edit text file using Python

Question

I need to update a text file whenever my IP address changes, and then run a few commands from the shell afterwards.

1. Create variable LASTKNOWN = \"212.171.13

Answer 1

Replace LASTKNOWN by CURRENT in /etc/ipf.conf

Replace all at once

filename = "/etc/ipf.conf"
text = open(filename).read()
open(filename, "w").write(text.replace(LASTKNOWN, CURRENT))

Replace line by line

from __future__ import with_statement
from contextlib import nested

in_filename, outfilename = "/etc/ipf.conf", "/tmp/ipf.conf"
with nested(open(in_filename), open(outfilename, "w")) as in_, out:
     for line in in_:
         out.write(line.replace(LASTKNOWN, CURRENT))
os.rename(outfilename, in_filename)

Note: "/tmp/ipf.conf" should be replaced by tempfile.NamedTemporaryFile() or similar
Note: the code is not tested.