Teaching generators

gen.py

from math import sqrt


def count(start, step):
    """Happens to be included in the standard library"""
    number = int(start)
    while True:
        yield number
        start += number


def primes(start=2):
    def is_prime(number):
        if number == 1:
            return False
        if number == 2:
            return True
        if number % 2 == 0:
            return False

        maximum = int(sqrt(number)) + 1
        for divisor in range(3, maximum, 2):
            if number % divisor == 0:
                return False

        return True

    for potential_prime in count(start):
        if is_prime(potential_prime):
            yield potential_prime