Insert line at middle of file with Python?

If you want to search a file for a substring and add a new text to the next line, one of the elegant ways to do it is the following:

import fileinput
for line in fileinput.FileInput(file_path,inplace=1):
    if "TEXT_TO_SEARCH" in line:
        line=line.replace(line,line+"NEW_TEXT")
    print line,

This is a way of doing the trick.

with open("path_to_file", "r") as f:
    contents = f.readlines()

contents.insert(index, value)

with open("path_to_file", "w") as f:
    contents = "".join(contents)
    f.write(contents)

index and value are the line and value of your choice, lines starting from 0.