Fix monkeypatch.setattr() failing to undo patches to __bases__

changelog/1938.bugfix.rst

@@ -0,0 +1 @@
+``monkeypatch.undo()`` now restores ``__bases__`` and similar type-level attributes patched via ``monkeypatch.setattr()``, instead of raising ``TypeError``.

src/_pytest/monkeypatch.py

@@ -256,8 +256,10 @@ def setattr(
             raise AttributeError(f"{target!r} has no attribute {name!r}")
 
         # avoid class descriptors like staticmethod/classmethod
-        if inspect.isclass(target):
-            oldval = target.__dict__.get(name, NOTSET)
+        is_class = inspect.isclass(target)
+        if is_class:
+            in_class_dict = name in target.__dict__
+            oldval = target.__dict__.get(name, oldval)
         elif not _is_data_descriptor(type(target), name):
             # With no data descriptor in the way, the `setattr()` below writes
             # into the instance `__dict__`, so `undo()` has to restore that
@@ -269,6 +271,13 @@ def setattr(
             if isinstance(target_dict, Mapping):
                 oldval = target_dict.get(name, NOTSET)
         setattr(target, name, value)
+        if is_class and not in_class_dict:
+            if name in target.__dict__:
+                # setattr() created a new entry shadowing an inherited
+                # attribute: undo() must remove it with delattr() (#156).
+                oldval = NOTSET
+            # setattr() did not add a __dict__ entry: undo() restores the old
+            # value via setattr() (#1938).
         self._setattr.append((target, name, oldval))
 
     def delattr(

testing/test_monkeypatch.py

@@ -443,6 +443,26 @@ def test_issue156_undo_staticmethod(Sample: type[Sample]) -> None:
     assert Sample.hello()
 
 
+def test_issue1938_patch_class_bases() -> None:
+    class Loud:
+        def thing(self):
+            return "!!!!"
+
+    class Quiet:
+        def thing(self):
+            return "sssh..."
+
+    class ThingToTest(Loud):
+        pass
+
+    monkeypatch = MonkeyPatch()
+    monkeypatch.setattr(ThingToTest, "__bases__", (Quiet,))
+    assert ThingToTest().thing() == "sssh..."
+    monkeypatch.undo()
+    assert ThingToTest.__bases__ == (Loud,)
+    assert ThingToTest().thing() == "!!!!"
+
+
 def test_undo_class_descriptors_delattr() -> None:
     class SampleParent:
         @classmethod