yum error[ImportError: cannot import name _]

[root@shenyang tmp]# yum
Traceback (most recent call last):
  File "/usr/bin/yum", line 28, in ?
    import yummain
  File "/usr/share/yum-cli/yummain.py", line 30, in ?
    from yum import _
ImportError: cannot import name _

[root@shenyang yum]# rpm -q yum
yum-3.2.8-9.el5.centos.2.1
my os is centos5.2


Can anyone help me?please!!
thanks in advance

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