Python(七)进程与线程

1.子进程
创建进程池同时启动多个子进程

from multiprocessing import Pool
import os,time,random
#子进程要执行的代码
def long_time_task(name):
    print('Run task %s (%s)...' % (name,os.getpid()))
    start=time.time()
    time.sleep(random.random()*3)
    end=time.time()
    print('Task %s runs %0.2f seconds.' % (name,(end-start)))
if __name__=='__main__':
    print('Parent process %s.' % os.getpid())
    p=Pool(4)
    for i in range(5):
        p.apply_async(long_time_task,args=(i,))
    print('Waiting for all subprocesses done...')
    p.close()
    p.join()
    print('All subprocesses done.')

Parent process 6868.
Waiting for all subprocesses done...
All subprocesses done.

2.创建线程

import time,threading
#新线程执行的代码:
def loop():
    print('thread %s is running...' % threading.current_thread().name)
    n=0
    while n<5:
        n=n+1
        print('thread %s >>> %s' % (threading.current_thread().name,n))
        time.sleep(1)
    print('thread %s ended.' % threading.current_thread().name)
print('thread %s is running...' % threading.current_thread().name)
print('thread %s is running...' % threading.current_thread().name)
t=threading.Thread(target=loop,name='LoopThread')
t.start()
t.join()
print('thread %s ended.' % threading.current_thread().name)

3.上锁
可以看做系统是随机的挑选线程执行，为了让某一段代码被一个线程从头到尾完整的执行，必须上锁

balance = 0
lock = threading.Lock()

def run_thread(n):
    for i in range(100000):
        # 先要获取锁:
        lock.acquire()
        try:
            # 放心地改吧:
            change_it(n)
        finally:
            # 改完了一定要释放锁:
            lock.release()

这样，change_it函数就会被从一个线程从头到尾完整执行