Python多进程编程详解
序. multiprocessing
Python中的多线程其实并不是真正的多线程，如果想要充分地使用多核CPU的资源，在Python中大部分情况需要使用多进程。Python提供了非常好用的多进程包multiprocessing，只需要定义一个函数，Python会完成其他所有事情。借助这个包，可以轻松完成从单进程到并发执行的转换。multiprocessing支持子进程、通信和共享数据、执行不同形式的同步，提供了Process、Queue、Pipe、Lock等组件。
1. Process
创建进程的类：Process([group [, target [, name [, args [, kwargs]]]]])，target表示调用对象，args表示调用对象的位置参数元组。kwargs表示调用对象的字典。name为别名。group实质上不使用。
方法：is_alive()、join([timeout])、run()、start()、terminate()。其中，Process以start()启动某个进程。
属性：authkey、daemon（要通过start()设置）、exitcode(进程在运行时为None、如果为–N，表示被信号N结束）、name、pid。其中daemon是父进程终止后自动终止，且自己不能产生新进程，必须在start()之前设置。
例1.1：创建函数并将其作为单个进程
import multiprocessing
import time
def worker(interval):
n = 5
while n > 0:
print("The time is {0}".format(time.ctime()))
time.sleep(interval)
n -= 1
if __name__ == "__main__":
p = multiprocessing.Process(target = worker, args = (3,))
p.start()
print ("p.pid:", p.pid)
print ("p.name:", p.name)
print ("p.is_alive:", p.is_alive())运行结果：
p.pid: 2322
p.name: Process-1
p.is_alive: True
The time is Sun May 5 07:36:18 2019
The time is Sun May 5 07:36:21 2019
The time is Sun May 5 07:36:24 2019
The time is Sun May 5 07:36:27 2019
The time is Sun May 5 07:36:30 2019
[Finished in 15.2s]
例1.2：创建函数并将其作为多个进程
import multiprocessing
import time
def worker_1(interval):
print ("worker_1")
time.sleep(interval)
print ("end worker_1")
def worker_2(interval):
print ("worker_2")
time.sleep(interval)
print ("end worker_2")
def worker_3(interval):
print ("worker_3")
time.sleep(interval)
print ("end worker_3")
if __name__ == "__main__":
p1 = multiprocessing.Process(target = worker_1, args = (2,))
p2 = multiprocessing.Process(target = worker_2, args = (3,))
p3 = multiprocessing.Process(target = worker_3, args = (4,))
p1.start()
p2.start()
p3.start()
print("The number of CPU is:" + str(multiprocessing.cpu_count()))
for p in multiprocessing.active_children():
print("child p.name:" + p.name + "\tp.id" + str(p.pid))
print ("END!!!!!!!!!!!!!!!!!")运行结果如下：
worker_2
worker_1
worker_3
The number of CPU is:1
child p.name:Process-3 p.id2783
child p.name:Process-1 p.id2781
child p.name:Process-2 p.id2782
END!!!!!!!!!!!!!!!!!
end worker_1
end worker_2
end worker_3
[Finished in 4.1s]
例1.3：将进程定义为类
import multiprocessing
import time
class ClockProcess(multiprocessing.Process):
def __init__(self, interval):
multiprocessing.Process.__init__(self)
self.interval = interval
def run(self):
n = 5
while n > 0:
print("the time is {0}".format(time.ctime()))
time.sleep(self.interval)
n -= 1
if __name__ == '__main__':
p = ClockProcess(3)
p.start()注：进程p调用start()时，自动调用run()
结果如下：
the time is Sun May 5 07:45:05 2019
the time is Sun May 5 07:45:08 2019
the time is Sun May 5 07:45:11 2019
the time is Sun May 5 07:45:14 2019
the time is Sun May 5 07:45:17 2019
[Finished in 15.1s]
例1.4：daemon程序对比结果
#1.4-1 不加daemon属性
import multiprocessing
import time
def worker(interval):
print("work start:{0}".format(time.ctime()));
time.sleep(interval)
print("work end:{0}".format(time.ctime()));
if __name__ == "__main__":
p = multiprocessing.Process(target = worker, args = (3,))
p.start()
print ("end!")结果如下：
end!
work start:Sun May 5 07:46:54 2019
work end:Sun May 5 07:46:57 2019
[Finished in 3.1s]
#1.4-2 加上daemon属性
import multiprocessing
import time
def worker(interval):
print("work start:{0}".format(time.ctime()));
time.sleep(interval)
print("work end:{0}".format(time.ctime()));
if __name__ == "__main__":
p = multiprocessing.Process(target = worker, args = (3,))
p.daemon = True
p.start()
print ("end!")结果如下：
end!
[Finished in 0.1s]
注：因子进程设置了daemon属性，主进程结束，它们就随着结束了。
#1.4-3 设置daemon执行完结束的方法
import multiprocessing
import time
def worker(interval):
print("work start:{0}".format(time.ctime()));
time.sleep(interval)
print("work end:{0}".format(time.ctime()));
if __name__ == "__main__":
p = multiprocessing.Process(target = worker, args = (3,))
p.daemon = True
p.start()
p.join()
print ("end!")结果如下：
work start:Sun May 5 07:49:59 2019
work end:Sun May 5 07:50:02 2019
end!
[Finished in 3.1s]
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