Fix monkeypatch undo of inherited attributes

AUTHORS

@@ -55,6 +55,7 @@ Antony Lee
 Arel Cordero
 Arias Emmanuel
 Ariel Pillemer
+Arpan Sahu
 Armin Rigo
 Aron Coyle
 Aron Curzon

changelog/10644.bugfix.rst

@@ -0,0 +1 @@
+Fix :class:`pytest.MonkeyPatch` restoring inherited attributes into an instance's ``__dict__`` after undoing ``setattr`` on an object.

src/_pytest/monkeypatch.py

@@ -244,6 +244,14 @@ def setattr(
         # avoid class descriptors like staticmethod/classmethod
         if inspect.isclass(target):
             oldval = target.__dict__.get(name, NOTSET)
+        else:
+            try:
+                target_vars = vars(target)
+            except TypeError:
+                pass
+            else:
+                if name not in target_vars:
+                    oldval = NOTSET
         setattr(target, name, value)
         self._setattr.append((target, name, oldval))
 

testing/test_monkeypatch.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 from collections.abc import Generator
+import inspect
 import os
 from pathlib import Path
 import re
@@ -51,6 +52,28 @@ class A:
         monkeypatch.setattr(A, "y")  # type: ignore[call-overload]
 
 
+def test_setattr_inherited_attribute_undo_restores_instance_dict() -> None:
+    class Descriptor:
+        def __get__(self, obj, objtype=None):
+            return 1
+
+    class Parent:
+        x = 1
+        descriptor = Descriptor()
+
+    for name in ("x", "descriptor"):
+        target = Parent()
+        monkeypatch = MonkeyPatch()
+        assert name not in vars(target)
+        assert isinstance(inspect.getattr_static(target, "descriptor"), Descriptor)
+
+        monkeypatch.setattr(target, name, 2)
+        monkeypatch.undo()
+
+        assert name not in vars(target)
+        assert isinstance(inspect.getattr_static(target, "descriptor"), Descriptor)
+
+
 class TestSetattrWithImportPath:
     def test_string_expression(self, monkeypatch: MonkeyPatch) -> None:
         with monkeypatch.context() as mp: