Fixed: ListField minus index assignment does not work #1119

Add code to detect '-1' as a integer.
Normalize negative index to regular list index
Added list assignment test case

mongoengine/base/document.py

@@ -606,7 +606,9 @@ class BaseDocument(object):
                 for p in parts:
                     if isinstance(d, (ObjectId, DBRef)):
                         break
-                    elif isinstance(d, list) and p.isdigit():
+                    elif isinstance(d, list) and p.lstrip('-').isdigit():
+                        if p[0] == '-':
+                            p = str(len(d)+int(p))
                         try:
                             d = d[int(p)]
                         except IndexError:
@@ -640,7 +642,9 @@ class BaseDocument(object):
                 parts = path.split('.')
                 db_field_name = parts.pop()
                 for p in parts:
-                    if isinstance(d, list) and p.isdigit():
+                    if isinstance(d, list) and p.lstrip('-').isdigit():
+                        if p[0] == '-':
+                            p = str(len(d)+int(p))
                         d = d[int(p)]
                     elif (hasattr(d, '__getattribute__') and
                           not isinstance(d, dict)):

tests/fields/fields.py

@@ -1022,6 +1022,54 @@ class FieldTest(unittest.TestCase):
         self.assertEqual(BlogPost.objects(info=['1', '2', '3', '4', '1', '2', '3', '4']).count(), 1)
         BlogPost.drop_collection()
 
+    def test_list_assignment(self):
+        """Ensure that list field element assignment and slicing work 
+        """
+        class BlogPost(Document):
+            info = ListField()
+
+        BlogPost.drop_collection()
+
+        post = BlogPost()
+        post.info = ['e1', 'e2', 3, '4', 5]
+        post.save()
+        
+        post.info[0] = 1
+        post.save()
+        post.reload()
+        self.assertEqual(post.info[0], 1)
+        
+        post.info[1:3] = ['n2', 'n3']
+        post.save()
+        post.reload()
+        self.assertEqual(post.info, [1, 'n2', 'n3', '4', 5])
+
+        post.info[-1] = 'n5'
+        post.save()
+        post.reload()
+        self.assertEqual(post.info, [1, 'n2', 'n3', '4', 'n5'])
+
+        post.info[-2] = 4
+        post.save()
+        post.reload()
+        self.assertEqual(post.info, [1, 'n2', 'n3', 4, 'n5'])
+
+        post.info[1:-1] = [2]
+        post.save()
+        post.reload()
+        self.assertEqual(post.info, [1, 2, 'n5'])
+
+        post.info[:-1] = [1, 'n2', 'n3', 4]
+        post.save()
+        post.reload()
+        self.assertEqual(post.info, [1, 'n2', 'n3', 4, 'n5'])
+
+        post.info[-4:3] = [2, 3]
+        post.save()
+        post.reload()
+        self.assertEqual(post.info, [1, 2, 3, 4, 'n5'])
+
+
     def test_list_field_passed_in_value(self):
         class Foo(Document):
             bars = ListField(ReferenceField("Bar"))