Show a window before calling a terminal command in python

[gioullis]

Hey i am a newbie in in python and qt so forgive me if my question is silly.In the following code i have a single window with exit and execute button from the class example. When i click the execute button i try to create a new window from the class newwin and then execute some terminal commands. In this case i execute three simple commands. The problem is that the commands is executed first and the window shows up after the commands ended . How i can fix that?
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import sys
import os
from PyQt4 import QtCore, QtGui

class newwin(QtGui.QWidget):
def cancel(self):
self.close()
def init(self,parent=None):
QtGui.QWidget.init(self, parent)

    self.resize(320, 128)
    self.setWindowTitle('Waiting')

    self.label = QtGui.QLabel('Pleace wait. This may take a while', self)
    self.label.setGeometry(60, 30, 225, 31)

self.label.show()
self.quit=QtGui.QPushButton('Cancel', self)
self.quit.setGeometry(190, 90, 88, 27)
self.quit.show()
self.quit.clicked.connect(self.cancel)

class Example(QtGui.QWidget):

def __init__(self, parent=None):

self.Window2 = newwin()
QtGui.QWidget.init(self, parent)
self.initUI()

def initUI(self):                     
    self.pushButton_4 = QtGui.QPushButton("Execute",self)
    self.pushButton_4.setGeometry(QtCore.QRect(20, 300, 88, 27))
    self.pushButton_5 = QtGui.QPushButton("Exit",self)
    self.pushButton_5.setGeometry(QtCore.QRect(270, 300, 88, 27))
    
    self.pushButton_5.clicked.connect(QtCore.QCoreApplication.instance().quit)
    self.pushButton_4.clicked.connect(self.execute)
   
    self.resize(394, 345)
    self.setWindowTitle('QtSimple')    
    self.show()

def execute(self,state):

self.Window2.show()
os.system("sleep 2s")
os.system("echo the window is not appeared yet")
os.system("sleep 2s")

def main():

app = QtGui.QApplication(sys.argv)
ex = Example()    
sys.exit(app.exec_())

if name == 'main':
main()
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