pyqt5打开图片

from PyQt5 import QtCore, QtGui, QtWidgets
import sys
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.graphicsView = QtWidgets.QGraphicsView(self.centralwidget)
        self.graphicsView.setGeometry(QtCore.QRect(10, 0, 131, 192))
        self.graphicsView.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.graphicsView.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.graphicsView.setObjectName("graphicsView")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(30, 220, 75, 23))
        self.pushButton.setObjectName("btn")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 23))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        self.pushButton.clicked.connect(self.btn1)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "btn"))

    def btn1(self):
        filename,_=QFileDialog.getOpenFileName(None,'open',r"E:\images1")
        img=QImage()
        img.load(filename)
        img=img.scaled(self.graphicsView.width(),self.graphicsView.height())
        scene=QGraphicsScene()
        scene.addPixmap(QPixmap().fromImage(img))
        self.graphicsView.setScene(scene)

if __name__=='__main__':
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

存在的一个问题为，如果将main函数改为：

if __name__=='__main__':
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    Ui_MainWindow().setupUi(MainWindow) #两步合为一步打开图片出错，？
    MainWindow.show()
    sys.exit(app.exec_())

这样无法成功运行。留坑待填