@staticmethod
def _output_request_type_check(req):
    """Output request type check and ensure list for manipulation.

        Parameters
        ----------
        req : list | tuple | str
            Output request of variable type.

        Returns
        -------
        output_request : list
            Output request.
        """
    if isinstance(req, list):
        output_request = req
    elif isinstance(req, tuple):
        output_request = list(req)
    elif isinstance(req, str):
        output_request = [req]
    else:
        raise TypeError(
            'Output request must be str, list, or tuple but received: {}'.
            format(type(req)))
    return output_request
