def _check_selection(selection):
    """Handle default and validation of selection"""
    available = ["counts", "exposure", "background"]

    if selection is None:
        selection = available

    if not isinstance(selection, list):
        raise TypeError("Selection must be a list of str")

    for name in selection:
        if name not in available:
            raise ValueError("Selection not available: {!r}".format(name))

    return selection