snip.py

import sys
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5.QtGui import QPainter, QBrush, QColor
from PyQt5 import QtCore

class MySnip(QWidget):
    def __init__(self):
        super().__init__()
        self.setGeometry(30,30,600,400)
        self.begin = QtCore.QPoint()
        self.end = QtCore.QPoint()
        self.show()

    def paintEvent(self, event):
        qp = QPainter(self)
        br = QBrush(QColor(100, 10, 10, 40))  
        qp.setBrush(br)   
        qp.drawRect(QtCore.QRect(self.begin, self.end))       

    def mousePressEvent(self, event):
        self.begin = event.pos()
        self.end = event.pos()
        self.update()

    def mouseMoveEvent(self, event):
        self.end = event.pos()
        self.update()

    def mouseReleaseEvent(self, event):
        self.begin = event.pos()
        self.end = event.pos()
        self.update()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MySnip()
    window.show()
    app.aboutToQuit.connect(app.deleteLater)
    sys.exit(app.exec_())