PyQt5的QPainter在设置CompositionMode为CompositionMode_Clear时透明部分是黑的

是重写的QLabel的paintEvent方法

def paintEvent(self, event: QtGui.QPaintEvent) -> None:

        self.painter = QPainter(self)
        self.painter.begin(self)

        rect = QRect(500, 500, 400, 400)
        self.painter.drawImage(rect, self.facebookImage)
        self.painter.setCompositionMode(QPainter.CompositionMode_Clear)
        rect = QRect(500, 500, 300, 300)
        self.painter.drawImage(rect, self.twitterImage)

        self.painter.end()

有哪位大佬能告诉我这是为什么吗

https://www.cnblogs.com/findumars/p/7487893.html