How to write to a specific line in file in Python?

Question

I have a file as the format:

xxxxx
yyyyy
zzzzz
ttttt

And I need to write in file between xxxxx and yyyyy lines as:

xxxxx
my         


        

Answer 1

If the file is small then you can simply use str.replace():

>>> !cat abc.txt
xxxxx
yyyyy
zzzzz
ttttt

>>> with open("abc.txt") as f,open("out.txt",'w') as o:
    data=f.read()
    data=data.replace("xxxxx\nyyyyy","xxxxx\nyourline\nyyyyy")
    o.write(data)
   ....:     

>>> !cat out.txt
xxxxx
yourline
yyyyy
zzzzz
ttttt

For a huge file use mgilson's approach.