如何创建元组的循环引用？

Question

由于历史原因（阅读：可怕的type(t) == tuple 检查），我发现自己需要将圆形图冻结为tuple 对象的集合。显然，这并不理想：

>>> head = ("head", None)
>>> a = ("a", ("b", ("c", head)))
>>> head[1] = a
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    head[1] = a
TypeError: 'tuple' object does not support item assignment

然而，我并不是真正的 TypeErrors 的忠实信徒，我怀疑通过特定于实现的骇客技术可以做到这一点。

• 如何在不冒越界或导致其他 C 未定义行为的风险的情况下生成这种怪物？
• 垃圾收集器的循环依赖释放部分能否应对这种情况？

Answer 1

然而，我并不是真正相信 TypeErrors，并且怀疑通过特定于实现的骇客技术可以做到这一点。

很遗憾，你是对的：

from ctypes import Structure, c_ssize_t, c_void_p, py_object, pythonapi

pythonapi.Py_DecRef.argtypes = py_object,


def mutable(tup):
    # We are generating this class dynamically because the size of ob_item
    # varies according to the size of the given tuple
    class PyTupleObject(Structure):
        _fields_ = [('ob_refcnt', c_ssize_t),
                    ('ob_type', c_void_p),
                    ('ob_size', c_ssize_t),
                    ('ob_item', py_object * len(tup))]

        @classmethod
        def from_tuple(cls, tup):
            instance = cls.from_address(id(tup))
            # Save a reference to tup on the instance, as we are using it directly from memory
            # and don't want it to be garbage collected
            instance.original = tup
            return instance

        def __setitem__(self, idx, val):
            # Replacing a value in self.ob_item doesn't decref the old value but does indref the new value
            pythonapi.Py_DecRef(self.ob_item[idx])
            self.ob_item[idx] = val

        def __getitem__(self, idx):
            return self.ob_item[idx]

        def __iter__(self):
            return iter(self.ob_item)

        def __len__(self):
            return len(self.ob_item)

        def __contains__(self, val):
            return val in self.ob_item

    return PyTupleObject.from_tuple(tup)


if __name__ == '__main__':
    tup = (None,)
    mut_tup = mutable(tup)
    mut_tup[0] = tup
    print(tup is tup[0]) # Outputs "True"

我怎样才能在不冒超出界限或导致其他 C 未定义行为的情况下产生这种怪物？

我们通过将成员 ob_item 定义为 py_object * len(tup) 来防止越界访问。

垃圾收集器的循环依赖释放部分可以处理这样的事情吗？

不！元组应该是不可变的，因此不应该对它们自己进行循环引用。这就是为什么他们没有实现tp_clear 方法，python 垃圾收集器使用该方法来打破引用循环并收集所涉及的垃圾。更多详情here