运行PyCon2015的Python Super示例代码遇TypeError：缺少self参数

我正在学习Raymond Hettinger在YouTube上的PyCon 2015演讲《Raymond Hettinger - Super considered super! - PyCon 2015》，输入了他用Python3.5演示的第一个示例代码pizza.py：

class DoughFactory(object):

  def get_dough(self):
    return 'insecticide treated wheat dough'


class Pizza(DoughFactory):

  def order_pizza(self, *toppings):
    print('Getting Dough')
    dough = DoughFactory.get_dough()
    print('Making pie with %s' % dough)
    for topping in toppings:
      print('Adding: %s' % topping)


if __name__ == "__main__":
  Pizza().order_pizza("pepperoni", "Bell Pepper")

使用Python3.5运行时出现以下报错，且在Python3.9环境及两台不同电脑上测试结果一致：

Getting Dough
Traceback (most recent call last):
  File "pizza.py", line 18, in <module>
    Pizza().order_pizza("pepperoni", "Bell Pepper")
  File "pizza.py", line 11, in order_pizza
    dough = DoughFactory.get_dough()
TypeError: get_dough() missing 1 required positional argument: 'self'

问题原因 #

get_dough是实例方法，定义时第一个参数为self，调用时需要自动传入当前实例对象。直接通过DoughFactory.get_dough()调用属于未绑定方法调用，没有传递实例参数，因此触发TypeError。

修复方案 #

有两种可靠的修复方式：

1. 通过当前实例调用父类方法（推荐）
   将dough = DoughFactory.get_dough()改为dough = self.get_dough()。由于Pizza继承自DoughFactory，实例会自动向上查找父类的方法，并传递self参数。
   修改后的完整代码：

   class DoughFactory(object):
   
     def get_dough(self):
       return 'insecticide treated wheat dough'
   
   
   class Pizza(DoughFactory):
   
     def order_pizza(self, *toppings):
       print('Getting Dough')
       dough = self.get_dough()
       print('Making pie with %s' % dough)
       for topping in toppings:
         print('Adding: %s' % topping)
   
   
   if __name__ == "__main__":
     Pizza().order_pizza("pepperoni", "Bell Pepper")
   

2. 显式传递当前实例参数
   将dough = DoughFactory.get_dough()改为dough = DoughFactory.get_dough(self)，手动把当前Pizza实例作为参数传给父类的get_dough方法。

修复后运行结果 #

执行修改后的代码会输出：

Getting Dough
Making pie with insecticide treated wheat dough
Adding: pepperoni
Adding: Bell Pepper

内容的提问来源于stack exchange，提问作者Questaware