fibs = [0, 1]
for i in range(8):
 fibs.append(fibs[-2] + fibs[-1])

class Fibs:
 def __init__(self):
  self.a = 0
  self.b = 1

 def next(self):
  self.a, self.b = self.b, self.a + self.b
  return self.a

 def __iter__(self):
  return self

fibs = Fibs()
for f in fibs:
 if f > 1000:
  print f
  break
 else:
  print f

class Fib(object):
 def __getitem__(self, n):
  if isinstance(n, int):
   a, b = 1, 1
   for x in range(n):
    a, b = b, a + b
   return a
  elif isinstance(n, slice):
   start = n.start
   stop = n.stop
   a, b = 1, 1
   L = []
   for x in range(stop):
    if x >= start:
     L.append(a)
    a, b = b, a + b
   return L
  else:
   raise TypeError("Fib indices must be integers")

f = Fib()
print f[0:5]
print f[:10]

0 1 1 2 3 5 8 13 21 34 55 89 144 233...

i, j = 0, 1

i = 0
j = 1

while i < 10000:

 print i,
 i, j = j, i+j

i = j
j = i+j

0 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192

i, j = 0, 1
while i < 10000:
 print i,
 i, j = j, i+j

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765