):
+        monkeypatch.setitem(d, "x", 1)
+
+    monkeypatch.undo()
+
+
 @pytest.mark.parametrize("before", [True, False])
 def test_setenv_deleted_meanwhile(before: bool) -> None:
     key = "qwpeoip123"
@@ -196,6 +229,23 @@ def test_delitem() -> None:
     assert d == {"hello": "world", "x": 1}
 
 
+def test_delitem_does_not_record_failed_mutation() -> None:
+    class BrokenDict(dict[str, object]):
+        def __setitem__(self, key: str, value: object) -> None:
+            raise RuntimeError("set failed")
+
+        def __delitem__(self, key: str) -> None:
+            raise RuntimeError("delete failed")
+
+    monkeypatch = MonkeyPatch()
+    d = BrokenDict({"x": 1})
+
+    with pytest.raises(RuntimeError, match="delete failed"):
+        monkeypatch.delitem(d, "x")
+
+    monkeypatch.undo()
+
+
 def test_setenv() -> None:
     monkeypatch = MonkeyPatch()
     with pytest.warns(pytest.PytestWarning):