Fixes 2 bugs in no_subclasses context mgr (__exit__ swallows exception + repair feature)

tests/test_context_managers.py

@@ -140,8 +140,6 @@ class ContextManagersTest(unittest.TestCase):
     def test_no_sub_classes(self):
         class A(Document):
             x = IntField()
-            y = IntField()
-
             meta = {'allow_inheritance': True}
 
         class B(A):
@@ -152,29 +150,29 @@ class ContextManagersTest(unittest.TestCase):
 
         A.drop_collection()
 
-        A(x=10, y=20).save()
-        A(x=15, y=30).save()
-        B(x=20, y=40).save()
-        B(x=30, y=50).save()
-        C(x=40, y=60).save()
+        A(x=10).save()
+        A(x=15).save()
+        B(x=20).save()
+        B(x=30).save()
+        C(x=40).save()
 
         self.assertEqual(A.objects.count(), 5)
         self.assertEqual(B.objects.count(), 3)
         self.assertEqual(C.objects.count(), 1)
 
-        with no_sub_classes(A) as A:
+        with no_sub_classes(A):
             self.assertEqual(A.objects.count(), 2)
 
             for obj in A.objects:
                 self.assertEqual(obj.__class__, A)
 
-        with no_sub_classes(B) as B:
+        with no_sub_classes(B):
             self.assertEqual(B.objects.count(), 2)
 
             for obj in B.objects:
                 self.assertEqual(obj.__class__, B)
 
-        with no_sub_classes(C) as C:
+        with no_sub_classes(C):
             self.assertEqual(C.objects.count(), 1)
 
             for obj in C.objects:
@@ -185,6 +183,32 @@ class ContextManagersTest(unittest.TestCase):
         self.assertEqual(B.objects.count(), 3)
         self.assertEqual(C.objects.count(), 1)
 
+    def test_no_sub_classes_modification_to_document_class_are_temporary(self):
+        class A(Document):
+            x = IntField()
+            meta = {'allow_inheritance': True}
+
+        class B(A):
+            z = IntField()
+
+        self.assertEqual(A._subclasses, ('A', 'A.B'))
+        with no_sub_classes(A):
+            self.assertEqual(A._subclasses, ('A',))
+        self.assertEqual(A._subclasses, ('A', 'A.B'))
+
+        self.assertEqual(B._subclasses, ('A.B',))
+        with no_sub_classes(B):
+            self.assertEqual(B._subclasses, ('A.B',))
+        self.assertEqual(B._subclasses, ('A.B',))
+
+    def test_no_subclass_context_manager_does_not_swallow_exception(self):
+        class User(Document):
+            name = StringField()
+
+        with self.assertRaises(TypeError):
+            with no_sub_classes(User):
+                raise TypeError()
+
     def test_query_counter(self):
         connect('mongoenginetest')
         db = get_db()