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。

实现代码：
复制代码 代码如下:
#!/usr/bin/python
# -*- coding: utf-8 -*-
# filename: paxel.py
# FROM: http://jb51.net/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={})