0.目录

2. threading.Lock() 的必要性
3.观察block
4.threading.RLock() 的应用场景

1.参考

Thread Synchronization Mechanisms in Python

count += 1 不是原子操作，三步操作可能被中断，通过lock将三步操作“封装”为一步操作，要么执行，要么不执行。

#!usr/bin/env python
#coding:utf-8
import sys
import time
import random
import logging

import threading
import Queue

lock = threading.Lock()     #'function-call ownership'
rlock = threading.RLock()   #thread ownership

logging.basicConfig(level=logging.DEBUG,
format = '%(asctime)s - %(threadName)-10s - %(levelname)s - %(message)s')
logger = logging.getLogger()

count = 0

class MyThread(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)

def run(self):
global count

for i in range(100):
count += 1
logger.debug('{} finished, count is {}'.format(self.name, count))

def main():
logger.debug('initial count: {}'.format(count))

thread_list = [MyThread() for i in range(2)]
for t in thread_list:
t.start()
for t in thread_list:
t.join()

logger.debug('final count: {}'.format(count))

if __name__ == '__main__':
main()

修改run函数代码的不同输出：

def run(self):
global count

for i in range(100):
count += 1
logger.debug('{} finished, count is {}'.format(self.name, count))
# 在切换线程之前，某一线程已经完成，两个线程顺序完成，结果几乎不会有误
# 2017-08-20 12:19:30,857 - MainThread - DEBUG - initial count: 0
# 2017-08-20 12:19:30,858 - Thread-1   - DEBUG - Thread-1 finished, count is 100
# 2017-08-20 12:19:30,858 - Thread-2   - DEBUG - Thread-2 finished, count is 200
# 2017-08-20 12:19:30,858 - MainThread - DEBUG - final count: 200

time.sleep(0.001)
for i in range(100):
count += 1
logger.debug('{} finished, count is {}'.format(self.name, count))
# 开头sleep导致两个线程几乎同时启动，结果可能有误
# 2017-08-20 12:24:59,046 - MainThread - DEBUG - initial count: 0
# 2017-08-20 12:24:59,048 - Thread-2   - DEBUG - Thread-2 finished, count is 124
# 2017-08-20 12:24:59,048 - Thread-1   - DEBUG - Thread-1 finished, count is 153
# 2017-08-20 12:24:59,048 - MainThread - DEBUG - final count: 153

for i in range(10000):
count += 1
logger.debug('{} finished, count is {}'.format(self.name, count))
# bytecodes足够导致两个线程交替运行，结果大概率有误
# 2017-08-20 12:20:17,719 - MainThread - DEBUG - initial count: 0
# 2017-08-20 12:20:17,723 - Thread-1   - DEBUG - Thread-1 finished, count is 12438
# 2017-08-20 12:20:17,723 - Thread-2   - DEBUG - Thread-2 finished, count is 12616
# 2017-08-20 12:20:17,723 - MainThread - DEBUG - final count: 12616

with lock:
for i in range(10000):
count += 1
logger.debug('{} finished, count is {}'.format(self.name, count))
# lock直到某一线程完成，结果正确
# 2017-08-20 12:20:37,630 - MainThread - DEBUG - initial count: 0
# 2017-08-20 12:20:37,631 - Thread-1   - DEBUG - Thread-1 finished, count is 10000
# 2017-08-20 12:20:37,632 - Thread-2   - DEBUG - Thread-2 finished, count is 20000
# 2017-08-20 12:20:37,634 - MainThread - DEBUG - final count: 20000

for i in range(10000):
with lock:
count += 1
logger.debug('{} finished, count is {}'.format(self.name, count))
# 两个线程交替lock，结果正确
# 2017-08-20 12:21:03,921 - MainThread - DEBUG - initial count: 0
# 2017-08-20 12:21:03,973 - Thread-1   - DEBUG - Thread-1 finished, count is 19979
# 2017-08-20 12:21:03,973 - Thread-2   - DEBUG - Thread-2 finished, count is 20000
# 2017-08-20 12:21:03,973 - MainThread - DEBUG - final count: 20000

3.观察block

def run(self):
global count

all = range(10000)  #确保每个线程 +1 的次数
while all != []:
if not lock.acquire(False):  #假设没有参数会导致block，则马上返回false当不block；否则返回true且acquire
logger.debug('{} wait...{}'.format(self.name, len(all)))
else:
try:
count += 1
all.pop()
except Exception as err:
logger.debug('{} err, count is {}'.format(self.name, count))
finally:
# logger.debug('{} release {} {}'.format(self.name, count, len(all)))  #导致两个线程顺序执行？？？
lock.release()
logger.debug('{} finished, count is {}'.format(self.name, count))

输出：

2017-08-20 12:32:55,204 - MainThread - DEBUG - initial count: 0
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,210 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,211 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,213 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,213 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,213 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,213 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,214 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,214 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,214 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,214 - Thread-1   - DEBUG - Thread-1 wait...9925
2017-08-20 12:32:55,216 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,216 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,216 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,216 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,216 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,217 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,219 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,219 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,219 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,219 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,219 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,220 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,221 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,221 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,221 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,221 - Thread-2   - DEBUG - Thread-2 wait...6036
2017-08-20 12:32:55,226 - Thread-1   - DEBUG - Thread-1 finished, count is 13964
2017-08-20 12:32:55,236 - Thread-2   - DEBUG - Thread-2 finished, count is 20000
2017-08-20 12:32:55,236 - MainThread - DEBUG - final count: 20000

4.threading.RLock() 的应用场景

Problems with Simple Locking

lock = threading.Lock()

def get_first_part():
with lock: # any thread that attempts to acquire the lock will block, even if the same thread is already holding the lock.
... fetch data for first part from shared object
return data

def get_second_part():
with lock:
... fetch data for second part from shared object
return data

def get_both_parts():
with lock:  # other thread may modify the resource between the two calls
first = get_first_part()
# between the two calls
second = get_second_part()
return first, second

# While simple locks will block if the same thread attempts to acquire the same lock twice,
# a re-entrant lock only blocks if another thread currently holds the lock.
rlock = threading.RLock()