Update forms.py to fix #284

An exception `AttributeError` is raised when `child.targets[0]` does not have an attribute `name`, as stated in the issue #284 .

`meta.get_children()` returns all children of the `Meta` class, which in the sample code contains a dictionary assignment.

The expected children are of type `Assign` whose first item in the attribute `targets` is of type `AssignName`, like `AssignName.widgets(name='widgets')`.

An instruction like `widget["field"] = forms.SomeWidget` will result in a `Assign` child containing a `Subscript(ctx=<Context.Store: 2>, value=<Name.widgets ...>, slice=<Index ...>)`. This is of course unexpected.

I guess other unexpected instructions in the `Meta` class can trigger similar errors, so probably the most economic solution is to enclose the code in `try... except` and ignore the `AttributeError`. Trying to identify the type of object before accessing the `name` attribute would probably incur in a slower execution.

Author: uy-rrodriguez

pylint_django/checkers/forms.py

@@ -44,6 +44,11 @@ def visit_classdef(self, node):
             if not isinstance(child, Assign):
                 continue
 
-            if child.targets[0].name == 'exclude':
-                self.add_message('W%s04' % BASE_ID, node=child)
-                break
+            # Capture and ignore AttributeError raised when targets[0] does not
+            # have an attribute "name"
+            try:
+                if child.targets[0].name == 'exclude':
+                    self.add_message('W%s04' % BASE_ID, node=child)
+                    break
+            except AttributeError:
+                pass