# Python Source Code Gems

## Apr 4, 2016 09:03 · 240 words · 2 minute read

So fairly regularly I read about and dip into the python source code. For some reason there seems to be a great deal of beautiful and interesting code in both the c and the standard libraries and tests for python. The other day I was doing some reading about generators (I was thinking about how to marry the ideas of functional and object oriented programming) and I came across something that I found fascinating as well beautiful so I decided I should share it here on this silly little blog. It’s a full implementation of a generator built Sieve of Eratosthenes in test_generators.py. If you are writing python and you aren’t using generators regularly then you are probably missing something.

```
Build up to a recursive Sieve of Eratosthenes generator.
>>> def firstn(g, n):
... return [g.next() for i in range(n)]
>>> def intsfrom(i):
... while 1:
... yield i
... i += 1
>>> firstn(intsfrom(5), 7)
[5, 6, 7, 8, 9, 10, 11]
>>> def exclude_multiples(n, ints):
... for i in ints:
... if i % n:
... yield i
>>> firstn(exclude_multiples(3, intsfrom(1)), 6)
[1, 2, 4, 5, 7, 8]
>>> def sieve(ints):
... prime = ints.next()
... yield prime
... not_divisible_by_prime = exclude_multiples(prime, ints)
... for p in sieve(not_divisible_by_prime):
... yield p
>>> primes = sieve(intsfrom(2))
>>> firstn(primes, 20)
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71]
```