How do I write a string to a specific line number?

Question

I am having trouble finding the answer to this after quite a bit of searching. What I want to do is, do a string search and write on the line above or below it, depending on my string.

Here is something I've done so far:

file = open('input.txt', 'r+')  
f = enumerate(file)  
for num, line in f:    
    if 'string' in line:    
        linewrite = num - 1   
            ???????

EDIT EXTENSION OF INITIAL QUESTION: I already picked the answer that best solved my initial question. But now using Ashwini's method where I rewrote the file, how can I do a search AND REPLACE a string. To be more specific.

I have a text file with

SAMPLE  
AB  
CD  
..  
TYPES  
AB  
QP  
PO  
..  
RUNS  
AB  
DE  
ZY

I want to replace AB with XX, ONLY UNDER lines SAMPLE and RUNS I've already tried multiple ways of using replace(). I tried something like

if  'SAMPLE' in line:  
f1.write(line.replace('testsample', 'XX'))  
if 'RUNS' in line:      
f1.write(line.replace('testsample', 'XX'))  

and that didn't work

Answer 1

The following can be used as a template:

import fileinput

for line in fileinput.input('somefile', inplace=True):
    if 'something' in line:
        print 'this goes before the line'
        print line,
        print 'this goes after the line'
    else:
        print line, # just print the line anyway

Answer 2

You may have to read all the lines in a list first, and if the condition is matched you can then store your string at a particular index using list.insert

with open('input.txt', 'r+') as f:
   lines = f.readlines()
   for i, line in enumerate(lines):
       if 'string' in line:
          lines.insert(i,"somedata")  # inserts "somedata" above the current line
   f.truncate(0)         # truncates the file
   f.seek(0)             # moves the pointer to the start of the file
   f.writelines(lines)   # write the new data to the file

or without storing all the lines you'll need a temporary file to store the data, and then rename the temporary file to the original file:

import os
with open('input.txt', 'r') as f, open("new_file",'w') as f1:
   for line in f:
       if 'string' in line:
          f1.write("somedate\n")  # Move f1.write(line) above, to write above instead
       f1.write(line)
os.remove('input.txt')  # For windows only 
os.rename("newfile", 'input.txt')  # Rename the new file