def display_list_by_prefix(names_list, starting_spaces=0):
    """Creates a help string for ``names_list`` grouped by prefix."""
    cur_prefix, result_lines = None, []
    space = ' ' * starting_spaces
    for name in sorted(names_list):
        split = name.split('_', 1)
        prefix = split[0]
        if cur_prefix != prefix:
            result_lines.append(space + prefix + ':')
            cur_prefix = prefix
        result_lines.append(space + '  * ' + name)
    return '\n'.join(result_lines)
