Fix private method hidden by ancestor class attribute (#4177)

* Fix private method hidden by ancestor class attribute

A class defined private method won't be hidden by an ancestor class private
attribute as the class will mangle the latter to avoid naming collisions.

Add tests to assert method-hidden : Although private attributes from the
parent class should not hide methods in the child class, this is not true for
protected attributes. This test ensures that ``method-hidden`` catches the
error when private attributes are not in use.

Signed-off-by: Tiago Honorato <tiagohonorato1@gmail.com>
Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

3 files changed, 35 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index a23865228..9174c31a0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -17,6 +17,9 @@ Release date: TBA
 ..
   Put bug fixes that will be cherry-picked to latest major version here
 
+* Fix false positive for ``method-hidden`` when using private attribute and method
+
+  Closes #3936
 * ``use-symbolic-message-instead`` now also works on legacy messages like ``C0111`` (``missing-docstring``).
 
 * Remove unwanted print to stdout from ``_emit_no_member``
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index d475cfcfb..c96cce007 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -1000,6 +1000,8 @@ a metaclass class method.",
 
             # If a subclass defined the method then it's not our fault.
             for ancestor in klass.ancestors():
+                if node.name in ancestor.instance_attrs and is_attr_private(node.name):
+                    return
                 for obj in ancestor.lookup(node.name)[1]:
                     if isinstance(obj, astroid.FunctionDef):
                         return
diff --git a/tests/checkers/unittest_classes.py b/tests/checkers/unittest_classes.py
index ecbc72c4a..4cf0a6cb1 100644
--- a/tests/checkers/unittest_classes.py
+++ b/tests/checkers/unittest_classes.py
@@ -187,3 +187,33 @@ class TestVariablesChecker(CheckerTestCase):
             Message("protected-access", node=attribute_in_fake_2, args="__private"),
         ):
             self.walk(node.root())
+
+    def test_private_attribute_hides_method(self):
+        node = astroid.extract_node(
+            """
+            class Parent:
+                def __init__(self):
+                    self.__private = None
+
+            class Child(Parent):
+                def __private(self): #@
+                    pass
+            """
+        )
+        with self.assertNoMessages():
+            self.checker.visit_functiondef(node)
+
+    def test_protected_attribute_hides_method(self):
+        node = astroid.extract_node(
+            """
+            class Parent:
+                def __init__(self):
+                    self._protected = None
+
+            class Child(Parent):
+                def _protected(self): #@
+                    pass
+            """
+        )
+        with self.assertAddsMessages(Message("method-hidden", node=node, args=("", 4))):
+            self.checker.visit_functiondef(node)