python 锁问题

问题：
 写两个线程，一个线程打印1~52，另一个线程打印A~Z，打印顺序是12A34B...5152Z；

from threading import Thread,Lock
import time
num = 1

def print_num():
    global num
    for x in range(1,53):
        if num%3!=0:
            c.acquire()
            print(x)
            num+=1
            c.release()




def print_alpha():
    global num
    for i in range(ord("A"),ord("Z")+1):
         if num%3==0:
             c.acquire()
             print(chr(i))
             num+=1
             c.release()

c = Lock()
a = Thread(target=print_num)
a.start()
b = Thread(target=print_alpha)
b.start()

运行结果显示1，2。不知道错在哪
求大神指教