m4a 转MP3

import os
for filename in os.listdir(r\'.\'):
    print filename
    os.rename(filename,filename.replace(\' \',\'_\'))
    cmd= "avconv -i "+filename.replace(\' \',\'_\') + " ./mp3/"+ filename.replace(\'m4a\',\'mp3\').replace(\' \',\'_\')
    print cmd
    os.system(cmd)