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

with open('input') as fin, open('output','w') as fout:
    for line in fin:
        fout.write(line)
        if line == 'xxxxx\n':
           next_line = next(fin)
           if next_line == 'yyyyy\n':
              fout.write('my_line\n')
           fout.write(next_line)

This will insert your line between every occurrence of xxxxx\n and yyyyy\n in the file.

An alternate approach would be to write a function to yield lines until it sees an xxxxx\nyyyyy\n

 def getlines(fobj,line1,line2):
     for line in iter(fobj.readline,''):  #This is necessary to get `fobj.tell` to work
         yield line
         if line == line1:
             pos = fobj.tell()
             next_line = next(fobj):
             fobj.seek(pos)
             if next_line == line2:
                 return

Then you can use this passed directly to writelines:

with open('input') as fin, open('output','w') as fout:
    fout.writelines(getlines(fin,'xxxxx\n','yyyyy\n'))
    fout.write('my_line\n')
    fout.writelines(fin)

Answer 2

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.