锁

python的内置数据结构比如列表和字典等是线程安全的，但是简单数据类型比如整数和浮点数则不是线程安全的，要这些简单数据类型的通过操作，就需要使用锁。

#!/usr/bin/env python3 # coding=utf-8 import threading shared_resource_with_lock = 0 shared_resource_with_no_lock = 0 COUNT = 100000 shared_resource_lock = threading.Lock() ####LOCK MANAGEMENT## def increment_with_lock(): global shared_resource_with_lock for i in range(COUNT): shared_resource_lock.acquire() shared_resource_with_lock += 1 shared_resource_lock.release() def decrement_with_lock(): global shared_resource_with_lock for i in range(COUNT): shared_resource_lock.acquire() shared_resource_with_lock -= 1 shared_resource_lock.release() ####NO LOCK MANAGEMENT ## def increment_without_lock(): global shared_resource_with_no_lock for i in range(COUNT): shared_resource_with_no_lock += 1 def decrement_without_lock(): global shared_resource_with_no_lock for i in range(COUNT): shared_resource_with_no_lock -= 1 ####the Main program if __name__ == "__main__": t1 = threading.Thread(target = increment_with_lock) t2 = threading.Thread(target = decrement_with_lock) t3 = threading.Thread(target = increment_without_lock) t4 = threading.Thread(target = decrement_without_lock) t1.start() t2.start() t3.start() t4.start() t1.join() t2.join() t3.join() t4.join() print ("the value of shared variable with lock management is %s" %shared_resource_with_lock) print ("the value of shared variable with race condition is %s" %shared_resource_with_no_lock)

执行结果：

$ ./threading_lock.py

the value of shared variable with lock management is 0 the value of shared variable with race condition is 0

又如：

import random import threading import time logging.basicConfig(level=logging.DEBUG, format='(%(threadName)-10s) %(message)s', ) class Counter(object): def __init__(self, start=0): self.lock = threading.Lock() self.value = start def increment(self): logging.debug(time.ctime(time.time())) logging.debug('Waiting for lock') self.lock.acquire() try: pause = random.randint(1,3) logging.debug(time.ctime(time.time())) logging.debug('Acquired lock') self.value = self.value + 1 logging.debug('lock {0} seconds'.format(pause)) time.sleep(pause) finally: self.lock.release() def worker(c): for i in range(2): pause = random.randint(1,3) logging.debug(time.ctime(time.time())) logging.debug('Sleeping %0.02f', pause) time.sleep(pause) c.increment() logging.debug('Done') counter = Counter() for i in range(2): t = threading.Thread(target=worker, args=(counter,)) t.start() logging.debug('Waiting for worker threads') main_thread = threading.currentThread() for t in threading.enumerate(): if t is not main_thread: t.join() logging.debug('Counter: %d', counter.value)

执行结果：

$ python threading_lock.py

(Thread-1 ) Tue Sep 15 15:49:18 2015 (Thread-1 ) Sleeping 3.00 (Thread-2 ) Tue Sep 15 15:49:18 2015 (MainThread) Waiting for worker threads (Thread-2 ) Sleeping 2.00 (Thread-2 ) Tue Sep 15 15:49:20 2015 (Thread-2 ) Waiting for lock (Thread-2 ) Tue Sep 15 15:49:20 2015 (Thread-2 ) Acquired lock (Thread-2 ) lock 2 seconds (Thread-1 ) Tue Sep 15 15:49:21 2015 (Thread-1 ) Waiting for lock (Thread-2 ) Tue Sep 15 15:49:22 2015 (Thread-1 ) Tue Sep 15 15:49:22 2015 (Thread-2 ) Sleeping 2.00 (Thread-1 ) Acquired lock (Thread-1 ) lock 1 seconds (Thread-1 ) Tue Sep 15 15:49:23 2015 (Thread-1 ) Sleeping 2.00 (Thread-2 ) Tue Sep 15 15:49:24 2015 (Thread-2 ) Waiting for lock (Thread-2 ) Tue Sep 15 15:49:24 2015 (Thread-2 ) Acquired lock (Thread-2 ) lock 1 seconds (Thread-1 ) Tue Sep 15 15:49:25 2015 (Thread-1 ) Waiting for lock (Thread-1 ) Tue Sep 15 15:49:25 2015 (Thread-1 ) Acquired lock (Thread-1 ) lock 2 seconds (Thread-2 ) Done (Thread-1 ) Done (MainThread) Counter: 4

acquire()中传入False值，可以检查是否获得了锁。比如：

import logging import threading import time logging.basicConfig(level=logging.DEBUG, format='(%(threadName)-10s) %(message)s', ) def lock_holder(lock): logging.debug('Starting') while True: lock.acquire() try: logging.debug('Holding') time.sleep(0.5) finally: logging.debug('Not holding') lock.release() time.sleep(0.5) return def worker(lock): logging.debug('Starting') num_tries = 0 num_acquires = 0 while num_acquires < 3: time.sleep(0.5) logging.debug('Trying to acquire') have_it = lock.acquire(0) try: num_tries += 1 if have_it: logging.debug('Iteration %d: Acquired', num_tries) num_acquires += 1 else: logging.debug('Iteration %d: Not acquired', num_tries) finally: if have_it: lock.release() logging.debug('Done after %d iterations', num_tries) lock = threading.Lock() holder = threading.Thread(target=lock_holder, args=(lock,), name='LockHolder') holder.setDaemon(True) holder.start() worker = threading.Thread(target=worker, args=(lock,), name='Worker') worker.start()

执行结果：

$ python threading_lock_noblock.py

(LockHolder) Starting (LockHolder) Holding (Worker ) Starting (LockHolder) Not holding (Worker ) Trying to acquire (Worker ) Iteration 1: Acquired (LockHolder) Holding (Worker ) Trying to acquire (Worker ) Iteration 2: Not acquired (LockHolder) Not holding (Worker ) Trying to acquire (Worker ) Iteration 3: Acquired (LockHolder) Holding (Worker ) Trying to acquire (Worker ) Iteration 4: Not acquired (LockHolder) Not holding (Worker ) Trying to acquire (Worker ) Iteration 5: Acquired (Worker ) Done after 5 iterations

线程安全锁

threading.RLock()

返回可重入锁对象。重入锁必须由获得它的线程释放。一旦线程获得了重入锁，同一线程可不阻塞地再次获得，获取之后必须释放。

通常一个线程只能获取一次锁：

import threading lock = threading.Lock() print 'First try :', lock.acquire() print 'Second try:', lock.acquire(0)

执行结果：

$ python threading_lock_reacquire.py

First try : True Second try: False

使用RLock可以获取多次锁：

import threading lock = threading.RLock() print 'First try :', lock.acquire() print 'Second try:', lock.acquire(0)

执行结果：

python threading_rlock.py

First try : True Second try: 1

再来看一个例子：

#!/usr/bin/env python3 # coding=utf-8 import threading import time class Box(object): lock = threading.RLock() def __init__(self): self.total_items = 0 def execute(self,n): Box.lock.acquire() self.total_items += n Box.lock.release() def add(self): Box.lock.acquire() self.execute(1) Box.lock.release() def remove(self): Box.lock.acquire() self.execute(-1) Box.lock.release() ## These two functions run n in separate ## threads and call the Box's methods def adder(box,items): while items > 0: print ("adding 1 item in the boxn") box.add() time.sleep(5) items -= 1 def remover(box,items): while items > 0: print ("removing 1 item in the box") box.remove() time.sleep(5) items -= 1 ## the main program build some ## threads and make sure it works if __name__ == "__main__": items = 5 print ("putting %s items in the box " % items) box = Box() t1 = threading.Thread(target=adder,args=(box,items)) t2 = threading.Thread(target=remover,args=(box,items)) t1.start() t2.start() t1.join() t2.join() print ("%s items still remain in the box " % box.total_items)

执行结果：

$ python3 threading_rlock2.py

putting 5 items in the box adding 1 item in the box removing 1 item in the box adding 1 item in the box removing 1 item in the box adding 1 item in the box removing 1 item in the box removing 1 item in the box adding 1 item in the box removing 1 item in the box adding 1 item in the box 0 items still remain in the box