def callback_to_list(callback):
    """Cast callback to list.
    
    Parameters
    ----------
    callback : callable or list of callables
        Callable object(s).
    
    Returns
    -------
    list
        List of callable objects.
    """
    check_callable = True
    
    # convert callback to list
    if not isinstance(callback, list):
        if callback is None:
            callback = []
            check_callable = False
        elif callable(callback):
            callback = [callback]
        else:
            raise TypeError("'callback' must be callables or list of "
                            "callables.")
    
    # check if all callbacks are callable
    if check_callable is True:
        for c in callback:
            if callable(c) is False:
                raise TypeError("'callback' is not callable.")
    
    return callback