docs: def validate(self):
          """
          Check the value of the config attributes.
          """
          for client in self.clients:
              for key in REQUIRED_KEYS:
                  if key not in client:
                      raise MissingConfigValue(key)
  
              if 'revision_file' not in client:
                  client.revision_file = DEFAULT_REVISION_FILEPATH.format(
                      client.key
                  )