真开五十个线程

真开 50 个线程，每个只做一件小事就结束。看做完之后完成计数和还活着的线程数：

import threading

done = []
lock = threading.Lock()

def work(i):
    with lock:
        done.append(i)

ts = [threading.Thread(target=work, args=(i,)) for i in range(50)]
for t in ts:
    t.start()
for t in ts:
    t.join()
print(str(len(done)) + "/" + str(threading.active_count()))