python文件下载与进度条

#encoding=utf-8
import urllib
import sys
 
def cbk(a,b,c):
    per = 100.0*a*b/c
    if per >100:
        per=100
    def cls(str):
        return '\r'*len(str)
    out='%.2f%%'%per
    sys.stdout.write(u'下载进度：')
    sys.stdout.write(out)
    sys.stdout.write(cls(out))
     
url=""
urllib.urlretrieve(url,'download_123.mp3',cbk)