一 進程池與線程池

在剛開始學多進程或多線程時，我們迫不及待地基於多進程或多線程實現並發的套接字通信，然而這種實現方式的致命缺陷是：服務的開啟的進程數或線程數都會隨著並發的客戶端數目地增多而增多，這會對服務端主機帶來巨大的壓力，甚至於不堪重負而癱瘓，於是我們必須對服務端開啟的進程數或線程數加以控制，讓機器在一個自己可以承受的範圍內運行，這就是進程池或線程池的用途，例如進程池，就是用來存放進程的池子，本質還是基於多進程，只不過是對開啟進程的數目加上了限制

介紹

官網：https://docs.python.org/dev/library/concurrent.futures.html

concurrent.futures模塊提供了高度封裝的異步調用接口
ThreadPoolExecutor：線程池，提供異步調用
ProcessPoolExecutor: 進程池，提供異步調用
Both implement the same interface, which is defined by the abstract Executor class.
 

基本方法

1、submit(fn, *args, **kwargs)
異步提交任務

2、map(func, *iterables, timeout=None, chunksize=1) 
取代for循環submit的操作

3、shutdown(wait=True) 
相當於進程池的pool.close()+pool.join()操作
wait=True，等待池內所有任務執行完畢回收完資源後才繼續
wait=False，立即返回，並不會等待池內的任務執行完畢
但不管wait參數為何值，整個程序都會等到所有任務執行完畢
submit和map必須在shutdown之前

4、result(timeout=None)
取得結果

5、add_done_callback(fn)
回調函數
 

二 進程池

介紹

The ProcessPoolExecutor class is an Executor subclass that uses a pool of processes to execute calls asynchronously. ProcessPoolExecutor uses the multiprocessing module, which allows it to side-step the Global Interpreter Lock but also means that only picklable objects can be executed and returned.

class concurrent.futures.ProcessPoolExecutor(max_workers=None, mp_context=None)
An Executor subclass that executes calls asynchronously using a pool of at most max_workers processes. If max_workers is None or not given, it will default to the number of processors on the machine. If max_workers is lower or equal to 0, then a ValueError will be raised.
 

用法

from concurrent.futures import ThreadPoolExecutor,ProcessPoolExecutor

import os,time,random
def task(n):
    print(‘%s is runing‘ %os.getpid())
    time.sleep(random.randint(1,3))
    return n**2

if __name__ == ‘__main__‘:

    executor=ProcessPoolExecutor(max_workers=3)

    futures=[]
    for i in range(11):
        future=executor.submit(task,i)
        futures.append(future)
    executor.shutdown(True)
    print(‘+++>‘)
    for future in futures:
        print(future.result())

三 線程池

介紹

ThreadPoolExecutor is an Executor subclass that uses a pool of threads to execute calls asynchronously.
class concurrent.futures.ThreadPoolExecutor(max_workers=None, thread_name_prefix=‘‘)
An Executor subclass that uses a pool of at most max_workers threads to execute calls asynchronously.

Changed in version 3.5: If max_workers is None or not given, it will default to the number of processors on the machine, multiplied by 5, assuming that ThreadPoolExecutor is often used to overlap I/O instead of CPU work and the number of workers should be higher than the number of workers for ProcessPoolExecutor.

New in version 3.6: The thread_name_prefix argument was added to allow users to control the threading.Thread names for worker threads created by the pool for easier debugging.

用法

把ProcessPoolExecutor換成ThreadPoolExecutor，其余用法全部相同

四 map方法

from concurrent.futures import ThreadPoolExecutor,ProcessPoolExecutor

import os,time,random
def task(n):
    print(‘%s is runing‘ %os.getpid())
    time.sleep(random.randint(1,3))
    return n**2

if __name__ == ‘__main__‘:

    executor=ThreadPoolExecutor(max_workers=3)

    # for i in range(11):
    #     future=executor.submit(task,i)

    executor.map(task,range(1,12)) #map取代了for+submit

五 回調函數

可以為進程池或線程池內的每個進程或線程綁定一個函數，該函數在進程或線程的任務執行完畢後自動觸發，並接收任務的返回值當作參數，該函數稱為回調函數

from concurrent.futures import ThreadPoolExecutor,ProcessPoolExecutor
from multiprocessing import Pool
import requests
import json
import os

def get_page(url):
    print(‘<進程%s> get %s‘ %(os.getpid(),url))
    respone=requests.get(url)
    if respone.status_code == 200:
        return {‘url‘:url,‘text‘:respone.text}

def parse_page(res):
    res=res.result()
    print(‘<進程%s> parse %s‘ %(os.getpid(),res[‘url‘]))
    parse_res=‘url:<%s> size:[%s]\n‘ %(res[‘url‘],len(res[‘text‘]))
    with open(‘db.txt‘,‘a‘) as f:
        f.write(parse_res)


if __name__ == ‘__main__‘:
    urls=[
        ‘https://www.baidu.com‘,
        ‘https://www.python.org‘,
        ‘https://www.openstack.org‘,
        ‘https://help.github.com/‘,
        ‘http://www.sina.com.cn/‘
    ]

    p=ProcessPoolExecutor(3)
    for url in urls:
        p.submit(get_page,url).add_done_callback(parse_page) #parse_page拿到的是一個future對象obj，需要用obj.result()拿到結果

進程池與線程池