Search text file and insert line

Question

What I want to do is (using the text below as an example), search for the string “Text2” in a text file, and then insert a line (“Inserted Text”) two lines after \"Text 2\".

Answer 1

You could use

f = open("file.txt","rw")
lines = f.readlines()
for i in range(len(lines)):
     if lines[i].startswith("Text2"):
            lines.insert(i+3,"Inserted text") #Before the line three lines after this, i.e. 2 lines later.

print "\n".join(lines)

Answer 2

If you load the file contents into a list, it would be easier to manipulate:

searchline = 'Text4'
lines = f.readlines() # f being the file handle
i = lines.index(searchline) # Make sure searchline is actually in the file

Now i contains the index of the line Text4. You can use that and list.insert(i,x) to insert before:

lines.insert(i, 'Random text to insert')

Or after:

lines.insert(i+1, 'Different random text')

Or three lines after:

lines.insert(i+3, 'Last example text')

Just make sure to include error handling for IndexErrors and you can do with that whatever you want.

Answer 3

The quick-n-dirty way would be something like that

before=-1
for line in fileinput.input('file.txt', inplace=1,backup='.bak'):
    if line.startswith('Text 2'):
        before = 2
    if before == 0
        print "Inserted Text"
    if before > -1
        before = before - 1
    print line,