Fix crash for unused-private-member when there are nested attributes

Author: yushao2

ChangeLog

@@ -60,6 +60,10 @@ Release date: TBA
 
   Closes #4673
 
+* Fix crash for ``unused-private-member`` that occurs when there is a presence of nested attributes
+
+  Closes #4755
+
 
 What's New in Pylint 2.9.6?
 ===========================

pylint/checkers/classes.py

@@ -974,9 +974,10 @@ def _check_unused_private_variables(self, node: astroid.ClassDef) -> None:
 
     def _check_unused_private_attributes(self, node: astroid.ClassDef) -> None:
         for assign_attr in node.nodes_of_class(astroid.AssignAttr):
-            if not is_attr_private(assign_attr.attrname):
+            if not is_attr_private(assign_attr.attrname) or not isinstance(
+                assign_attr.expr, astroid.Name
+            ):
                 continue
-
             # Logic for checking false positive when using __new__,
             # Get the returned object names of the __new__ magic function
             # Then check if the attribute was consumed in other instance methods

tests/functional/u/unused/unused_private_member.py

@@ -201,3 +201,19 @@ def __inner_4():  # [unused-private-member]
 
         fn_to_return = __inner_1 if flag else __inner_3(__inner_2)
         return fn_to_return
+
+
+# Test cases for crash reported in #4755
+# https://github.com/PyCQA/pylint/issues/4755
+class Crash4755Context:
+    def __init__(self):
+        self.__messages = None  # [unused-private-member]
+
+class Crash4755Command:
+    def __init__(self):
+        self.context = Crash4755Context()
+
+    def method(self):
+        self.context.__messages = []
+        for message in self.context.__messages:
+            print(message)

tests/functional/u/unused/unused_private_member.txt

@@ -9,3 +9,4 @@ unused-private-member:132:8:FalsePositive4657.__init__:Unused private member `Fa
 undefined-variable:137:15:FalsePositive4657.attr_c:Undefined variable 'cls':HIGH
 unused-private-member:179:8:FalsePositive4673.do_thing.__true_positive:Unused private member `FalsePositive4673.do_thing.__true_positive(in_thing)`:HIGH
 unused-private-member:199:8:FalsePositive4673.complicated_example.__inner_4:Unused private member `FalsePositive4673.complicated_example.__inner_4()`:HIGH
+unused-private-member:210:8:Crash4755Context.__init__:Unused private member `Crash4755Context.__messages`:HIGH