Add exceptions.DeletedObject and ObjectBase.assert_not_deleted.

etiquette/exceptions.py

@@ -181,6 +181,12 @@ class DatabaseOutOfDate(EtiquetteException):
     '''
     error_message = OUTOFDATE
 
+class DeletedObject(EtiquetteException):
+    '''
+    For when thing.deleted == True.
+    '''
+    error_message = '{} has already been deleted by another caller.'
+
 class FeatureDisabled(EtiquetteException):
     '''
     For when features of the system have been disabled by the configuration.

etiquette/objects.py

@@ -68,6 +68,10 @@ class ObjectBase:
 
         return author_id
 
+    def assert_not_deleted(self):
+        if self.deleted:
+            raise exceptions.DeletedObject(self)
+
     def get_author(self):
         '''
         Return the User who created this object, or None if it is unassigned.