Merge branch 'cherry-pick-d0fca0ec-2' into 'release-20.2'

release-20.2:Backport!4269

See merge request noc/noc!4271

core/datastream/base.py

@@ -189,7 +189,7 @@ class DataStream(object):
         if not is_changed(doc, hash):
             logger.info("[%s] Object hasn't been changed", l_name)
             return False  # Not changed
-        if not format and cls.on_change(data):
+        if not fmt and cls.on_change(data):
             hash = cls.get_hash(data)
             if not is_changed(doc, hash):
                 logger.info("[%s] Object hasn't been changed", l_name)
@@ -455,6 +455,7 @@ class DataStream(object):
             raise ValueError("Invalid instance")
         return {"_id": {"$mod": [n_instances, instance]}}
 
+    @classmethod
     def get_format_role(cls, fmt: str) -> Optional[str]:
         """
         Returns format role, if any