fork

Python的os模塊封裝了常見的系統(tǒng)調(diào)用横堡，其中就包括fork，可以在Python程序中輕松創(chuàng)建子進程：

import os

print('Process (%s) start...' % os.getpid())
# Only works on Unix/Linux/Mac:
pid = os.fork()
if pid == 0:
    print('I am child process (%s) and my parent is %s.' % (os.getpid(), os.getppid()))
else:
    print('I (%s) just created a child process (%s).' % (os.getpid(), pid))
執(zhí)行結(jié)果
Process (876) start...
I (876) just created a child process (877).
I am child process (877) and my parent is 876.

multiprocessing

由于Python是跨平臺的迁客，自然也應(yīng)該提供一個跨平臺的多進程支持遭笋。multiprocessing模塊就是跨平臺版本的多進程模塊聋庵。

from multiprocessing import Process
import os

# 子進程要執(zhí)行的代碼
def run_proc(name):
    print('Run child process %s (%s)...' % (name, os.getpid()))

if __name__=='__main__':
    print('Parent process %s.' % os.getpid())
    p = Process(target=run_proc, args=('test',))
    print('Child process will start.')
    p.start()
    p.join()
    print('Child process end.')

執(zhí)行結(jié)果
Parent process 928.
Process will start.
Run child process test (929)...
Process end.
創(chuàng)建子進程時饱苟，只需要傳入一個執(zhí)行函數(shù)和函數(shù)的參數(shù)，創(chuàng)建一個Process實例编矾，用start()方法啟動
join()方法可以等待子進程結(jié)束后再繼續(xù)往下運行熟史，通常用于進程間的同步。

Pool

如果要啟動大量的子進程窄俏，可以用進程池的方式批量創(chuàng)建子進程：

from multiprocessing import Pool
import os, time, random

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.')

執(zhí)行結(jié)果：
Parent process 669.
Waiting for all subprocesses done...
Run task 0 (671)...
Run task 1 (672)...
Run task 2 (673)...
Run task 3 (674)...
Task 2 runs 0.14 seconds.
Run task 4 (673)...
Task 1 runs 0.27 seconds.
Task 3 runs 0.86 seconds.
Task 0 runs 1.41 seconds.
Task 4 runs 1.91 seconds.
All subprocesses done.

對Pool對象調(diào)用join()方法會等待所有子進程執(zhí)行完畢蹂匹，調(diào)用join()之前必須先調(diào)用close()，調(diào)用close()之后就不能繼續(xù)添加新的Process了凹蜈。

請注意輸出的結(jié)果限寞，task 0，1仰坦，2履植，3是立刻執(zhí)行的，而task 4要等待前面某個task完成后才執(zhí)行悄晃，pool的默認(rèn)大小與cpu核心數(shù)量相同玫霎，這里為4,

子進程

很多時候，子進程并不是自身，而是一個外部進程鼠渺。我們創(chuàng)建了子進程后鸭巴，還需要控制子進程的輸入和輸出。subprocess模塊可以讓我們非常方便地啟動一個子進程拦盹，然后控制其輸入和輸出鹃祖。

import subprocess

print('$ nslookup www.python.org')
r = subprocess.call(['nslookup', 'www.python.org'])
print('Exit code:', r)
運行結(jié)果：

$ nslookup www.python.org
Server:        192.168.19.4
Address:    192.168.19.4#53

Non-authoritative answer:
www.python.org    canonical name = python.map.fastly.net.
Name:    python.map.fastly.net
Address: 199.27.79.223

Exit code: 0

如果子進程還需要輸入，則可以通過communicate()方法輸入：

import subprocess

print('$ nslookup')
p = subprocess.Popen(['nslookup'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output, err = p.communicate(b'set q=mx\npython.org\nexit\n')
print(output.decode('utf-8'))
print('Exit code:', p.returncode)

上面的代碼相當(dāng)于在命令行執(zhí)行命令nslookup普舆，然后手動輸入：

set q=mx
python.org
exit

運行結(jié)果如下：

$ nslookup
Server:        192.168.19.4
Address:    192.168.19.4#53

Non-authoritative answer:
python.org    mail exchanger = 50 mail.python.org.

Authoritative answers can be found from:
mail.python.org    internet address = 82.94.164.166
mail.python.org    has AAAA address 2001:888:2000:d::a6

Exit code: 0

進程間通信

Python的multiprocessing模塊包裝了底層的機制恬口，提供了Queue、Pipes等多種方式來交換數(shù)據(jù)沼侣。我們以Queue為例祖能，在父進程中創(chuàng)建兩個子進程，一個往Queue里寫數(shù)據(jù)蛾洛，一個從Queue里讀數(shù)據(jù)

from multiprocessing import Process, Queue
import os, time, random

# 寫數(shù)據(jù)進程執(zhí)行的代碼:
def write(q):
    print('Process to write: %s' % os.getpid())
    for value in ['A', 'B', 'C']:
        print('Put %s to queue...' % value)
        q.put(value)
        time.sleep(random.random())

# 讀數(shù)據(jù)進程執(zhí)行的代碼:
def read(q):
    print('Process to read: %s' % os.getpid())
    while True:
        value = q.get(True)
        print('Get %s from queue.' % value)

if __name__=='__main__':
    # 父進程創(chuàng)建Queue养铸，并傳給各個子進程：
    q = Queue()
    pw = Process(target=write, args=(q,))
    pr = Process(target=read, args=(q,))
    # 啟動子進程pw，寫入:
    pw.start()
    # 啟動子進程pr轧膘，讀取:
    pr.start()
    # 等待pw結(jié)束:
    pw.join()
    # pr進程里是死循環(huán)钞螟，無法等待其結(jié)束，只能強行終止:
    pr.terminate()

運行結(jié)果如下：
Process to write: 50563
Put A to queue...
Process to read: 50564
Get A from queue.
Put B to queue...
Get B from queue.
Put C to queue...
Get C from queue