can not create window win transparent background

import sys
from PyQt5.QtCore import Qt
from PyQt5.QtGui import QPainter, QBrush
from PyQt5.QtWidgets import QDialog, QApplicationWIDTH=400
HEIGHT=400class MyWindow(QDialog):    def __init__(self):
        super().__init__()        self.setModal(True)
        self.setWindowOpacity(0.5)
        self.setStyleSheet('background: transparent; background-color: none; opacity: 0.5;')
        self.setAttribute(Qt.WA_TranslucentBackground, True)
        self.setWindowFlags(Qt.Window | Qt.WindowStaysOnTopHint | Qt.FramelessWindowHint)
        self.setAttribute(Qt.WA_NoSystemBackground, True)
        self.setAttribute(Qt.WA_OpaquePaintEvent, False)
        #self.setAttribute(Qt.WA_PaintOnScreen)        self.setMinimumWidth(WIDTH)
        self.setMinimumHeight(HEIGHT)
        self.setMaximumWidth(WIDTH)
        self.setMaximumHeight(HEIGHT)    def paintEvent(self, event):
        painter = QPainter(self)
        painter.setRenderHint(QPainter.Antialiasing)
        painter.setBrush(QBrush(Qt.gray, Qt.SolidPattern))
        painter.drawEllipse(0, 0, WIDTH, HEIGHT)def main():
    app = QApplication(sys.argv)
    win = MyWindow()
    win.show()
    sys.exit(app.exec_())