Note that if full is True known_absent must be empty

synapse/util/caches/dictionary_cache.py

@@ -39,6 +39,8 @@
 class DictionaryEntry:  # should be: Generic[DKT, DV].
     """Returned when getting an entry from the cache
 
+    If `full` is true then `known_absent` will be the empty set.
+
     Attributes:
         full: Whether the cache has the full or dict or just some keys.
             If not full then not all requested keys will necessarily be present