使用 urllib2 进行节流

Question

使用urllib2 时是否可以轻松限制 kbps？ 如果是这样，您可以指导我访问的任何代码示例或资源将不胜感激。

Answer 1

urllib 模块中有urlretrieve(url, filename=None, reporthook=None, data=None) 函数。 如果您将reporthook-function/object 实现为token bucket 或漏桶，则您有全局速率限制。

编辑：经过仔细检查，我发现使用reporthook 进行全局速率限制并不像我想象的那么容易。 reporthook 仅给出下载量和总大小，仅凭这些信息不足以与令牌桶一起使用。解决此问题的一种方法是将最后下载的数量存储在每个速率限制器中，但使用全局令牌桶。

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编辑 2： 将两个代码合并为一个示例。

"""Rate limiters with shared token bucket."""

import os
import sys
import threading
import time
import urllib
import urlparse

class TokenBucket(object):
    """An implementation of the token bucket algorithm.
    source: http://code.activestate.com/recipes/511490/

    >>> bucket = TokenBucket(80, 0.5)
    >>> print bucket.consume(10)
    True
    >>> print bucket.consume(90)
    False
    """
    def __init__(self, tokens, fill_rate):
        """tokens is the total tokens in the bucket. fill_rate is the
        rate in tokens/second that the bucket will be refilled."""
        self.capacity = float(tokens)
        self._tokens = float(tokens)
        self.fill_rate = float(fill_rate)
        self.timestamp = time.time()
        self.lock = threading.RLock()

    def consume(self, tokens):
        """Consume tokens from the bucket. Returns 0 if there were
        sufficient tokens, otherwise the expected time until enough
        tokens become available."""
        self.lock.acquire()
        tokens = max(tokens,self.tokens)
        expected_time = (tokens - self.tokens) / self.fill_rate
        if expected_time <= 0:
            self._tokens -= tokens
        self.lock.release()
        return max(0,expected_time)

    @property
    def tokens(self):
        self.lock.acquire()
        if self._tokens < self.capacity:
            now = time.time()
            delta = self.fill_rate * (now - self.timestamp)
            self._tokens = min(self.capacity, self._tokens + delta)
            self.timestamp = now
        value = self._tokens
        self.lock.release()
        return value

class RateLimit(object):
    """Rate limit a url fetch.
    source: http://mail.python.org/pipermail/python-list/2008-January/472859.html
    (but mostly rewritten)
    """
    def __init__(self, bucket, filename):
        self.bucket = bucket
        self.last_update = 0
        self.last_downloaded_kb = 0

        self.filename = filename
        self.avg_rate = None

    def __call__(self, block_count, block_size, total_size):
        total_kb = total_size / 1024.

        downloaded_kb = (block_count * block_size) / 1024.
        just_downloaded = downloaded_kb - self.last_downloaded_kb
        self.last_downloaded_kb = downloaded_kb

        predicted_size = block_size/1024.

        wait_time = self.bucket.consume(predicted_size)
        while wait_time > 0:
            time.sleep(wait_time)
            wait_time = self.bucket.consume(predicted_size)

        now = time.time()
        delta = now - self.last_update
        if self.last_update != 0:
            if delta > 0:
                rate = just_downloaded / delta
                if self.avg_rate is not None:
                    rate = 0.9 * self.avg_rate + 0.1 * rate
                self.avg_rate = rate
            else:
                rate = self.avg_rate or 0.
            print "%20s: %4.1f%%, %5.1f KiB/s, %.1f/%.1f KiB" % (
                    self.filename, 100. * downloaded_kb / total_kb,
                    rate, downloaded_kb, total_kb,
                )
        self.last_update = now


def main():
    """Fetch the contents of urls"""
    if len(sys.argv) < 4:
        print 'Syntax: %s rate url1 url2 ...' % sys.argv[0]
        raise SystemExit(1)
    rate_limit  = float(sys.argv[1])
    urls = sys.argv[2:]
    bucket = TokenBucket(10*rate_limit, rate_limit)

    print "rate limit = %.1f" % (rate_limit,)

    threads = []
    for url in urls:
        path = urlparse.urlparse(url,'http')[2]
        filename = os.path.basename(path)
        print 'Downloading "%s" to "%s"...' % (url,filename)
        rate_limiter = RateLimit(bucket, filename)
        t = threading.Thread(
            target=urllib.urlretrieve,
            args=(url, filename, rate_limiter))
        t.start()
        threads.append(t)

    for t in threads:
        t.join()

    print 'All downloads finished'

if __name__ == "__main__":
    main()