from PyQt4 import QtGui,QtCore
import sys
class Web_Browser(QtGui.QDialog):
  def __init__(self,parent=None):
    QtGui.QWidget.__init__(self)
    self.resize(550,400)
    open=QtGui.QPushButton('OPEN')
    self.connect(open,QtCore.SIGNAL('clicked()'),self.ok)
    grid=QtGui.QGridLayout()
    grid.addWidget(open,1,0,1,1)
    self.setLayout(grid)
    self.setWindowTitle('main windows')  
  def ok(self):
    web=Web_Browser()
    web.setModal(False)
    web.setWindowTitle('sub window')
    web.exec_()
 
app=QtGui.QApplication(sys.argv)
 
app.setFont(QtGui.QFont("Helvetica", 16))
main=Web_Browser()
main.show()
 
sys.exit(app.exec_())