Don't emit ``attribute-defined-outside-init`` for variables defined in setters.

Close #409

3 files changed, 30 insertions, 6 deletions

diff --git a/ChangeLog b/ChangeLog
index 1f9ab6425..b00e0ca86 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,6 +7,10 @@ What's New in Pylint 2.4.0?
 
 Release date: TBA
 
+* Don't emit ``attribute-defined-outside-init`` for variables defined in setters.
+
+  Close #409
+
 * Syntax errors report the column number.
 
   Close #2914
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index df39331d3..1313f4830 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -811,7 +811,11 @@ a metaclass class method.",
         defining_methods = self.config.defining_attr_methods
         current_module = cnode.root()
         for attr, nodes in cnode.instance_attrs.items():
-            # skip nodes which are not in the current module and it may screw up
+            # Exclude `__dict__` as it is already defined.
+            if attr == "__dict__":
+                continue
+
+            # Skip nodes which are not in the current module and it may screw up
             # the output, while it's not worth it
             nodes = [
                 n
@@ -821,11 +825,14 @@ a metaclass class method.",
             ]
             if not nodes:
                 continue  # error detected by typechecking
-            # check if any method attr is defined in is a defining method
-            if any(node.frame().name in defining_methods for node in nodes):
-                continue
-            # Exclude `__dict__` as it is already defined.
-            if attr == "__dict__":
+
+            # Check if any method attr is defined in is a defining method
+            # or if we have the attribute defined in a setter.
+            frames = (node.frame() for node in nodes)
+            if any(
+                frame.name in defining_methods or is_property_setter(frame)
+                for frame in frames
+            ):
                 continue
 
             # check attribute is defined in a parent's __init__
diff --git a/pylint/test/functional/attribute_defined_outside_init.py b/pylint/test/functional/attribute_defined_outside_init.py
index ab5cf5f19..ff31e955d 100644
--- a/pylint/test/functional/attribute_defined_outside_init.py
+++ b/pylint/test/functional/attribute_defined_outside_init.py
@@ -65,3 +65,16 @@ class Mixin(object):
 class F:
     def func(self):
         self.__dict__ = {'foo': 'bar'}
+
+
+class Mine:
+    def __init__(self, param):
+        self.prop = param
+
+    @property
+    def prop(self):
+        return self.__prop
+
+    @prop.setter
+    def prop(self, value):
+        self.__prop = value