property_as_callable.py

class CartItem:
  def __init__(self, weight):
    self.weight = weight
    
  def get_weight(self):
    return self.__weight
    
  def set_weight(self, val):
    if val > 0:
      self.__weight = val
    else:
      raise ValueError()
      
  weight = property(get_weight, set_weight)