測試一下

import os
import sys
import urllib
from time import time, sleep

class RateLimit(object):
    """Rate limit a url fetch"""
    def __init__(self, rate_limit):
        """rate limit in kBytes / second"""
        self.rate_limit = rate_limit
        self.start = time()
    def __call__(self, block_count, block_size, total_size):
        total_kb = total_size / 1024
        downloaded_kb = (block_count * block_size) / 1024
        elapsed_time = time() - self.start
        if elapsed_time != 0:
            #實際下載速度
            rate = downloaded_kb / elapsed_time
            print "%d kb of %d kb downloaded %f.1 kBytes/s\n" % (downloaded_kb ,total_kb, rate),
            expected_time = downloaded_kb / self.rate_limit
            sleep_time = expected_time - elapsed_time
            print "Sleep for", sleep_time
            #等待時間
            if sleep_time > 0:
                sleep(sleep_time)