I'm running this code:

import pywikibot

site = pywikibot.Site("wikidata", "wikidata")
repo = site.data_repository()
item_id = "Q254" # Wolfgang Amadeus Mozart

item = pywikibot.ItemPage(repo, item_id)
if item.exists():
    print(f"item {item_id} exists")

item.get()

It produces these warnings at item.exists() and item.get():

WARNING: entity-schema datatype is not supported yet.

The reason may be that https://www.wikidata.org/wiki/Q254 has a statement with the property EntitySchema for this class. However, showing this warning at .exists() and .get() looks like an exaggeration, because these methods don't seem to do anything with the statement with this property.