如何并行文件下载？

Question

我可以一次下载一个文件，使用：

import urllib.request

urls = ['foo.com/bar.gz', 'foobar.com/barfoo.gz', 'bar.com/foo.gz']

for u in urls:
  urllib.request.urlretrieve(u)

我可以尝试这样做：

import subprocess
import os

def parallelized_commandline(command, files, max_processes=2):
    processes = set()
    for name in files:
        processes.add(subprocess.Popen([command, name]))
        if len(processes) >= max_processes:
            os.wait()
            processes.difference_update(
                [p for p in processes if p.poll() is not None])

    #Check if all the child processes were closed
    for p in processes:
        if p.poll() is None:
            p.wait()

urls = ['http://www.statmt.org/wmt15/training-monolingual-nc-v10/news-commentary-v10.en.gz',
'http://www.statmt.org/wmt15/training-monolingual-nc-v10/news-commentary-v10.cs.gz', 
'http://www.statmt.org/wmt15/training-monolingual-nc-v10/news-commentary-v10.de.gz']

parallelized_commandline('wget', urls)

有没有任何方法可以并行化urlretrieve，而不使用os.system或subprocess进行欺骗？

既然我现在必须求助于“作弊”，那么subprocess.Popen是下载数据的正确方式吗？

当使用上面的parallelized_commandline()时，它对wget使用多线程而不是多核，这正常吗？有没有办法让它多核而不是多线程？

Answer 1

可以使用线程池并行下载文件：

#!/usr/bin/env python3
from multiprocessing.dummy import Pool # use threads for I/O bound tasks
from urllib.request import urlretrieve

urls = [...]
result = Pool(4).map(urlretrieve, urls) # download 4 files at a time

您还可以使用asyncio在单个线程中同时下载多个文件：

#!/usr/bin/env python3
import asyncio
import logging
from contextlib import closing
import aiohttp # $ pip install aiohttp

@asyncio.coroutine
def download(url, session, semaphore, chunk_size=1<<15):
    with (yield from semaphore): # limit number of concurrent downloads
        filename = url2filename(url)
        logging.info('downloading %s', filename)
        response = yield from session.get(url)
        with closing(response), open(filename, 'wb') as file:
            while True: # save file
                chunk = yield from response.content.read(chunk_size)
                if not chunk:
                    break
                file.write(chunk)
        logging.info('done %s', filename)
    return filename, (response.status, tuple(response.headers.items()))

urls = [...]
logging.basicConfig(level=logging.INFO, format='%(asctime)s %(message)s')
with closing(asyncio.get_event_loop()) as loop, \
     closing(aiohttp.ClientSession()) as session:
    semaphore = asyncio.Semaphore(4)
    download_tasks = (download(url, session, semaphore) for url in urls)
    result = loop.run_until_complete(asyncio.gather(*download_tasks))

其中^{} is defined here。