sed -i -e 's/old_word/new_word/' /path/to/filename.conf in python

sed_file.py

# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
#
# sed_file.py is;
#      
#       sed -i -e 's/old_word/new_word/' /path/to/filename.conf
#

import fileinput, sys
for line in fileinput.input('/path/to/filename.conf', inplace=True):
    line = line.replace('old_word', 'new_word')
    # sys.stdout is redirected to the file
    sys.stdout.write(line)

# refered links: http://stackoverflow.com/questions/1582750/edit-text-file-using-python,
#     http://docs.python.org/2/library/fileinput.html