Don't emit protected-acces for single underscore prefixed attributes in special methods

This PR prevents the emission of protected-access message when a single
underscore prefixed attribute is accessed inside a special  method.

Close #1802

6 files changed, 73 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index a0af1f5c6..6f5406114 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,6 +7,11 @@ What's New in Pylint 2.4.0?
 
 Release date: TBA
 
+* Don't emit ``protected-access`` when a single underscore prefixed attribute
+  is used inside a special method
+
+  Close #1802
+
 * Fix the "statement" values in the PyLinter's stats reports by module.
 
 * Added a new check, ``invalid-overridden-method``
diff --git a/doc/whatsnew/2.4.rst b/doc/whatsnew/2.4.rst
index a01635146..ceaf437bf 100644
--- a/doc/whatsnew/2.4.rst
+++ b/doc/whatsnew/2.4.rst
@@ -103,6 +103,11 @@ New checkers
 Other Changes
 =============
 
+* Don't emit ``protected-access`` when a single underscore prefixed attribute is used 
+  inside a special method
+
+  Close #1802
+
 * ``len-as-condition`` now only fires when a ``len(x)`` call is made without an explicit comparison.
 
   The message and description accompanying this checker has been changed
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index 79724656a..1bd298b35 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -1351,8 +1351,25 @@ a metaclass class method.",
                     if _is_attribute_property(name, klass):
                         return
 
+                #  A licit use of protected member is inside a special method
+                if not attrname.startswith(
+                    "__"
+                ) and self._is_called_inside_special_method(node):
+                    return
+
                 self.add_message("protected-access", node=node, args=attrname)
 
+    @staticmethod
+    def _is_called_inside_special_method(node: astroid.node_classes.NodeNG) -> bool:
+        """
+        Returns true if the node is located inside a special (aka dunder) method
+        """
+        try:
+            frame_name = node.frame().name
+        except AttributeError:
+            return False
+        return frame_name and frame_name in PYMETHODS
+
     def _is_type_self_call(self, expr):
         return (
             isinstance(expr, astroid.Call)
diff --git a/tests/functional/access_to_protected_members.py b/tests/functional/access_to_protected_members.py
index 1c97a3b50..a88dee82b 100644
--- a/tests/functional/access_to_protected_members.py
+++ b/tests/functional/access_to_protected_members.py
@@ -57,3 +57,45 @@ class Issue1031(object):
         if self._attr == 1:
             return type(INST)._protected  # [protected-access]
         return None
+
+
+class Issue1802(object):
+    """Test for GitHub issue 1802"""
+    def __init__(self, value):
+        self._foo = value
+        self.__private = 2 * value
+
+    def __eq__(self, other):
+        """Test a correct access as the access to protected member is in a special method"""
+        if isinstance(other, self.__class__):
+            answer = self._foo == other._foo
+            return answer and self.__private == other.__private  # [protected-access]
+        return False
+
+    def not_in_special(self, other):
+        """
+        Test an incorrect access as the access to protected member is not inside a special method
+        """
+        if isinstance(other, self.__class__):
+            return self._foo == other._foo  # [protected-access]
+        return False
+
+    def __le__(self, other):
+        """
+        Test a correct access as the access to protected member
+        is inside a special method even if it is deeply nested
+        """
+        if 2 > 1:
+            if isinstance(other, self.__class__):
+                if "answer" == "42":
+                    return self._foo == other._foo
+        return False
+
+    def __fake_special__(self, other):
+        """
+        Test an incorrect access as the access
+        to protected member is not inside a licit special method
+        """
+        if isinstance(other, self.__class__):
+            return self._foo == other._foo  # [protected-access]
+        return False
diff --git a/tests/functional/access_to_protected_members.txt b/tests/functional/access_to_protected_members.txt
index fb5c152e6..f1a9ceaa4 100644
--- a/tests/functional/access_to_protected_members.txt
+++ b/tests/functional/access_to_protected_members.txt
@@ -4,3 +4,6 @@ protected-access:42::Access to a protected member _protected of a client class
 protected-access:43::Access to a protected member _cls_protected of a client class
 protected-access:44::Access to a protected member _cls_protected of a client class
 protected-access:58:Issue1031.incorrect_access:Access to a protected member _protected of a client class
+protected-access:72:Issue1802.__eq__:Access to a protected member __private of a client class
+protected-access:80:Issue1802.not_in_special:Access to a protected member _foo of a client class
+protected-access:100:Issue1802.__fake_special__:Access to a protected member _foo of a client class
diff --git a/tests/unittest_checker_classes.py b/tests/unittest_checker_classes.py
index 3925cc36d..da880362b 100644
--- a/tests/unittest_checker_classes.py
+++ b/tests/unittest_checker_classes.py
@@ -105,7 +105,7 @@ class TestVariablesChecker(CheckerTestCase):
                 return 1 if self._nargs else 2
 
         class Application(metaclass=MC):
-            def __new__(cls):
+            def __no_special__(cls):
                 nargs = obj._nargs #@
         """
         )