Transparent rectangle inside a translucent/opaque window

Question

I am currently working on a "snipping tool" app. During the snipping process, the entire screen should become a translucent white, meanwhile the snipped area should be transparent. So far, I have managed to make the whole screen a translucent white, but I am not sure how to make the snipped area transparent. How can I achieve this?

Here is the code I have so far, it produces a translucent window

from PyQt5 import QtWidgets, QtGui, QtCore
from PyQt5.QtWidgets import QApplication, QMainWindow, QDesktopWidget, QSizeGrip
import cv2
import sys
import time
import numpy as np


class Window(QMainWindow):
    def __init__(self):
        super().__init__()
        self.title = "Frameless"
        self.xpos = 0
        self.ypos = 0
        self.width = QDesktopWidget().availableGeometry().bottomRight().x()
        self.height = QDesktopWidget().availableGeometry().bottomRight().y()
        self.initUI()

    def initUI(self):
        self.setWindowTitle(self.title)
        self.setGeometry(self.xpos, self.ypos, self.width, self.height)
        self.setWindowOpacity(0.5)

        flags = QtCore.Qt.WindowFlags(
            QtCore.Qt.FramelessWindowHint | QtCore.Qt.WindowStaysOnTopHint)
        self.setWindowFlags(flags)
        self.show()


def window():
    app = QApplication(sys.argv)
    win = Window()
    sys.exit(app.exec_())


window()