XML : ImportError: No module named xml.dom.minidom

I try to export xml.dom to my python program.

  from xml.dom import minidom    

But when i run the code, i got this error:

  import xml.dom.minidom  ImportError: No module named xml.dom.minidom    

This happened before when i rename my file to xml.py, but it's clear when i change name to other. But this is another problem i got. i didn't rename my file. It happened after i run apt-get update and apt-get upgrade ubuntu.

What's wrong?