Don't emit 'assigning-non-slot' for descriptors. Closes issue #652.

4 files changed, 62 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index 6c9a2cc..e1e71c0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -325,6 +325,8 @@ ChangeLog for Pylint
     * Add a new convention warning, 'singleton-comparison', emitted when 
       comparison to True, False or None is found.
 
+    * Don't emit 'assigning-non-slot' for descriptors. Closes issue #652.
+
 
 
 2015-03-14 -- 1.4.3
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index 71041ea..0fc0fae 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -57,6 +57,25 @@ def _is_invalid_base_class(cls):
     return cls.name in INVALID_BASE_CLASSES and is_builtin_object(cls)
 
 
+def _has_data_descriptor(cls, attr):
+    attributes = cls.getattr(attr)
+    for attribute in attributes:
+        try:
+            for inferred in attribute.infer():
+                if isinstance(inferred, astroid.Instance):
+                    try:
+                        inferred.getattr('__get__')
+                        inferred.getattr('__set__')
+                    except astroid.NotFoundError:
+                        continue
+                    else:
+                        return True
+        except astroid.InferenceError:
+            # Can't infer, avoid emitting a false positive in this case.
+            return True
+    return False
+
+
 def _called_in_methods(func, klass, methods):
     """ Check if the func was called in any of the given methods,
     belonging to the *klass*. Returns True if so, False otherwise.
@@ -578,6 +597,10 @@ a metaclass class method.'}
                         # Properties circumvent the slots mechanism,
                         # so we should not emit a warning for them.
                         return
+                    if (node.attrname in klass.locals
+                            and _has_data_descriptor(klass, node.attrname)):
+                        # Descriptors circumvent the slots mechanism as well.
+                        return
                     self.add_message('assigning-non-slot',
                                      args=(node.attrname, ), node=node)
 
diff --git a/pylint/test/functional/assigning_non_slot.py b/pylint/test/functional/assigning_non_slot.py
index b80c32e..7163ac9 100644
--- a/pylint/test/functional/assigning_non_slot.py
+++ b/pylint/test/functional/assigning_non_slot.py
@@ -1,10 +1,10 @@
 """ Checks assigning attributes not found in class slots
 will trigger assigning-non-slot warning.
 """
-# pylint: disable=too-few-public-methods, no-init, missing-docstring, no-absolute-import
+# pylint: disable=too-few-public-methods, no-init, missing-docstring, no-absolute-import, import-error
 from collections import deque
 
-__revision__ = 0
+from missing import Unknown
 
 class Empty(object):
     """ empty """
@@ -98,3 +98,36 @@ class UnicodeSlots(object):
     def __init__(self):
         self.first = 42
         self.second = 24
+
+
+class DataDescriptor(object):
+    def __init__(self, name, default=''):
+        self.__name = name
+        self.__default = default
+
+    def __get__(self, inst, cls):
+        return getattr(inst, self.__name, self.__default)
+
+    def __set__(self, inst, value):
+        setattr(inst, self.__name, value)
+
+
+class NonDataDescriptor(object):
+    def __get__(self, inst, cls):
+        return 42
+
+
+class SlotsWithDescriptor(object):
+    __slots__ = ['_err']
+    data_descriptor = DataDescriptor('_err')
+    non_data_descriptor = NonDataDescriptor()
+    missing_descriptor = Unknown()
+
+
+def dont_emit_for_descriptors():
+    inst = SlotsWithDescriptor()
+    # This should not emit, because attr is
+    # a data descriptor
+    inst.data_descriptor = 'foo'
+    inst.non_data_descriptor = 'lala' # [assigning-non-slot]
+    return
diff --git a/pylint/test/functional/assigning_non_slot.txt b/pylint/test/functional/assigning_non_slot.txt
index ad32066..3591c41 100644
--- a/pylint/test/functional/assigning_non_slot.txt
+++ b/pylint/test/functional/assigning_non_slot.txt
@@ -1,3 +1,4 @@
 assigning-non-slot:18:Bad.__init__:Assigning to attribute 'missing' not defined in class slots

 assigning-non-slot:26:Bad2.__init__:Assigning to attribute 'missing' not defined in class slots

-assigning-non-slot:36:Bad3.__init__:Assigning to attribute 'missing' not defined in class slots
\ No newline at end of file
+assigning-non-slot:36:Bad3.__init__:Assigning to attribute 'missing' not defined in class slots

+assigning-non-slot:131:dont_emit_for_descriptors:Assigning to attribute 'non_data_descriptor' not defined in class slots
\ No newline at end of file