day10_单线程和多线程下载文件

单线程下载文件：

import requests
import time
from hashlib import md5


def down_load_pic(url):
    req = requests.get(url)
    m = md5(url.encode())
    with open(m.hexdigest() + '.png', 'wb') as fw:  # md5后的名称作为图片的前缀
        fw.write(req.content)


url_list = [
                 'http://www.nnzhp.cn/wp-content/uploads/2019/10/f410afea8b23fa401505a1449a41a133.png',
                 'http://www.nnzhp.cn/wp-content/uploads/2019/11/481b5135e75c764b32b224c5650a8df5.png',
                 'http://www.nnzhp.cn/wp-content/uploads/2019/11/b23755cdea210cfec903333c5cce6895.png',
                 'http://www.nnzhp.cn/wp-content/uploads/2019/11/542824dde1dbd29ec61ad5ea867ef245.png',
               ]

start_time = time.time()
for url in url_list:
    down_load_pic(url)
end_time = time.time()
print(end_time - start_time)

多线程下载文件：

import requests
import time
import threading
from hashlib import md5


def down_load_pic(url):
    req = requests.get(url)
    m = md5(url.encode())
    with open(m.hexdigest() + '.png', 'wb') as fw:
        fw.write(req.content)


url_list = [
                 'http://www.nnzhp.cn/wp-content/uploads/2019/10/f410afea8b23fa401505a1449a41a133.png',
                 'http://www.nnzhp.cn/wp-content/uploads/2019/11/481b5135e75c764b32b224c5650a8df5.png',
                 'http://www.nnzhp.cn/wp-content/uploads/2019/11/b23755cdea210cfec903333c5cce6895.png',
                 'http://www.nnzhp.cn/wp-content/uploads/2019/11/542824dde1dbd29ec61ad5ea867ef245.png',
               ]


start_time = time.time()
for url in url_list:
    t = threading.Thread(target=down_load_pic, args=(url,))
    t.start()
while threading.activeCount() != 1:
    pass
end_time = time.time()
print(end_time - start_time)

GIL(Global Interpreter Lock)，全局解释器锁，导致python多线程不能利用多核cpu，只能用一个cpu，python多线程比较鸡肋，看着像并行工作，实际是cpu上下文切换，cpu负责调度，调度完了剩下的就是把图片存到磁盘跟cpu没有关系了，cpu再去调度其他的线程

如果url比较多，list里有1000个url，不能同时启动1000个线程，要用到线程池，pip install threadpool，再导入threadpool

import threadpool
import requests
from hashlib import md5


def down_load_pic(url):
    req = requests.get(url)
    m = md5(url.encode())
    with open(m.hexdigest() + '.png', 'wb') as fw:
        fw.write(req.content)


url_list = [
             'http://www.nnzhp.cn/wp-content/uploads/2019/10/f410afea8b23fa401505a1449a41a133.png',
             'http://www.nnzhp.cn/wp-content/uploads/2019/11/481b5135e75c764b32b224c5650a8df5.png',
             'http://www.nnzhp.cn/wp-content/uploads/2019/11/b23755cdea210cfec903333c5cce6895.png',
             'http://www.nnzhp.cn/wp-content/uploads/2019/11/542824dde1dbd29ec61ad5ea867ef245.png',
           ]

pool = threadpool.ThreadPool(20)  # 实例化一个线程池，开辟20个线程，根据自己电脑的性能设置
reqs = threadpool.makeRequests(down_load_pic, url_list)  # 分配数据，每个线程分配多少个url
[pool.putRequest(req) for req in reqs]  # 加到线程池里
pool.wait()  # 等待子线程执行完
print('end')