Insert text into a text file following specific text using Python

Question

I have to edit some text files to include new information, but I will need to insert that information at specific locations in the file based on the surrounding text.

This doesn't work the way I need it to:

 with open(full_filename, "r+") as f:
        lines = f.readlines() 
        for line in lines:
            if 'identifying text' in line:   
                offset = f.tell()
                f.seek(offset)  
                f.write('Inserted text')

...in that it adds the text to the end of the file. How would I write it to the next line following the identifying text?

(AFAICT, this is not a duplicate of similar questions, since none of those were able to provide this answer)

Answer 1

If you don't need to work in place, then maybe something like:

with open("old.txt") as f_old, open("new.txt", "w") as f_new:
    for line in f_old:
        f_new.write(line)
        if 'identifier' in line:
            f_new.write("extra stuff\n")

(or, to be Python-2.5 compatible):

f_old = open("old.txt")
f_new = open("new.txt", "w")

for line in f_old:
    f_new.write(line)
    if 'identifier' in line:
        f_new.write("extra stuff\n")

f_old.close()
f_new.close()

which turns

>>> !cat old.txt
a
b
c
d identifier
e

into

>>> !cat new.txt
a
b
c
d identifier
extra stuff
e

(Usual warning about using 'string1' in 'string2': 'name' in 'enamel' is True, 'hello' in 'Othello' is True, etc., but obviously you can make the condition arbitrarily complicated.)

Answer 2

You could use a regex and then replace the text.

import re
c = "This is a file's contents, apparently you want to insert text"
re.sub('text', 'text here', c)
print c

returns "This is a file's contents, apparently you want to insert text here"

Not sure if it'll work for your usecase but it's nice and simple if it fits.