Deleting n number of lines after specific line of file in python

Question

I am trying to remove a specific number of lines from a file. These lines always occur after a specific comment line. Anyways, talk is cheap, here is an example of what I have.

FILE: --

randomstuff
randomstuff2
randomstuff3

# my comment
extrastuff
randomstuff2
extrastuff2

#some other comment
randomstuff4

So, I am trying to remove the section after # my comment. Perhaps there is someway to delete a line in r+ mode?

Here is what I have so far

with open(file_name, 'a+') as f:
    for line in f:
        if line == my_comment_text:
            f.seek(len(my_comment_text)*-1, 1) # move cursor back to beginning of line
            counter = 4
        if counter > 0:
            del(line) # is there a way to do this?

Not exactly sure how to do this. How do I remove a specific line? I have looked at this possible dup and can't quite figure out how to do it that way either. The answer recommends you read the file, then you re-write it. The problem with this is they are checking for a specific line when they write. I cant do that exactly, plus I dont like the idea of storing the entire files contents in memory. That would eat up a lot of memory with a large file (since every line has to be stored, rather than one at a time).

Any ideas?

You want to remove all lines after the comment or only until another comment is found? — solarc, Aug 22 '14 at 17:26
Using a temporary file ? Read each line to a new file. WHen the is matched, stop writing. When the is matched, start writing again. in the end copy it to a new file ... — Cristiano Araujo, Aug 22 '14 at 17:30
@solarc only the section. A specific number of lines (as the title suggest) and NOT the whole file. Or until a blank line is found — Nick Humrich, Aug 22 '14 at 17:31

score 1 · Accepted Answer · edited May 23 '17 at 11:57

1

You can use the fileinput module for this and open the file in inplace=True mode to allow in-place modification:

import fileinput

counter = 0
for line in fileinput.input('inp.txt', inplace=True):
    if not counter:
        if line.startswith('# my comment'):
            counter = 4
        else:
            print line,
    else:
        counter -= 1

Edit per your comment "Or until a blank line is found":

import fileinput

ignore = False
for line in fileinput.input('inp.txt', inplace=True):
    if not ignore:
        if line.startswith('# my comment'):
            ignore = True
        else:
            print line,
    if ignore and line.isspace():
        ignore = False

edited May 23 '17 at 11:57

Community

1
1

answered Aug 22 '14 at 17:36

Ashwini Chaudhary

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why would I print the line to stdout? – Nick Humrich Aug 22 '14 at 17:42
@Humdinger When we use `inplace=True` the standard output is actually re-directed to the file, so this will work fine.(Check the docs for more details: [Optional in-place filtering](https://docs.python.org/2/library/fileinput.html#fileinput.FileInput)) – Ashwini Chaudhary Aug 22 '14 at 17:51
Wow.. thats actually really cool. Except the line read also includes a '/n' so the line needs to be striped before printing, or else this code creates extra new lines after every line – Nick Humrich Aug 22 '14 at 17:58
@Humdinger That's why I used that trailing `,` after print. If you're on Python 3 then use `print(line, end='')`. – Ashwini Chaudhary Aug 22 '14 at 17:59
What if I want it to be compatible with both 2 and 3? – Nick Humrich Aug 22 '14 at 18:00
@Humdinger import `print_function` from `__future__` at the top of the file. – Ashwini Chaudhary Aug 22 '14 at 18:00

score 0 · Answer 2 · answered Aug 22 '14 at 17:29

You can make a small modification to your code and stream the content from one file to the other very easily.

with open(file_name, 'r') as f:
    with open(second_file_name,'w') a t:
    counter = 0
    for line in f:
        if line == my_comment_text:
            counter = 3             
        elif: counter > 0
            counter -= 1
        else:
            w.write(line)

score 0 · Answer 3 · answered Aug 22 '14 at 18:46

I like the answer form @Ashwini. I was working on the solution also and something like this should work if you are OK to write a new file with filtered lines:

def rewriteByRemovingSomeLines(inputFile, outputFile):
unDesiredLines = []
count = 0
skipping = False
fhIn = open(inputFile, 'r')
line = fhIn.readline()
while(line):
    if line.startswith('#I'):
        unDesiredLines.append(count)
        skipping = True
    while (skipping):
        line = fhIn.readline()
        count = count + 1
        if (line == '\n' or line.startswith('#')):
            skipping=False
        else:
            unDesiredLines.append(count)
    count = count + 1
    line = fhIn.readline()
fhIn.close()

fhIn = open(inputFile, 'r')
count = 0
#Write the desired lines to a new file
fhOut = open(outputFile, 'w')
for line in fhIn:
    if not (count in unDesiredLines):
        fhOut.write(line)
    count = count + 1
fhIn.close()
fhOut.close

Deleting n number of lines after specific line of file in python

3 Answers3