python - 为什么我的代码在包含的项目中使用 __iadd__ 时会生成 __setitem__ 错误？

Question

谁能告诉我为什么这段代码提示容器上没有 __setitem__？我以为我只需要容器上的 __getitem__ 来获取项目，然后 __iadd__ 来设置值，不知道为什么它期待 __setitem__

class Item:
    def __init__(self):
        pass

    def __iadd__(self, value):
        print 'added: ' + value
        return self


class Container:

    def __init__(self):
        self.__items = {
            'foo': Item()
        }

    def __getitem__(self, name):
        return self.__items[name]

if __name__ == '__main__':

    # works!
    con = Container()
    item = con['foo']
    item += 'works!'

    # wtf?
    con['foo'] += "what's going on?"

    # output:

    # added: works!
    # added: what's going on?
    # Traceback (most recent call last):
    #   File "foo.py", line 27, in <module>
    #     con['foo'] += "what's going on?"
    # AttributeError: Container instance has no attribute '__setitem__'

Answer 1

基本上，

con['foo'] += "what's going on?"

编译成类似的东西:

item = con['foo']
item += "what's going on?"
conf['foo'] = item

可以看到，反编译代码，是这样的:

  2           0 LOAD_GLOBAL              0 (con)
              3 LOAD_CONST               1 ('foo')
              6 DUP_TOPX                 2
              9 BINARY_SUBSCR       
             10 LOAD_CONST               2 ("what's going on?")
             13 INPLACE_ADD         
             14 ROT_THREE           
             15 STORE_SUBSCR