yum 出现错误ImportError: No module named urlgrabber.grabber

yum 出现错误：

root@iZ23t4pnz63Z ~]# yum update
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirrors.aliyuncs.com
 * epel: mirrors.aliyuncs.com
 * extras: mirrors.aliyuncs.com
 * updates: mirrors.aliyuncs.com
Traceback (most recent call last):
  File "/usr/libexec/urlgrabber-ext-down", line 22, in <module>
    from urlgrabber.grabber import \
ImportError: No module named urlgrabber.grabber
Traceback (most recent call last):
  File "/usr/libexec/urlgrabber-ext-down", line 22, in <module>
    from urlgrabber.grabber import \
ImportError: No module named urlgrabber.grabber

vi /usr/libexec/urlgrabber-ext-down

把头部的python改成和/usr/bin/yum中一样的