在python的多线程和多进程中，当我们需要对多线程或多进程的共享资源或对象进行修改操作时，往往会出现因cpu随机调度而导致结果和我们预期不一致的问题，
线程举例：
from threading import Thread,Lock
x = 0
def task():
global x
for i in range(200000):
x = x+1
'''
假设
t1 的 x刚拿到0 保存状态 就被切了
t2 的 x拿到0 进行+1 1
t1 又获得运行了 x = 0 +1 1
思考:一共加了几次1? 加了两次1 真实运算出来的数字本来应该+2 实际只+1
这就产生了数据安全问题.
'''
if __name__ == '__main__':
t1 = Thread(target=task)
t2 = Thread(target=task)
t3 = Thread(target=task)
t1.start()
t2.start()
t3.start()
t1.join()
t2.join()
t3.join()
print(x)479261
from multiprocessing import Process,Lock
import json,time,os
def search():
time.sleep(1) # 模拟网络io
with open('db.txt',mode='rt',encoding='utf-8') as f:
res = json.load(f)
print(f'还剩{res["count"]}')
def get():
with open('db.txt',mode='rt',encoding='utf-8') as f:
res = json.load(f)
time.sleep(1) # 模拟网络io
if res['count'] > 0:
res['count'] -= 1
with open('db.txt',mode='wt',encoding='utf-8') as f:
json.dump(res,f)
print(f'进程{os.getpid()} 抢票成功')
time.sleep(1.5) # 模拟网络io
else:
print('票已经售空啦!!!!!!!!!!!')
def task():
search()
get()
if __name__ == '__main__':
for i in range(5):
p = Process(target=task)
p.start()
还剩1
还剩1
还剩1
还剩1
还剩1
进程6292 抢票成功
进程10604 抢票成功
进程19280 抢票成功
进程272 抢票成功
进程12272 抢票成功
这时就需要对线程或者进程加锁，以保证一个线程或进程在对共享对象进行修改时，其他的线程或进程无法访问这个对象，直至获取锁的线程的操作执行完毕后释放锁。所以，锁在多线程和多进程中起到一个同步的作用，以保护每个线程和进程必要操作的完整执行。
#线程锁
from threading import Thread,Lock
x = 0
mutex = Lock()
def task():
global x
mutex.acquire()
for i in range(200000):
x = x+1
mutex.release()
if __name__ == '__main__':
t1 = Thread(target=task)
t2 = Thread(target=task)
t3 = Thread(target=task)
t1.start()
t2.start()
t3.start()
t1.join()
t2.join()
t3.join()
print(x)600000
from multiprocessing import Process,Lock
import json,time,os
def search():
time.sleep(1) # 模拟网络io
with open('db.txt',mode='rt',encoding='utf-8') as f:
res = json.load(f)
print(f'还剩{res["count"]}')
def get():
with open('db.txt',mode='rt',encoding='utf-8') as f:
res = json.load(f)
# print(f'还剩{res["count"]}')
time.sleep(1) # 模拟网络io
if res['count'] > 0:
res['count'] -= 1
with open('db.txt',mode='wt',encoding='utf-8') as f:
json.dump(res,f)
print(f'进程{os.getpid()} 抢票成功')
time.sleep(1.5) # 模拟网络io
else:
print('票已经售空啦!!!!!!!!!!!')
def task(lock):
search()
# 锁住
lock.acquire()
get()
lock.release()
# 释放锁头
if __name__ == '__main__':
lock = Lock() # 写在主进程是为了让子进程拿到同一把锁.
for i in range(15):
p = Process(target=task,args=(lock,))
p.start()
# p.join()
# 进程锁 是把锁住的代码变成了串行
# join 是把所有的子进程变成了串行
# 为了保证数据的安全,串行牺牲掉效率.还剩1
还剩1
还剩1
还剩1
还剩1
进程16868 抢票成功
票已经售空啦!!!!!!!!!!!
票已经售空啦!!!!!!!!!!!
票已经售空啦!!!!!!!!!!!
票已经售空啦!!!!!!!!!!!