file operation in python

http://stackoverflow.com/questions/125703/how-do-i-modify-a-text-file-in-python

#Unfortunately there is no way to insert into the middle of a file without re-writing it.
 
import sys
import fileinput

# replace all occurrences of 'sit' with 'SIT' and insert a line after the 5th
for i, line in enumerate(fileinput.input('C:/Users/test/Desktop/test.txt', inplace=1)):
    sys.stdout.write(line.replace('sit', 'SIT'))  # replace 'sit' and write
#    if i == 4: sys.stdout.write('\n')  # write a blank line after the 5th line