Insert line at middle of file with Python?

Is there a way to do this? Say I have a file that's a list of names that goes like this:

1. Alfred
2. Bill
3. Donald

How could I insert the third name, "Charlie", at line x (in this case 3), and automatically send all others down one line? I've seen other questions like this, but they didn't get helpful answers. Can it be done, preferably with either a method or a loop?

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.

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,

There is a combination of techniques which I found useful in solving this issue:

with open(file, 'r+') as fd:
    contents = fd.readlines()
    contents.insert(index, new_string)  # new_string should end in a newline
    fd.seek(0)  # readlines consumes the iterator, so we need to start over
    fd.writelines(contents)  # No need to truncate as we are increasing filesize

In our particular application, we wanted to add it after a certain string:

with open(file, 'r+') as fd:
    contents = fd.readlines()
    if match_string in contents[-1]:  # Handle last line to prevent IndexError
        contents.append(insert_string)
    else:
        for index, line in enumerate(contents):
            if match_string in line and insert_string not in contents[index + 1]:
                contents.insert(index + 1, insert_string)
                break
    fd.seek(0)
    fd.writelines(contents)

If you want it to insert the string after every instance of the match, instead of just the first, remove the else: (and properly unindent) and the break.

Note also that the and insert_string not in contents[index + 1]: prevents it from adding more than one copy after the match_string, so it's safe to run repeatedly.