code: def popup(self, title, callfn, initialdir=None):
          
          self.cb = callfn
          dirname = QtGui.QFileDialog.getExistingDirectory(
              self.parent, title, initialdir)
          if dirname:
              self.cb(dirname)
docs: Let user select a directory.
  
          Parameters
          ----------
          title : str
              Title for the dialog.
  
          callfn : func
              Function used to handle selected directory.
  
          initialdir : str or `None`
              Directory for dialog.