code: def exists(self, key):
      
      if key in self._keystore and not self._keystore[key].is_expired:
        return True
      elif key in self._keystore and self._keystore[key].is_expired:
        self.delete(key)
        return False
  
      return False
docs: Check if the particular key exists in the storage
  
      :param key: name of the key which existence need to be checked
      :return:
  
      :type key str
      :rtype bool