open file, review it, process, behind - shortest slight python

i wish simple filtering file. review it, processing, back.

i'm looking "golfing", nonetheless wish simplest many glorious slight grasp this. i came adult with:

from __future__ import with_statement

filename = "..." # sys.argv...

with open(filename) f:
    new_txt = # ...some interpretation f.read() 

open(filename, 'w').write(new_txt)

the with matter creates things shorter given i don't have definitely open tie file.

any ideas ?