code: def key_exists(self, key):
          
          assert isinstance(key, str)
          cursor = self.conn.cursor()
          with contextlib.closing(cursor):
              cursor.execute('SELECT COUNT(*) FROM events WHERE uuid=?', (key,))
              res = cursor.fetchone()
              count = res[0]
          if count == 0:
              return False
          else:
              assert count in (0, 1), \
                  "Duplicate event ids detected: {0}".format(count)
              return True
docs: Check whether a key exists in the event store.
  
          Returns True if it does, False otherwise.