python线程使用场景 多线程下载

http://blog.xiayf.cn/2015/09/11/parallelism-in-one-line
http://python.jobbole.com/84327/
http://www.runoob.com/python/python-multithreading.html
http://www.jianshu.com/p/544d406e0875
http://www.jianshu.com/p/86b8e78c418a
http://www.imooc.com/article/6969?block_id=tuijian_wz
http://blog.csdn.net/zhaonanemail/article/details/7175449

实现简单的多线程下载，需要关注如下几点：
1.文件的大小：可以从reponse header中提取，如“Content-Length:911”表示大小是911字节
2.任务拆分：指定各个线程下载的文件的哪一块，可以通过request header中添加“Range: bytes=300-400”（表示下载300~400byte的内容），注意可以请求的文件的range是[0, size-1]字节的。
3.下载文件的聚合：各个线程将自己下载的文件块保存为临时文件，所有线程都完成后，再将这些临时文件按顺序聚合写入到最终的一个文件中。

网上看到一个不错的多线程下载文件的例子，是根据Linux上多线程下载工具axel的思想来写的，源代码在：http://fayaa.com/code/view/58/full/

#!/usr/bin/python
# -*- coding: utf-8 -*-
# filename: paxel.py
# FROM: http://fayaa.com/code/view/58/full/
# Jay modified it a little and save for further potential usage.

\'\'\'It is a multi-thread downloading tool

    It was developed following axel.
        Author: volans
        E-mail: volansw [at] gmail.com
\'\'\'

import sys
import os
import time
import urllib
from threading import Thread

# in case you want to use http_proxy
local_proxies = {\'http\': \'http://131.139.58.200:8080\'}


class AxelPython(Thread, urllib.FancyURLopener):
    \'\'\'Multi-thread downloading class.

        run() is a vitural method of Thread.
    \'\'\'
    def __init__(self, threadname, url, filename, ranges=0, proxies={}):
        Thread.__init__(self, name=threadname)
        urllib.FancyURLopener.__init__(self, proxies)
        self.name = threadname
        self.url = url
        self.filename = filename
        self.ranges = ranges
        self.downloaded = 0

    def run(self):
        \'\'\'vertual function in Thread\'\'\'
        try:
            self.downloaded = os.path.getsize(self.filename)
        except OSError:
            #print \'never downloaded\'
            self.downloaded = 0

        # rebuild start poind
        self.startpoint = self.ranges[0] + self.downloaded

        # This part is completed
        if self.startpoint >= self.ranges[1]:
            print \'Part %s has been downloaded over.\' % self.filename
            return

        self.oneTimeSize = 16384  # 16kByte/time
        print \'task %s will download from %d to %d\' % (self.name, self.startpoint, self.ranges[1])

        self.addheader("Range", "bytes=%d-%d" % (self.startpoint, self.ranges[1]))
        self.urlhandle = self.open(self.url)

        data = self.urlhandle.read(self.oneTimeSize)
        while data:
            filehandle = open(self.filename, \'ab+\')
            filehandle.write(data)
            filehandle.close()

            self.downloaded += len(data)
            #print "%s" % (self.name)
            #progress = u\'\r...\'

            data = self.urlhandle.read(self.oneTimeSize)


def GetUrlFileSize(url, proxies={}):
    urlHandler = urllib.urlopen(url, proxies=proxies)
    headers = urlHandler.info().headers
    length = 0
    for header in headers:
        if header.find(\'Length\') != -1:
            length = header.split(\':\')[-1].strip()
            length = int(length)
    return length


def SpliteBlocks(totalsize, blocknumber):
    blocksize = totalsize / blocknumber
    ranges = []
    for i in range(0, blocknumber - 1):
        ranges.append((i * blocksize, i * blocksize + blocksize - 1))
    ranges.append((blocksize * (blocknumber - 1), totalsize - 1))

    return ranges


def islive(tasks):
    for task in tasks:
        if task.isAlive():
            return True
    return False


def paxel(url, output, blocks=6, proxies=local_proxies):
    \'\'\' paxel
    \'\'\'
    size = GetUrlFileSize(url, proxies)
    ranges = SpliteBlocks(size, blocks)

    threadname = ["thread_%d" % i for i in range(0, blocks)]
    filename = ["tmpfile_%d" % i for i in range(0, blocks)]

    tasks = []
    for i in range(0, blocks):
        task = AxelPython(threadname[i], url, filename[i], ranges[i])
        task.setDaemon(True)
        task.start()
        tasks.append(task)

    time.sleep(2)
    while islive(tasks):
        downloaded = sum([task.downloaded for task in tasks])
        process = downloaded / float(size) * 100
        show = u\'\rFilesize:%d Downloaded:%d Completed:%.2f%%\' % (size, downloaded, process)
        sys.stdout.write(show)
        sys.stdout.flush()
        time.sleep(0.5)

    filehandle = open(output, \'wb+\')
    for i in filename:
        f = open(i, \'rb\')
        filehandle.write(f.read())
        f.close()
        try:
            os.remove(i)
            pass
        except:
            pass

    filehandle.close()

if __name__ == \'__main__\':
    url = \'http://dldir1.qq.com/qqfile/QQforMac/QQ_V3.1.1.dmg\'
    output = \'download.file\'
    paxel(url, output, blocks=4, proxies={})