title: How do I open sub window after I click on button on main screen in PyQt4
body: <p>HI I am trying to make a simple converter.
  I have used PyQt4 designed to make the Gui
  I want to know how launch a new window after I click on the individual button.</p>
  
  <p>This is the interface I have created using PyQt4 Designer.
  Here is the Image link :</p>
  
  <p><img src="https://i.stack.imgur.com/gzJKL.png" alt="a"></p>
  
  <p>and I want to launch this windows when I click on currency button.
  Here is the Image Link:</p>
  
  <p><img src="https://i.stack.imgur.com/Q5muH.png" alt="b"></p>
  
  <p>Here is my code for <strong>main.py</strong></p>
  
  <pre><code>from PyQt4 import QtGui
  from main_screen import mainscreen
  
  def main():
      import sys
      qApp = QtGui.QApplication(sys.argv)
      aw = mainscreen()
      aw.show()
      sys.exit(qApp.exec_())
  
  if __name__ == '__main__':
      main()
  </code></pre>
  
  <p>and code for <strong>mainscreen.py</strong></p>
  
  <pre><code>from PyQt4 import QtCore, QtGui
  from window_main import Ui_MainWindow
  
  class mainscreen(QtGui.QMainWindow, Ui_MainWindow):
          def __init__(self, parent=None):
                  super(mainscreen,self).__init__(parent)
                  self.ui = Ui_MainWindow()
                  self.ui.setupUi(self)
  </code></pre>
  
  <p>How can I open new window after I click on currency button (object name for currency button is "currency_bt")
  and do I have to write the code for currency in same window or I have to write in new window.
  How do I do it.</p>
  
  <p>I am new to Python Gui programming.</p>