问PyQt:为什么新窗口在打开后立即关闭

EN

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class MainWindow (QMainWindow):
    def __init__(self):
        win = QWidget()
        win.adjustSize()
        grid=QGridLayout()
        grid.setRowStretch(0, 1)
        grid.setRowStretch(1, 1)
        grid.setRowStretch(5, 1)
        for i in range(0,5):
            for j in range(0,4):
                if i==0 and j==2:
                    l1=grid.addWidget(QLabel("Choose an option:"),i,j, 2, 2)
                if i==2 and j==1:
                    b1= QPushButton("Get Best Match")
                    grid.addWidget(b1,i,j)
                elif i==2 and j==2:
                    b2=QPushButton("Button2")
                    grid.addWidget(b2,i,j)
                elif i==2 and j==3:
                    b3=QPushButton("Button3")
                    grid.addWidget(b3,i,j)
        b5=grid.addWidget(QLabel(""),3,4) 
        b4=QPushButton("Button4")
        grid.addWidget(b4,2,4)
        w1=b1.clicked.connect(window1)
        b2.clicked.connect(Win2)
        b3.clicked.connect(Win3)
        b4.clicked.connect(Win4)            
        win.setLayout(grid)
        win.setGeometry(100,100,width//2,height//2,)
        win.setWindowTitle("PYQT")
        win.show()
        win.setStyleSheet("""
        .QPushButton {
        height: 30px ;
        width: 20px ; 
        }
        .QLabel {
        qproperty-alignment: AlignCenter;
        font-size:12pt
         }

         """)
        sys.exit(app.exec_())

class window1():
    def __init__(self, pressed):
        super(window1, self).__init__()
        win1 = QWidget()
        win1.adjustSize()
        win1.setGeometry(100,100,width//2,height//2,)
        win1.setWindowTitle("Get Best Match")
        win1.show()

if __name__ == '__main__':

    app = QApplication(sys.argv)
    screen_resolution = app.desktop().screenGeometry()
    width, height = screen_resolution.width(), screen_resolution.height()
    main=MainWindow()