while reading line in a file of python i need to add the newline after specific index code example

Example: python insert on a specific line from file

f = open("path_to_file", "r")
contents = f.readlines()
f.close()

contents.insert(index, value)

f = open("path_to_file", "w")
contents = "".join(contents)
f.write(contents)
f.close()