'Qt Override.
Filter tab keys and process double tab keys.'

def event(self, event):

    if ((event.type() == QEvent.KeyPress) and (event.key() == Qt.Key_Tab)):
        self.sig_tab_pressed.emit(True)
        self.numpress += 1
        if (self.numpress == 1):
            self.presstimer = QTimer.singleShot(400, self.handle_keypress)
        return True
    return QComboBox.event(self, event)
