class ListBoxes (tk.Frame):
    """A frame containing a list of filenames, and controls to add or delete
    files from the list.
    """
    def __init__(self, master=None):
        tk.Frame.__init__(self, master)
        # some tk calls to use the undocumented "hidden files" button
        self.tk.call('namespace', 'import', '::tk::dialog::file::')
        self.tk.call('set', '::tk::dialog::file::showHiddenBtn',  '1')
        self.tk.call('set', '::tk::dialog::file::showHiddenVar',  '0')