Hi,

I needed reversed(enumerate(x: list)) in my code, and have discovered that it wound't work. This is disappointing because operation is well defined. It is also well defined for str type, range, and - in principle, but not yet in practice - on dictionary iterators - keys(), values(), items() as dictionaries are ordered now.

It would also be well defined on any user type implementing __iter__, __len__, __reversed__ - think numpy arrays, some pandas dataframes, tensors.

That's plenty of usecases, therefore I guess it would be quite useful to avoid hacky / inefficient solutions like described here: https://code.activestate.com/lists/python-list/706205/.

If deemed useful, I would be interested in implementing this, maybe together with __reversed__ on dict keys, values, items.

Best Regards,
--
Ilya Kamen

-----------

p.s.

Sketch of what I am proposing:

class reversible_enumerate:

    def __init__(self, iterable):
        self.iterable = iterable
        self.ctr = 0

    def __iter__(self):
        for e in self.iterable:
            yield self.ctr, e
            self.ctr += 1

    def __reversed__(self):
        try:
            ri = reversed(self.iterable)
        except Exception as e:
            raise Exception(
                "enumerate can only be reversed if iterable to enumerate can be reversed and has defined length."
            ) from e

        try:
            l = len(self.iterable)
        except Exception as e:
            raise Exception(
                "enumerate can only be reversed if iterable to enumerate can be reversed and has defined length."
            ) from e

        indexes = range(l-1, -1, -1)
        for i, e in zip(indexes, ri):
            yield i, e

for i, c in reversed(reversible_enumerate("Hello World")):
    print(i, c)

for i, c in reversed(reversible_enumerate([11, 22, 33])):

print(i, c)