Bind an instance of a base to a subclass - can this be done?

[Lou Pecora]

I've been scanning Python in a Nutshell, but this seems to be either 
undoable or so subtle that I don't know how to do it.

I want to subclass a base class that is returned from a Standard Library 
function (particularly, subclass file which is returned from open).  I 
would add some extra functionality and keep the base functions, too.  
But I am stuck.

E.g. 

class myfile(file):
   def myreadline():
      #code here to return something read from file

Then do something like (I know this isn't right, I'm just trying to 
convey the idea of what I would like)

mf=myfile()

mf=open("Afile","r")  

s=mf.myreadline()     # Use my added function

mf.close()            # Use the original file function


Possible in some way?  Thanks in advance for any clues.

-- Lou Pecora  (my views are my own) REMOVE THIS to email me.