Python限制多线程

Question

As you surely know, I can do multithreading to download files from the Internet faster. But if I send lots of requests to the same website, I could be black listed.

So could you help me to implement something like "I've got a list of urls. I want you to download all of these files but if 10 downloads are already running, wait for a slot."

I'll appreciate any help. Tk.

binoua

This is the code I'm using (doesn't work).

class PDBDownloader(threading.Thread):

    prefix = 'http://www.rcsb.org/pdb/files/'

    def __init__(self, queue):
        threading.Thread.__init__(self)
        self.queue = queue
        self.pdbid = None
        self.urlstr = ''
        self.content = ''

    def run(self):
        while True:
            self.pdbid = self.queue.get()
            self.urlstr = self.prefix + pdbid + '.pdb'
            print 'downloading', pdbid
            self.download()

            filename = '%s.pdb' %(pdbid)
            f = open(filename, 'wt')
            f.write(self.content)
            f.close()

            self.queue.task_done()

    def download(self):
        try:
            f = urllib2.urlopen(self.urlstr)
        except urllib2.HTTPError, e:
            msg = 'HTTPError while downloading file %s at %s. '\
                    'Details: %s.' %(self.pdbid, self.urlstr, str(e))
            raise OstDownloadException, msg
        except urllib2.URLError, e:
            msg = 'URLError while downloading file %s at %s. '\
                    'RCSB erveur unavailable.' %(self.pdbid, self.urlstr)
            raise OstDownloadException, msg
        except Exception, e:
            raise OstDownloadException, str(e)
        else:
            self.content = f.read()
if __name__ == '__main__':

    pdblist = ['1BTA', '3EAM', '1EGJ', '2BV9', '2X6A']

    for i in xrange(len(pdblist)):
        pdb = PDBDownloader(queue)
        pdb.setDaemon(True)
        pdb.start()

    while pdblist:
        pdbid = pdblist.pop()
        queue.put(pdbid)

    queue.join()

Answer 1

Using threads doesn't " download files from the Internet faster ". You have only one network card and one internet connection so that's just not true.

The threads are being used to wait , and you can't wait faster .

You can use a single thread and be as fast, or even faster -- Just don't wait for the response of one file before starting another. In other words, use asynchronous, non-blocking network programming.

Here's a complete script that uses twisted.internet.task.coiterate to start multiple downloads at the same time, without using any kind of threading, and respecting the pool size (I'm using 2 simultaneous downloads for the demonstration, but you can change the size):

from twisted.internet import defer, task, reactor
from twisted.web import client
from twisted.python import log

@defer.inlineCallbacks
def deferMap(job, dataSource, size=1):
    successes = []
    failures = []

    def _cbGather(result, dataUnit, succeeded):
        """This will be called when any download finishes"""
        if succeeded:
            # you could save the file to disk here
            successes.append((dataUnit, result))
        else:
            failures.append((dataUnit, result))

    @apply
    def work():
        for dataUnit in dataSource:
            d = job(dataUnit).addCallbacks(_cbGather, _cbGather,
                callbackArgs=(dataUnit, True),  errbackArgs=(dataUnit, False))
            yield d

    yield defer.DeferredList([task.coiterate(work) for i in xrange(size)])
    defer.returnValue((successes, failures))

def printResults(result):
    successes, failures = result
    print "*** Got %d pages total:" % (len(successes),)
    for url, page in successes:
        print '  * %s -> %d bytes' % (url, len(page))
    if failures:
        print "*** %d pages failed download:" % (len(failures),)
        for url, failure in failures:
            print '  * %s -> %s' % (url, failure.getErrorMessage())

if __name__ == '__main__':
    import sys
    log.startLogging(sys.stdout)
    urls = ['http://twistedmatrix.com',
            'XXX',
            'http://debian.org',
            'http://python.org',
            'http://python.org/foo',
            'https://launchpad.net',
            'noway.com',
            'somedata',
        ]
    pool = deferMap(client.getPage, urls, size=2) # download 2 at once
    pool.addCallback(printResults)
    pool.addErrback(log.err).addCallback(lambda ign: reactor.stop())
    reactor.run()

Note that I included some bad urls on purpose so we can see some failures in the result:

...
2010-06-29 08:18:04-0300 [-] *** Got 4 pages total:
2010-06-29 08:18:04-0300 [-]   * http://twistedmatrix.com -> 16992 bytes
2010-06-29 08:18:04-0300 [-]   * http://python.org -> 17207 bytes
2010-06-29 08:18:04-0300 [-]   * http://debian.org -> 13820 bytes
2010-06-29 08:18:04-0300 [-]   * https://launchpad.net -> 18511 bytes
2010-06-29 08:18:04-0300 [-] *** 4 pages failed download:
2010-06-29 08:18:04-0300 [-]   * XXX -> Connection was refused by other side: 111: Connection refused.
2010-06-29 08:18:04-0300 [-]   * http://python.org/foo -> 404 Not Found
2010-06-29 08:18:04-0300 [-]   * noway.com -> Connection was refused by other side: 111: Connection refused.
2010-06-29 08:18:04-0300 [-]   * somedata -> Connection was refused by other side: 111: Connection refused.
...

Answer 2

Use a thread pool with a shared list of urls. Each thread tries to pop a url from the list and download it until none are left. pop() from a list is threadsafe

while True:
    try:
        url = url_list.pop()
        # download URL here
    except IndexError:
        break