I was intrigued to see how this worked out in practice for Python 3, where range returns an iterator rather than a list— like, would reversing the iteration req

I was intrigued to see how this worked out in practice for Python 3, where range returns an iterator rather than a list— like, would reversing the iteration require allocating the whole list in memory? $ python3 -c 'print(reversed(range(10)))' <range_iterator object at 0x7fdb53ae96f0> It turns out that reversed() only works on objects that expose the __len__ and __getitem__ methods: https://www.python.org/dev/peps/pep-0322/ https://www.python.org/dev/peps/pep-0322/ If you try to use reversed() with your own generator it will fail with "TypeError: argument to reversed() must be a sequence", until you wrap the generator invocation in an explicit list(). Very nice.