特性工厂函数

def quantity(storage_name):
  def qty_getter(instance):
    return instance.__dict__[storage_name]

  def qty_setter(instance,value):
    if value > 0:
      instance.__dict__[storage_name] = value
    else:
      raise ValueError('value must be > 0')
  
  return property(qty_getter, qty_setter)


class LineItem:
  weight = quantity('weight')  #相当于就是使用了@property装饰器，为weight设置了get和set
  price = quantity('price') #相当于就是使用了@property装饰器，为weight设置了get和set
  
  def __init__(self, description, weight, price):
    self.description = description
    self.weight = weight
    self.price = price

  def subtotal(self):
    return self.weight * self.price

if __name__ == '__main__':
    item = LineItem("苹果", -1, -5)  # 会调用weight和price的set方法
    print(item)  #报错，weight和price的值必须要大于0
    item = LineItem("苹果", 1, 5)
    print(item.weight, item.price) # 会调用weight和price的get方法