'Returns the file selection dialog that may be shared among
different calls from this application.  This command will create a
file selection dialog widget when it is called the first time. This
dialog will be returned by all subsequent calls to tix_filedialog.
An optional dlgclass parameter can be passed to specified what type
of file selection dialog widget is desired. Possible options are
tix FileSelectDialog or tixExFileSelectDialog.'

def tix_filedialog(self, dlgclass=None):

    if (dlgclass is not None):
        return self.tk.call('tix', 'filedialog', dlgclass)
    else:
        return self.tk.call('tix', 'filedialog')
        return
