Define the bounds of a Item in a QGraphicsView

Hello, I made an application using the Qt Designer and PyQt4, I'm using QGraphicsView to display an image with a rectangle on it, I can move the rectangle over the image, but when I move it with the mouse I can pass the image boundaries and I wanted to limit the rectangle to the image boundary, how could I do that?
The answer can be in C++.
My image with rectangle
Thanks.

Code:

# -*- coding: utf-8 -*-
 
from PyQt4.QtCore import *
from PyQt4.QtGui import *
import sys
from screen import *
 
class Main(QWidget, Ui_Form):
    def __init__(self, parent=None):
        super(Main, self).__init__(parent)
        self.setupUi(self)
 
        self.scene = QGraphicsScene()
        self.cena.setScene(self.scene)
        self.scene.addPixmap(QPixmap("01.png"))
 
        pen = QPen(Qt.darkMagenta)
        pen.setWidth(4)
 
        rectangle = self.scence.addRect(5,5,300,150, pen)
        rectangle.setFlag(QGraphicsItem.ItemIsMovable)
 
    def showEvent(self, event):
        self.cena.fitInView(self.scene.sceneRect(), Qt.IgnoreAspectRatio)
 
app = QApplication(sys.argv)
window = Main()
window.show()