最近在看Python，虚拟机装的是CentOS 6.6，自带的Python版本是2.6.6，打算升级到2.7。
我的升级过程大致如下：
下载2.7源码包https://www.python.org/downloads/source/
卸载旧的python，rpm -e python
编译安装python2.7
整个过程没有遇到问题，但升级完后，再用yum安装软件时报如下错误
[root@localhost python]# yum
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:
No module named yum
Please install a package which provides this module, or
verify that the module is installed correctly.
It's possible that the above module doesn't match the
current version of Python, which is:
2.7.13 (r266:84292, Jan 22 2014, 09:37:14)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)]
If you cannot solve this problem yourself, please go to
the yum faq at:
yum不可用了，这下问题就大了，spacer.gif没有yum，安装rpm包特别费劲。
开始百度关键字“No module named yum”，出来很多帖子和博客，博客上边说的大概意思就是yum就基于python的，升级python后，yum与高版本的python不兼容，导致yum无法使用。博客上给出的方法都是修改/usr/bin/yum文件的头部，把/usr/bin/python修改为/usr/bin/python2.6就可以了，但我的情况是我把旧的python已经卸载了，于是又开始安装2.6版本的python。
安装好2.6.6版本的之后以为就好了，但是还是报有错误：
[root@localhost python]# yum
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:
No module named yum
Please install a package which provides this module, or
verify that the module is installed correctly.
It's possible that the above module doesn't match the
current version of Python, which is:
2.6.6 (r266:84292, Jan 22 2014, 09:37:14)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)]
If you cannot solve this problem yourself, please go to
the yum faq at:
这就奇怪了，都已经把旧版本的装上了，还是不行。这时看到一篇博客是把python和yum都全部卸载后重新安装的。想想这也是个办法于是就又全部下载
whereis python |xargs rm -rf
rpm -e --nodeps python
卸载后重新安装，从光盘镜像里找到python和yum的包
rpm -ivh --nodeps python*
rpm -ivh --nodeps yum*
这次总该好了吧，验证一下
[root@localhost python]# python
Python 2.6.6 (r266:84292, Jan 22 2014, 09:37:14)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import yum
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.6/site-packages/yum/__init__.py", line 23, in <module>
import rpm
ImportError: No module named rpm
还是报错。不过这次错误变了，变成了“No module named rpm”
再次百度，看到有人说是缺少包：rpm -ivh rpm-python-4.8.0-37.el6.i686.rpm
安装上面的包后一切yum恢复正常了。
那问题又来了难道因为yum就不升级python版本吗？当然不是。其实是我在安装新版python的时候把旧版本的也卸载了才导致这样的问题。
后来在不卸载2.6版本的python是，重新编译安装2.7版本，安装成功并且yum仍然可用。