Python: Print next x lines from text file when hitting string

Question

The situation is as follows:

I have a .txt file with results of several nslookups.

I want to loop tru the file and everytime it hits the string "Non-authoritative answer:" the scripts has to print the following 8 lines from that position. If it works I shoud get all the positive results in my screen :).

First I had the following code:

#!/bin/usr/python

file = open('/tmp/results_nslookup.txt', 'r')
f = file.readlines()

for positives in f:
        if 'Authoritative answers can be found from:' in positives:
                print positives
file.close()

But that only printed "Authoritative answers can be found from:" the times it was in the .txt.

The code what I have now:

#!/bin/usr/python

file = open('/tmp/results_nslookup.txt', 'r')
lines = file.readlines()

i = lines.index('Non-authoritative answer:\n')

for line in lines[i-0:i+9]:
        print line,

file.close()

But when I run it, it prints the first result nicely to my screen but does not print the other positve results.

p.s. I am aware of socket.gethostbyname("foobar.baz") but first I want to solve this basic problem.

Thank you in advance!

This is what grep was made for, in particular the -A option: grep -A8 'Non-authoritative answer:' /tmp/results_nslookup.txt — Kevin
– Kevin, Commented Oct 4, 2012 at 12:40
Or a simple awk one-liner: awk 'count-->0;/Non-authoritative answer:/{count=8;print}' /tmp/results_nslookup.txt — Kevin
– Kevin, Commented Oct 4, 2012 at 12:43
@Kevin I think you should have made that an answer. Of course, I have this rebellious streak for lateral answers... — Mattie B
– Mattie B, Commented Oct 4, 2012 at 14:21

Martijn Pieters · Accepted Answer · 2018-08-24 19:14:14Z

7

You can use the file as an iterator, then print the next 8 lines every time you find your sentence:

with open('/tmp/results_nslookup.txt', 'r') as f:
    for line in f:
        if line == 'Non-authoritative answer:\n':
            for i in range(8):
                print(next(lines).strip())

Each time you use the next() function on the file object (or loop over it in a for loop), it'll return the next line in that file, until you've read the last line.

Instead of the range(8) for loop, I'd actually use itertools.islice:

from itertools import islice

with open('/tmp/results_nslookup.txt', 'r') as f:
    for line in f:
        if line == 'Non-authoritative answer:\n':
            print(''.join(islice(f, 8)))

edited Aug 24, 2018 at 19:14

answered Oct 4, 2012 at 12:33

Martijn Pieters

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2 Comments

Shashi Over a year ago

how to print the line which has "Non-authoriative answer" as well?

Martijn Pieters Over a year ago

@Shashi just add print(line) as the first statement inside the if line == test.

nagylzs · Accepted Answer · 2012-10-04 12:33:52Z

4

file = open('/tmp/results_nslookup.txt', 'r')
for line in file:
    if line=='Non-authoritative answer:\n':
        for _ in range(8):
            print file.next()

By the way: don't ever use the name file for a variable because it is the name of a built-in function.

answered Oct 4, 2012 at 12:33

nagylzs

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2 Comments

Shashi Over a year ago

how to print the line which has "Non-authoriative answer" as well?

nagylzs Over a year ago

@Shasi: just put "print(line)" before the for loop. (But that will always print that fixed text.) You may also want to print (file.next())[:-1] instead, e.g. strip the newline to avoid printing double newlines

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