>>> a,b = 0,1
>>> while b < 20:
...     print b
...     a=b
...     b=a+b
...
1
2
4
8
16
>>> a,b = 0,1
>>> while b < 20:
...     print b
...     a,b = b,a+b
...
1
1
2
3
5
8
13