Python+PyQt5:如何对一个或多个QGroupBox使用QScrollArea？

Question

我一直在尝试将一个/多个QGroupBox插入一个QScrollArea。

问题是：滚动条不显示。

这是我的代码：

# -*- coding: utf-8 -*-

from PyQt5.QtWidgets import QWidget
from PyQt5.QtWidgets import QLabel
from PyQt5.QtWidgets import QCheckBox
from PyQt5.QtWidgets import QGroupBox
from PyQt5.QtWidgets import QScrollArea

from PyQt5.QtWidgets import QApplication
from PyQt5.QtWidgets import QHBoxLayout, QVBoxLayout, QGridLayout

import sys

lst = [u"D", u"E", u"EF", u"F", u"FG", u"G", u"H", u"JS", u"J", u"K", u"M", u"P", u"R", u"S", u"T", u"U", u"V", u"X", u"Y", u"Z"]

class MyApp(QWidget):
    def __init__(self):
        super(MyApp, self).__init__()

        window_width = 1200
        window_height = 600
        self.setFixedSize(window_width, window_height)

        self.initUI()

    def createLayout_group(self):
        self.groupbox = QGroupBox(u"Group1:")
        self.layout_groupbox = QVBoxLayout()

        for i in range(len(lst)):
            self.item = QCheckBox(lst[i], self.groupbox)
            self.layout_groupbox.addWidget(self.item)
        self.layout_groupbox.addStretch(1)

        self.groupbox.setLayout(self.layout_groupbox)


    def createLayout_Container(self):

        self.scrollarea = QScrollArea(self)
        self.scrollarea.setFixedSize(250, 6000)
        self.scrollarea.setWidgetResizable(False)

        self.layout_SArea = QVBoxLayout()
        self.layout_SArea.addWidget(self.groupbox)
        self.layout_SArea.addWidget(self.groupbox) # add groupbox one more to test
        self.layout_SArea.addWidget(self.groupbox) # add groupbox one more to test
        self.layout_SArea.addStretch(1)

        self.scrollarea.setLayout(self.layout_SArea)



    def initUI(self):
        self.createLayout_group()   # load one groupbox
        self.createLayout_Container()  # load groupbox container

        self.layout_All = QVBoxLayout()
        self.layout_All.addWidget(self.scrollarea)
        self.setLayout(self.layout_All)

        self.show()


if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = MyApp()
    sys.exit(app.exec_())

我想达到这个效果。但上面的代码没有显示滚动条。