Python 多進程實踐

多進程的方式可以增加腳本的并發處理能力， python 支持這種多進程的編程方式

在類unix系統中， python的os模塊內置了fork 函數用以創建子進程

fork 方式創建子進程

import os

print "Process %s start ..." %(os.getpid())

pid = os.fork()

if pid == 0:
    print "This is child process and my pid is %d, my father process is %d" %(os.getpid(), os.getppid())
else:
    print "This is Fater process, And Its child pid is %d" %(pid)

執行結果

Process 4276 start ...
This is Fater process, And Its child pid is 4277
This is child process and my pid is 4277, my father process is 4276

從結果可以看到， 從pid = os.fork()開始， 下面的部分代碼運行了兩次， 第一次是父進程運行， 第二次是子進程運行， 且子進程的fork的結果總是0， 所以這個也可以用來作為區分父進程或是子進程標志

那么變量在多個進程之間是否相互影響呢import os

print "Process %s start ..." %(os.getpid())
pid = os.fork()
source = 10
if pid == 0:
    print "This is child process and my pid is %d, my father process is %d" %(os.getpid(), os.getppid())
    source = source - 6
    print "child process source value is "+str(source)
else:
    print "This is Fater process, And Its child pid is %d" %(pid)
    source = source - 1
    print "father process source value is "+str(source)
print "source value is "+str(source)

執行的結果如下：

Process 4662 start ...
This is Fater process, And Its child pid is 4663
This is child process and my pid is 4663, my father process is 4662
father process source value is 9
child process source value is 4
source value is 9
source value is 4

很明顯， 初始值為10的source 在父進程中值 減少了 1, 為9， 而子進程明顯source的初始值 是10, 也就是說多進程之間并沒有什么相互影響

multiprocessing 方式創建子進程

fork 方式是僅在linux 下才有的接口， 在windows下并沒有， 那么在windows下如何實現多進程呢， 這就用到了multiprocessing

multiprocessing 模塊的Process 對象表示的是一個進程對象， 可以創建子進程并執行制定的函數

from multiprocessing import Process
import os
def pro_do(name, func):
    print "This is child process %d from parent process %d, and name is  %s which is used for %s" %(os.getpid(), os.getppid(), name, func)
if __name__ == "__main__":
    print "Parent process id %d" %(os.getpid())
    #process 對象指定子進程將要執行的操作方法(pro_do), 以及該函數的對象列表args(必須是tuple格式， 且元素與pro_do的參數一一對應)
    pro = Process(target=pro_do, args=("test", "dev"))
    print "start child process"
    #啟動子進程
    pro.start()
    #是否阻塞方式執行， 如果有， 則阻塞方式， 否則非阻塞
    pro.join() #if has this, it's synchronous operation or asynchronous operation
    print "Process end"

執行結果

Parent process id 4878
start child process
This is child process 4879 from parent process 4878, and name is  test which is used for dev
Process end

如果沒有pro.join()， 則表示非阻塞方式運行， 那么最終的Process end的輸出位置就有可能出現在pro_do 方法執行之前了

Parent process id 4903
start child process
Process end
This is child process 4904 from parent process 4903, and name is  test which is used for dev

通過multiprocessing 的process對象創建多進程， 還可以從主進程中向子進程傳遞參數， 例如上面例子中的pro_do的參數

Pool 進程池

from multiprocessing import Pool
import os, time
def pro_do(process_num):
    print "child process id is %d" %(os.getpid())
    time.sleep(6 - process_num)
    print "this is process %d" %(process_num)
if __name__ == "__main__":
    print "Current process is %d" %(os.getpid())
    p = Pool()
    for i in range(5):
        p.apply_async(pro_do, (i,))  #增加新的進程
    p.close() # 禁止在增加新的進程
    p.join()
    print "pool process done"

輸出：

Current process is 19138
child process id is 19139
child process id is 19140
this is process 1
child process id is 19140
this is process 0
child process id is 19139
this is process 2
child process id is 19140
this is process 3
this is process 4
pool process done

其中

child process id is 19139
   child process id is 19140

是立即輸出的， 后面的依次在等待了sleep的時間后輸出 ， 之所以立即輸出了上面兩個是因為誒Pool 進程池默認是按照cpu的數量開啟子進程的， 我是在虛擬機中運行， 只分配了兩核， 所以先立即啟動兩個子進程， 剩下的進程要等到前面的進程執行完成后才能啟動。不過也可以在p=Poo() 中使用Pool(5)來指定啟動的子進程數量， 這樣輸出就是下面的了：

Current process is 19184
child process id is 19185
child process id is 19186
child process id is 19188
child process id is 19189
child process id is 19187
this is process 4
this is process 3
this is process 2
this is process 1
this is process 0
pool process done

且

Current process is 19184
child process id is 19185
child process id is 19186
child process id is 19188
child process id is 19189
child process id is 19187

都是立即輸出的

進程間的通信

父進程可以指定子進程執行的方法及其參數， 達到父進程向子進程傳遞消息的單向通信的目的， 那子進程之間或子進程怎么向父進程通信呢

Queue

Queue 是一種方式

from multiprocessing import Process, Queue
import os, time
def write_queue(q):
    for name in ["Yi_Zhi_Yu", "Tony" ,"San"]:
        print "put name %s to queue" %(name)
        q.put(name)
        time.sleep(2)
    print "write data finished"
def read_queue(q):
    print "begin to read data"
    while True:
        name = q.get()
        print "get name %s from queue" %(name)
if __name__ == "__main__":
    q = Queue()
    pw = Process(target=write_queue, args=(q,))
    pr = Process(target=read_queue,args=(q,))
    pw.start()
    pr.start()
    pw.join() #這個表示是否阻塞方式啟動進程， 如果要立即讀取的話， 兩個進程的啟動就應該是非阻塞式的， 所以pw在start后不能立即使用pw.join(), 要等pr start后方可
    pr.terminate() #服務進程，強制停止

結果

put name Yi_Zhi_Yu to queue
begin to read data
get name Yi_Zhi_Yu from queue
put name Tony to queue
get name Tony from queue
put name San to queue
get name San from queue
write data finished

Pipe

#!/usr/bin/env python
#encoding=utf-8
from multiprocessing import Process,Pipe
import os,time,sys
def send_pipe(p):
     names = ["Yi_Zhi_Yu", "Tony", "San"]
     for name in names:
          print "put name %s to Pipe" %(name)
          p.send(name)
          time.sleep(1)
def recv_pipe(p):
     print "Try to read data in pipe"
     while True:
                name = p.recv()
                print "get name %s from pipe" %(name)
if __name__ == "__main__":
    #pipe, one for send, one for read
    ps_pipe, pr_pipe = Pipe()
    #process
    ps = Process(target=send_pipe, args=(ps_pipe,))
    pr = Process(target=recv_pipe, args=(pr_pipe,))
    pr.start()
    ps.start()
    ps.join()
    pr.terminate()

在實例化Pipe的時候， 會產生兩個ps_pipe(read-write Connection, handle 5)， pr_pipe(read-write Connection, handle 5) , 都可以作為發送或者接受方， 一旦一個確認為攻， 另一個自然就是受了(之所以Pipe只能作為兩個進程之間的通信方式， 原因也許就是這個)，產生的結果如下

Try to read data in pipe
put name Yi_Zhi_Yu to Pipe
get name Yi_Zhi_Yu from pipe
put name Tony to Pipe
get name Tony from pipe
put name San to Pipe
get name San from pipe

還有一種Array, Value 的形式， 暫且不表， 有時間在折騰

以上均為python 學習筆記和練習， 如有錯誤， 歡迎指出