infinite sequence of primes generator in Python (unbounded Sieve of Eratosthenes)

primesieve.py

def primes():
    prime_multiple_factors = {}
    def add_factor(composite, factor):
        if composite not in prime_multiple_factors:
            prime_multiple_factors[composite] = set([factor])
        else:
            prime_multiple_factors[composite].add(factor)

    x = 2
    while True:
        if x in prime_multiple_factors:
            for factor in prime_multiple_factors[x]:
                add_factor(x + factor, factor)

            del prime_multiple_factors[x]
        else:
            add_factor(x * 2, x)
            yield x

        x += 1