为什么__getattribute__中存在无限递归循环风险？

Question

请参考现有问题的第二个最高答案：Difference between __getattr__ vs __getattribute__，其中包括某人建议的代码：

class Count(object):
    def __init__(self, mymin, mymax):
        self.mymin = mymin
        self.mymax = mymax
        self.current = None

    def __getattr__(self, item):
        self.__dict__[item] = 0
        return 0

    def __getattribute__(self, item):
        if item.startswith('cur'):
            raise AttributeError
        return super(Count, self).__getattribute__(item)

obj1 = Count(1, 10)
print(obj1.mymin)
print(obj1.mymax)
print(obj1.current)

我的问题是：

当我运行代码时，它没有 not 运行到无限递归深度（通过超过最大递归深度结束）。为什么？而且，如果我将代码super(Count, self).__getattribute__(item)更改为super(object, self).__getattribute__(item)，它的确陷入了无限循环。又为什么呢？

请通过详细的呼叫流程说明原因。

Answer 1

我将通过将self.__dict__[item]分为两部分来使其更简单：

class Count(object):
    def __getattr__(self, item):
        print('__getattr__:', item)
        d = self.__dict__
        print('resolved __dict__')
        d[item] = 0
        return 0

    def __getattribute__(self, item):
        print('__getattribute__:', item)
        if item.startswith('cur'):
            raise AttributeError
        return super(Count, self).__getattribute__(item)

obj1 = Count()
print(obj1.current)

输出为

__getattribute__: current
__getattr__: current
__getattribute__: __dict__
resolved __dict__
0

现在，如果我们将super(Count, self)替换为错误的构造 super(object, self)，则不会打印该消息。这是因为__getattribute__ 将也掩盖了对__dict__的访问。但是super对象将指向不存在的object基类，因此我们的__getattribute__函数将始终抛出AttributeError。

现在，在__getattribute__失败之后，正在尝试__getattr__……好吧，它不仅仅是将__dict__解析为某个值，而是尝试将其获取为属性-并最终再次调用__getattribute__。这样我们得到了。

....
__getattribute__:  __dict__
__getattr__: __dict__
__getattribute__:  __dict__
__getattr__: __dict__
__getattribute__:  __dict__
__getattr__: __dict__
__getattribute__:  __dict__
__getattr__: __dict__
__getattribute__:  __dict__
__getattr__: __dict__
Traceback (most recent call last):
  File "getattribute.py", line 15, in <module>
    print(obj1.current)
  File "getattribute.py", line 4, in __getattr__
    d = self.__dict__
  File "getattribute.py", line 4, in __getattr__
    d = self.__dict__
  File "getattribute.py", line 4, in __getattr__
    d = self.__dict__
  [Previous line repeated 328 more times]
  File "getattribute.py", line 8, in __getattribute__
    print('__getattribute__: ', item)
RecursionError: maximum recursion depth exceeded while calling a Python object

如果您使用setattr(self, item, 0)而不是查找self.__dict__，这可能是“避免的”：

class Count(object):
    def __getattr__(self, item):
        setattr(self, item, 0)
        return 0

    def __getattribute__(self, item):
        if item.startswith('cur'):
            raise AttributeError
        return super(object, self).__getattribute__(item)

obj1 = Count()
print(obj1.current)

当然，这样的代码不是正确的-尝试访问任何其他属性仍然会失败。