Ignore __all__ elements that don't have a parent

There used to be a bug which involved astroid and extension modules,
where the elements of the __all__ declaration didn't have a parent set.
This problem was fixed though in astroid 2.0 and we can infer properly
these declarations with the parent being set. Unfortunately, we can't
rely on astroid 2.0 right now, so the solution for not having crashes
is to skip these elements in the undefined-all-variable pattern.

Close #846

Author: PCManticore

pylint/checkers/variables.py

@@ -384,6 +384,8 @@ def _check_all(self, node, not_consumed):
                 continue
             if elt_name is astroid.YES:
                 continue
+            if not elt_name.parent:
+                continue
 
             if (not isinstance(elt_name, astroid.Const)
                     or not isinstance(elt_name.value, six.string_types)):

pylint/test/unittest_checker_variables.py

@@ -38,6 +38,14 @@ def test_no_name_in_module_skipped(self):
         with self.assertNoMessages():
             self.checker.visit_importfrom(node)
 
+    def test_all_elements_without_parent(self):
+        node = test_utils.extract_node('__all__ = []')
+        node.value.elts.append(astroid.Const('test'))
+        root = node.root()
+        with self.assertNoMessages():
+            self.checker.visit_module(root)
+            self.checker.leave_module(root)
+
     @set_config(callbacks=('callback_', '_callback'))
     def test_custom_callback_string(self):
         """ Test the --calbacks option works. """