Prime generator - draft concept of 6n±1

primes.py

prev = []
start = 5
num = start
lastPrime = (0,0)
ctr = 2 # 5 is the 3rd prime

while ctr < 50000:
    isPrime = not any(num % n == 0 for n in prev)
    if isPrime:
        ctr += 1
        lastPrime = (ctr, num)
        prev.append(num)
    if (num+1) % 6 == 0:
        num += 2
    else:
        num += 4
        
print "%d\t%d" % lastPrime