how to write file with configparser

[hlt2000 at hotmail.com]

I,m using the ConfigParser module to read my ini file. After my
program has ran I want to update the ini file. With configdict.set I
modify an option to a new value. 

How do I write this back onto my file system?  

cfgfile=open('config.ini',' rw')
...
cfgdict.set(....)
...
cfgdict.write(cfgfile)

This gives an IOerror: (0,Error)

What is the right way to do this?

TIA,
Hans