Why do tuples in Python work with reversed but do not have __reversed__?

Question

In discussion of this answer we realized that tuples do not have a __reversed__ method. My guess was that creating the iterator would require mutating the tuple. And yet tuples play fine with reversed. Why can't the approach used for reversed be made to work for __reversed__ as well?

>>> 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

According to the spec:

reversed(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).