Why do tuples in Python work with reversed but do not have __reversed__?
immutability, iterator, python, reverse, tuples
Solution
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).
Problem
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) ```