python - 为什么 Python 中的元组使用 reversed 但没有 __reversed__？

Question

在讨论this answer我们意识到元组没有 __reversed__ 方法。我的猜测是创建迭代器需要改变元组。然而，元组与 reversed 配合得很好。为什么不能使用于 reversed 的方法也适用于 __reversed__？

>>> foo = range(3)
>>> foo
[0, 1, 2]
>>> list(foo.__reversed__())
[2, 1, 0]
>>> foo
[0, 1, 2]
>>> bar = (0, 1, 2)
>>> list(bar.__reversed__())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'tuple' object has no attribute '__reversed__'
>>> reversed(bar)
<reversed object at 0x10603b310>
>>> tuple(reversed(bar))
(2, 1, 0)

Answer 1

根据spec :

反转(seq)

Return a reverse iterator. seq must be an object which has a reversed() method or supports the sequence protocol (the __len__() method and the __getitem__() method with integer arguments starting at 0).