Add attrs special attribute to prevent false positve in pylint

Astroid cannot infer this attribute because of the way
attr builds its classes

Close PyCQA/pylint#1884

ChangeLog

@@ -14,6 +14,10 @@ Change log for the astroid package (used to be astng)
         Close #472
+   * Add missing attrs special attribute
+
+	Close PyCQA/pylint#1884
+
 2018-01-23 -- 1.6.1

astroid/brain/brain_attrs.py

@@ -30,6 +30,10 @@ def attr_attributes_transform(node):
     """Given that the ClassNode has an attr decorator,
     rewrite class attributes as instance attributes
     """
+    # Astroid can't infer this attribute properly
+    # Prevents https://github.com/PyCQA/pylint/issues/1884
+    node.locals["__attrs_attrs__"] = [astroid.Unknown(parent=node.body)]
+
     for cdefbodynode in node.body:
         if not isinstance(cdefbodynode, astroid.Assign):
             continue

astroid/tests/unittest_brain.py

@@ -841,6 +841,22 @@ class AttrsTest(unittest.TestCase):
         should_be_attribute = next(module.getattr('f')[0].infer()).getattr('d')[0]
         self.assertIsInstance(should_be_attribute, astroid.Unknown)
+    def test_special_attributes(self):
+        """Make sure special attrs attributes exist"""
+
+        code = """
+        import attr
+
+        @attr.s
+        class Foo:
+            pass
+        Foo()
+        """
+        foo_inst = next(astroid.extract_node(code).infer())
+        [attr_node] = foo_inst.getattr("__attrs_attrs__")
+        # Prevents https://github.com/PyCQA/pylint/issues/1884
+        assert isinstance(attr_node, nodes.Unknown)
+
 if __name__ == '__main__':
     unittest.main()