求第N个质数算法

def calc_prime(prime,num):
    i,gab=7,2
    while num>3:
        flag=True
        for x in prime:
            if x*x>i:
                break
            if i%x==0:
                flag=False
                break
        if flag:
            prime.append(i)
            if len(prime)>=num:
                break
        gab=6-gab
        i+=gab
    return prime[num-1]

#调用
print calc_prime([2,3,5],1000)