def METHOD_NAME(self, obs_vars: list) -> tuple:
    """
    Get lists of obs / mod variables to be processed
    Parameters
    ----------
    obs_vars : list
        list of observation variables
    Returns
    -------
    list
        list of observation variables (potentially extended from input
        list)
    list
        corresponding model variables which are mapped based on content
        of :attr:`model_add_vars` and :attr:`model_use_vars`.
    """
    obsout, modout = [], []
    for obsvar in obs_vars:
        obsout.append(obsvar)
        if obsvar in self.model_use_vars:
            modout.append(self.model_use_vars[obsvar])
        else:
            modout.append(obsvar)
    for ovar, mvars in self.model_add_vars.items():
        if not isinstance(mvars, list):
            raise AttributeError(
                f"values of model_add_vars need to be lists, even if "
                f"only single variables are to be added: "
                f"{self.model_add_vars}"
            )
        for mvar in mvars:
            obsout.append(ovar)
            modout.append(mvar)
    return (obsout, modout)