How can I inherit the parant class' function in Qt model?

Question

When I try this code, the child class(addwindow) can't recorgnize self.load_region(). How can I inherit the parant class's load_region() function?

class MainWindow(QMainWindow, main_ui):

  def __init__(self):
      super().__init__()
      self.load_region()
  def clicked_add(self):
      addwindow(self)
  def load_region(self):
      self.region_list.clear()

class addwindow(QDialog):
  def __init__(self, parent):
      super(addwindow, self).__init__(parent)
  def add_data(self):
      self.load_region()