http://d.hatena.ne.jp/flalin/20130411/1365703599

zigzag.py

#! /usr/bin/env python
import sys
def zigzag(sig, steps):
	while True:
		yield (sig, steps)
		sig *= -1
		steps += 1

def locus(x, motion):
	i = 0
	for (dx, steps) in motion:
		for _ in xrange(steps):
			yield (x, i)
			x += dx
			i += 1

# http://d.hatena.ne.jp/odz/20070718/1184769027
def take(n, iterable):
	i = 0
	for item in iterable:
		if i >= n:
			break
		else:
			i += 1
			yield item

for (indent, i) in take(50, locus(20, zigzag(-1, 1))):
	sys.stdout.write("{}{}\n".format(" " * indent, i))