When I am upgrading my Fedora 10 to Fedora rawhide, my desktop was locked. Then I restart the computer, rerun the yum upgrade. some error like the following comes out.

There was a problem importing one of the Python modules
  required to run yum. The error leading to this problem was:
    No module named rpm
 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:
 2.5 (r25:51908, Apr 10 2007, 10:29:13)
 [GCC 4.1.2 20070403 (Red Hat 4.1.2-8)]
 If you cannot solve this problem yourself, please go to
 the yum faq at:

someone in fedoraforum said to reinstall yum and python, but i found it didn’t work well.

then i try uninstall some yum plugin such as yum-metadata-parser, then the error mesage becomes

    No module named iniparser

it seems that the error comes from here. then i find a package called python-iniparser is out of date. so i download it by hand and update it. try to yum upgrade again, it works! everything comes to there original way.

after all, you just need to reinstall 2 packages: yum-metadata-parser and python-iniparser.