如何使类和 getattr 可选取

Question

如何修改下面的类以使其可挑选？

本题：类似，但在使用getattr时引用了错误的异常。

This other question seems to provide meaningful insight Why does pickle.dumps call __getattr__?, 但是它没有提供一个例子，老实说我不明白我应该实现什么。

import pickle
class Foo(object):
    def __init__(self, dct):
        for key in dct:
            setattr(self, key, dct[key])


class Bar(object):
    def __init__(self, dct):
        for key in dct:
            setattr(self, key, dct[key])

    def __getattr__(self, attr):
        """If attr is not in channel, look in timing_data
        """
        return getattr(self.foo, attr)

if __name__=='__main__':
    dct={'a':1,'b':2,'c':3}
    foo=Foo(dct)
    dct2={'d':1,'e':2,'f':3,'foo':foo}
    bar=Bar(dct2)
    pickle.dump(bar,open('test.pkl','w'))
    bar=pickle.load(open('test.pkl','r'))

结果：

     14         """If attr is not in channel, look in timing_data
     15         """
---> 16         return getattr(self.foo, attr)
     17
     18 if __name__=='__main__':

RuntimeError: maximum recursion depth exceeded while calling a Python object

Answer 1

这里的问题是您的 __getattr__ 方法执行不佳。它假定 self.foo 存在。如果 self.foo 不存在，尝试访问它最终会调用 __getattr__ - 这会导致无限递归：

>>> bar = Bar({})  # no `foo` attribute
>>> bar.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "untitled.py", line 19, in __getattr__
    return getattr(self.foo, attr)
  File "untitled.py", line 19, in __getattr__
    return getattr(self.foo, attr)
  File "untitled.py", line 19, in __getattr__
    return getattr(self.foo, attr)
  [Previous line repeated 329 more times]
RecursionError: maximum recursion depth exceeded while calling a Python object

要解决此问题，如果不存在 foo 属性，则必须抛出 AttributeError：

def __getattr__(self, attr):
    """If attr is not in channel, look in timing_data
    """
    if 'foo' not in vars(self):
        raise AttributeError
    return getattr(self.foo, attr)

（我使用vars函数获取了对象的字典，因为它看起来比self.__dict__好看。）

现在一切正常：

dct={'a':1,'b':2,'c':3}
foo=Foo(dct)
dct2={'d':1,'e':2,'f':3,'foo':foo}
bar=Bar(dct2)
data = pickle.dumps(bar)
bar = pickle.loads(data)
print(vars(bar))
# output:
# {'d': 1, 'e': 2, 'f': 3, 'foo': <__main__.Foo object at 0x7f040fc7e7f0>}

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