Jak zamknąć okno dialogowe?

Mam okno dialogowe.

class StartQT4_Captcha(QtGui.QDialog):
      def __init__(self, parent=None):
          super(StartQT4_Captcha, self).__init__(parent)
          self.ui2 = Ui_MainWindow2()
          self.ui2.setupUi(self)
          self.ui2.closeEvent = self.closeEvent
          self.ui2.lineEdit.setFocus()

          self.ui2.lineEdit.returnPressed.connect(self.close)

      def closeEvent(self, event):
          global CAPTCHA
          CAPTCHA = self.ui2.lineEdit.text()
          event.accept()

I otwieram:

    CaptchaWindow = StartQT4_Captcha()
    CaptchaWindow.show()

    CaptchaWindow.exec_()

Po naciśnięciu Enter program idzie dalej, a okno się nie zamyka. Pyqt4 i python 2.7.