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)