Change parameter file, in place

change_par_file.py

import fileinput
import os

# create a copy of the template parameter file
os.system('cp template_file new_file')

# for example, in the template you have
# par1 = 10
# par2 = 'some path'
# and you want to change to
# par1 = 20
# par2 = '/home/vardan'

# change the parameter file (the copy you just created)
for line in fileinput.input('new_file', inplace=True):
    if 'par1 = 10' in line:
        print 'par1 = 20',   # be careful to put the commas in the end
    elif "par2 = 'some path'" in line:
    	print "par2 = '/home/vardan'",
    else:
        print line,   # every other line is unchanged

# in the end, new_file is changed