def METHOD_NAME(self, widget):
    self.fileselectionDialog = gui.FileSelectionDialog('File Selection Dialog', 'Select an image file', False, '.')
    self.fileselectionDialog.confirm_value.do(
        self.on_image_file_selected)
    self.fileselectionDialog.cancel_dialog.do(
        self.on_dialog_cancel)
    # here is shown the dialog as root widget
    self.fileselectionDialog.show(self)