python - how to make a iterable object like this?

Question

Question 1

how to make an iterable object like this:

0 1 2 3 4 5 4 3 2 1 0 1 2 3 4 5 4 3 2 1 0 1 2 3 4 5 4 3 2 1 ....

Question 2

if use list(obj) on the object above would this eat up machine's memory? how to prevent it?

Please don't use python2

Answer 1

You can make an infinite generator that counts up and down:

def updown(n):
    while True:
        for i in range(n):
            yield i
        for i in range(n - 2, 0, -1):
            yield i

uptofive = updown(6)
for i in range(20):
    print uptofive.next(),

would output:

0 1 2 3 4 5 4 3 2 1 0 1 2 3 4 5 4 3 2 1

You cannot prevent list(updown(6)) from trying to consume all memory, no. As the doctor would say: "Then don't do that!".

Use .next() calls instead, or use your generator with another statement that limits the number of times you iterate over the generator. The itertools.islice() function would do just that:

import itertools
list(itertools.islice(updown(6), 20))