code: def _credentials(self):
          
  
          if self.aws_account_id and self.aws_arn_role_name:
              return 'aws_iam_role=arn:aws:iam::{id}:role/{role}'.format(
                  id=self.aws_account_id,
                  role=self.aws_arn_role_name
              )
          elif self.aws_access_key_id and self.aws_secret_access_key:
              return 'aws_access_key_id={key};aws_secret_access_key={secret}{opt}'.format(
                  key=self.aws_access_key_id,
                  secret=self.aws_secret_access_key,
                  opt=';token={}'.format(self.aws_session_token) if self.aws_session_token else ''
              )
          else:
              raise NotImplementedError("Missing Credentials. "
                                        "Ensure one of the pairs of auth args below are set "
                                        "in a configuration file, environment variables or by "
                                        "being overridden in the task: "
                                        "'aws_access_key_id' AND 'aws_secret_access_key' OR "
                                        "'aws_account_id' AND 'aws_arn_role_name'")
docs: Return a credential string for the provided task. If no valid
          credentials are set, raise a NotImplementedError.