Question

How would you iterate a number backwards without using a list in Python 3?

Using lists, I would probably do something like:

li = list(range(100))
for i in li[::-1]:
    print(i)

This is a fine solution, but it does not work with huge numbers.
Right now, I’m trying to iterate a number backwards with large numbers, but an overflow error happens, so I think I need to somehow find a way to iterate a number backwards without using lists.

Is there any way?

Answer 1

Iterators that provide the __reversed__() special method also support reversed iteration. In Python 3.x, range() iterators have this method, so you can also use

for i in reversed(range(100)):
    print(i)

In Python 2.x, xarnge() allows reversed iteration:

for i in reversed(xrange(100)):
    print i

Both versions do not store the whole list in memory.