python中的RLock和Lock

爲了保證多線程任務執行時，共享資源的同步，出現了鎖這個東西。
Lock是阻塞其餘線程對共享資源的訪問，且同一線程只能acquire一次，如多於一次就出現了死鎖，程序沒法繼續執行。
爲了保證線程對共享資源的獨佔，又避免死鎖的出現，就有了RLock。RLock容許在同一線程中被屢次acquire，線程對共享資源的釋放須要把全部鎖都release。即n次acquire，須要n次release。

import threading,time,random
lock=threading.RLock()
result=[]
def step1():
    global result
    if lock.acquire():
        result.append('step1')
        time.sleep(2)
        lock.release()
def step2():
    global result
    if lock.acquire():
        result.append('step2')
        time.sleep(2)
        lock.release()

def showresult():
    if lock.acquire():
        step1()
        step2()
        lock.release()
    print result

def clearresult():
    global result
    if lock.acquire():
        result=None
        time.sleep(2)
        lock.release()
    print result

t1=threading.Thread(target=showresult)
t2=threading.Thread(target=clearresult)

t1.start()
t2.start()