Launch window when the process ends in ffmpeg. PyQt

In Python, PyQt and ffmpeg: I need to learn how to make a window appear with the typical message: "The process has finished" That window has to appear when the ffmpeg process ends. Ffmpeg launched it with subprocess. I can not know how to execute "something" when a process ends. This is my code (for now):

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



   fileinput = ''

   def window():
      app = QApplication(sys.argv)
      win = QDialog()
      b1 = QPushButton(win)
      b1.setText("Select file")
      b1.move(50,20)
      b1.clicked.connect(b1_clicked)

      b2 = QPushButton(win)
      b2.setText("Output file")
      b2.move(50,50)
      b2.clicked.connect(b2_clicked)

      b3 = QPushButton(win)
      b3.setText("Encode!")
      b3.move(50,80)
      b3.clicked.connect(b3_clicked)

      win.setGeometry(100,100,200,200)
      win.setWindowTitle("Encoder")
      win.show()
      sys.exit(app.exec_())

   def b1_clicked():
      global fileinput
      fileinput = QFileDialog.getOpenFileName(None, "Selecciona archivo a convertir", "/home/salva", "video files (*.avi *.mkv *.mp4 *.mov *.mpg);; All files (*)")
      print (fileinput)


   def b2_clicked():
      filesaved = QFileDialog.getSaveFileName(None, "Archivo de salida", "/home/salva", "video files (*.avi *.mkv *.mp4 *.mov *.mpg);; All files (*)")
      print (filesaved)


   def b3_clicked():
       subprocess.call(['ffmpeg', '-i', fileinput, filesaved])


   if __name__ == '__main__':
      window()

EDIT:

To understand my problem better, I build a very simple code and try to pop-up but it does not work:

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


a = 20
if a == 20:
    print (a)
    QtGui.QMessageBox.information(self, "Información", "Se ha procesado {0}".format(a),QtGui.QMessageBox.Ok)

The error that gives me is this:

  

QtGui.QMessageBox.information (self, "Information", "Processed   {0} ". Format (a), QtGui.QMessageBox.Ok) NameError: name 'QtGui' is not   defined

EDITO II:

I have worked on a simpler and more customized code for what I am doing to find out how to launch a window when button 3 finishes its process. I've tried but it does not work for me. I have a main window: window and another one that has to appear when button 3 finishes its process: winok I need to close the window window and winok window appear at the end of the process button 3. Also know how to check the return value of button 3 (which will be what ffmpeg do in the future) and show error or success message . This is what I have for now oriented to solve the window:

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


def window():

   win = QDialog()

   b1 = QPushButton(win)
   b1.setText("Select file")
   b1.move(50,20)
   b1.clicked.connect(b1_clicked)

   b2 = QPushButton(win)
   b2.setText("Output file")
   b2.move(50,50)
   b2.clicked.connect(b2_clicked)

   b3 = QPushButton(win)
   b3.setText("Encode!")
   b3.move(50,80)
   b3.clicked.connect(b3_clicked)

   win.setGeometry(100,100,200,200)
   win.setWindowTitle("Encoder")
   win.show()
   sys.exit(app.exec_())

def b1_clicked():
    print('boton 1 pulsado')


def b2_clicked():
    print('boton 2 pulsado')


def b3_clicked():
   print ('boton 3 pulsado')
   winok()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window()


def winok():

    win = QDialog()

    b10 = QPushButton(win)
    b10.setText("Do another job?")
    b10.move(50,20)
    b10.clicked.connect(b10_clicked)

    b11 = QPushButton(win)
    b11.setText("Exit")
    b11.move(50,50)
    b11.clicked.connect(b11_clicked)

    win.setGeometry(100,100,300,100)
    win.setWindowTitle("Work completed. Oh Yeah!")
    win.show()
    sys.exit(app.exec_())

def b10_clicked():
    print ('Volviendo a codificar...')


def b11_clicked():
    exit()