def _var_names(var_names):
    """Handle var_names input across arviz.

    Parameters
    ----------
    var_names: str, list, or None

    Returns
    -------
    var_name: list or None
    """
    if var_names is None:
        return None

    elif isinstance(var_names, str):
        return [var_names]

    else:
        return var_names