Python的reverse()函数如何工作？

Question

根据Python的文档,reversed()使用__getitem__和__len__if __reversed__未实现.

我遇到了一个奇怪的行为,但未能解释它:

>>> class A(dict):
...     pass
...
>>> reversed(A())
Traceback (most recent call last):
    ...
TypeError: 'A' object is not reversible

>>> class B(dict):
...     def __getitem__(self, key):
...         return super().__getitem__(key)
...     def __len__(self):
...         return super().__len__()
...
>>> reversed(B())
Traceback (most recent call last):
    ...
TypeError: 'B' object is not reversible

>>> class C:
...     def __getitem__(self, key):
...         return "item"
...     def __len__(self):
...         return 1
...
>>> reversed(C())
<reversed object at 0x00000000022BB9B0>

尽管调用reversed()映射类型没有意义,但它如何知道它是映射？它在内部检查isinstance(inst, dict)吗？它会检查任何一般的映射collections.abc.Mapping吗？有没有办法在没有实现的情况下覆盖这种行为__reversed__？

我认为这可能是由于dict实现了一个__reversed__抛出一个TypeError,或者一个None与你如何禁用相似的东西__hash__,但dict.__reversed__结果是空的AttributeError抛出.

Answer 1

是的,有一个检查dict型使用.PySequence_Check reversed

// cpython/Objects/enumobject.c
if (!PySequence_Check(seq)) {
    PyErr_Format(PyExc_TypeError,
                 "'%.200s' object is not reversible",
                 Py_TYPE(seq)->tp_name);
    return NULL;
}


// cpython/Objects/abstract.c

int
PySequence_Check(PyObject *s)
{
    if (PyDict_Check(s))
        return 0;
    return s != NULL && s->ob_type->tp_as_sequence &&
        s->ob_type->tp_as_sequence->sq_item != NULL;
}