Fix false positive for invalid-class-object when inference fails

ChangeLog

@@ -215,6 +215,9 @@ Release date: TBA
 
   Closes #3793
 
+* Fixed a false positive for ``invalid-class-object`` when the object
+  being assigned to the ``__class__`` attribute is uninferable.
+
 * Fixed false positive for ``used-before-assignment`` with self-referential type
   annotation in conditional statements within class methods.
 

doc/whatsnew/2.13.rst

@@ -283,6 +283,9 @@ Other Changes
 
   Closes #3793
 
+* Fixed a false positive for ``invalid-class-object`` when the object
+  being assigned to the ``__class__`` attribute is uninferable.
+
 * Added a ``testutil`` extra require to the packaging, as ``gitpython`` should not be a dependency
   all the time but is still required to use the primer helper code in ``pylint.testutil``. You can
   install it with ``pip install pylint[testutil]``.

pylint/checkers/classes/class_checker.py

@@ -1482,8 +1482,12 @@ def _check_invalid_class_object(self, node: nodes.AssignAttr) -> None:
         if not node.attrname == "__class__":
             return
         inferred = safe_infer(node.parent.value)
-        if isinstance(inferred, nodes.ClassDef) or inferred is astroid.Uninferable:
-            # If is uninferrable, we allow it to prevent false positives
+        if (
+            isinstance(inferred, nodes.ClassDef)
+            or inferred is astroid.Uninferable
+            or inferred is None
+        ):
+            # If is uninferable, we allow it to prevent false positives
             return
         self.add_message("invalid-class-object", node=node)
 

tests/functional/i/invalid/invalid_class_object.py

@@ -16,3 +16,17 @@ class B:
 A.__class__ = defaultdict
 A.__class__ = defaultdict(str)  # [invalid-class-object]
 A.__class__ = 1  # [invalid-class-object]
+
+
+# Here, ambiguity is found when inferring self.__class__
+class C:
+    @classmethod
+    def _new_instance(cls):
+        obj = C()
+        obj.__class__ = cls
+        return obj
+
+    def __deepcopy__(self, memo):
+        obj = C()
+        obj.__class__ = self.__class__
+        return obj