Python: Prime Factors

from collections import defaultdict

def prime_factors(n):
    if n < 2:
        return None
    d = defaultdict(lambda: 0)
    i = 2
    while i <= n:
        if n % i == 0:
            d[i] += 1
            n /= i
            i = 2
        else:
            i += 1
    return d