sieve.py

>>> def naturals(start):
...     yield start
...     yield from naturals(start+1)
...
>>> def sieve(prev_sieve):
...     prime = next(prev_sieve)
...     yield prime
...     yield from (num for num in prev_sieve if num % prime != 0)
... 
>>> s = sieve(naturals(2))
>>> next(s)
2
>>> next(s)
3
>>> next(s)
5
>>> next(s)
7
>>> next(s)
9
>>> next(s)
11
>>> next(s)
13
>>> next(s)
15
>>> next(s)
17
>>> next(s)
19
>>> next(s)
21
>>> next(s)
23
>>> next(s)
25
>>> next(s)
27
>>> next(s)
29
>>> next(s)
31
>>> next(s)
33