title: How to create a pop-up window (link a MainWindow with a Dialog) in Qt Designer (Python)?
body: <p>I'm trying to build an "About" dialog window for my app. My main window is a "MainWindow" type and the "About" window is a Dialog, both created with QT Designer and transformed to ".py" files with pyuic4 --> gui_v1.py and about_v1.py</p>
  
  <p>In my the main file of my app, I built this second class for the about dialog:</p>
  
  <pre><code>import gui_v1
  import about_v1
  
  class AboutDialog(QtGui.QDialog, about_v1.Ui_Dialog):
      def __init__(self, parent=None):
          super(AboutDialog, self).__init__(parent)
          self.setupUi(self)
  </code></pre>
  
  <p>I added the open_about() function to my main class in the main file, and linked this function to the menu button responsible for opening the about window:</p>
  
  <pre><code>class MainDialog(QtGui.QMainWindow, gui_v1.Ui_MainWindow):
      def __init__(self, parent=None):
          super(MainDialog, self).__init__(parent)
          self.setupUi(self)
          QtCore.pyqtRemoveInputHook()
  
          [...]
  
          def open_about(self):
             app = QtGui.QApplication(sys.argv)
             form = AboutDialog()
             form.show()
             app.exec_()
  </code></pre>
  
  <p>My main app is running from this last function:</p>
  
  <pre><code>if __name__ == '__main__':
      def main():
          app=QtGui.QApplication(sys.argv)
          form=MainDialog()
          form.show()
          app.exec_()
  
      main()
  </code></pre>
  
  <p>When clicking on the About menu item, nothing happens. I'm guessing the problem is in the open_about() function, but I can't seem to find what it is. Thanks for your help!</p>