code: def popup(self, title, callfn, initialdir=None, filename=None):
          
          self.cb = callfn
          self.filew.set_title(title)
          if initialdir:
              self.filew.set_current_folder(initialdir)
  
          if filename:
              #self.filew.set_filename(filename)
              self.filew.set_current_name(filename)
  
          self.filew.show()
docs: Let user select and load file.