Python: finding and deleting strings in a text file

Question

I would like to periodically check whether a certain word has been saved to a text file with python. Whenever I run the function it deletes anything in the text file regardless of what I tell it to look for specifically.

import threading

def ban():
        threading.Timer(0.1, ban).start()
        ff=open("htfl.txt","r")
        liney = ff.readlines()
        ff.close()
        ff=open("htfl.txt","w")
        for line in liney:
            if line!=("HTFL"):
                liney = liney[:-1]
                print("true")
ban()

It has to run every few seconds as it is part of an instant messenger I am working on which ought to ban rude words for example or certain usernames.

Why are you opening the file again? You never do anything with the second `ff = open(..., 'w')` object. — Nick T, Feb 20 '17 at 23:10
Why are you opening the file to write? You don't appear to write to the file anywhere. — tobassist, Feb 20 '17 at 23:12
You probably should also compare with `'HTFL\n'` as the line has the newline at the end. — Peter Wood, Feb 20 '17 at 23:14
@PeterWood Replaced it with ff.write(line) and \n now it seems to be deleting the string still regardless of what it contains and writing it again and deleting it over and over again. — douglas rouse, Feb 20 '17 at 23:21
Your function `ban` creates a thread which calls `ban`, ad infinitum. — Peter Wood, Feb 20 '17 at 23:24
@PeterWood True, I suppose that would be the cause of it looping the whole time. Still can't quite figure out why it deletes a random string which doesn't contain any of the character of htfl. — douglas rouse, Feb 20 '17 at 23:27
Well, when you open the file to write in the other threads it deletes the whole file. — Peter Wood, Feb 20 '17 at 23:28
@PeterWood I could split it into two functions. one which simply checks and one which deletes it. — douglas rouse, Feb 20 '17 at 23:28
See [Is it possible to modify lines in a file in-place?](http://stackoverflow.com/questions/5453267/is-it-possible-to-modify-lines-in-a-file-in-place) — Peter Wood, Feb 20 '17 at 23:31

score 1 · Answer 1 · answered Feb 20 '17 at 23:36

import threading

def ban():
        threading.Timer(0.1, ban).start()
        ff=open("htfl.txt","r")
        liney = ff.readlines()
        ff.close()
        ff=open("htfl.txt","w")
        for line in liney:
            if line!=("HTFL"):
                ff.write(line)
ban()

You opened the original file in write mode, which deletes everything. So you have to rewrite every line. Also doing liney[:-1] didn't make sense because your goal is to get rid of specific lines, not just mindlessly copy everything but the first line.

Peter Wood · Accepted Answer · 2017-02-20T23:50:43.723

1

You can edit a file in-place using the fileinput module:

import threading
import fileinput
import time

def ban():
    for line in fileinput.input('htfl.txt', inplace=True):
        if line.rstrip() != "HTFL":
            print line,

edited Feb 20 '17 at 23:50

answered Feb 20 '17 at 23:38

Peter Wood

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Hi I am not familiar with that module, is it possible to do this without creating a new file? As in target an existing file? When I tried executing this i got an error: Cannot create a file when that file already exists: 'htfl.txt' -> 'htfl.txt.bak' – douglas rouse Feb 20 '17 at 23:46
See [the documentation](https://docs.python.org/2/library/fileinput.html#fileinput.FileInput), particularly the part about in-place filtering and the temporary backup filename. – Peter Wood Feb 20 '17 at 23:48
Not sure where I went wrong seems to be working well for me. I need to use threading though. If I have a .sleep in there it freezes my tkinter gui – douglas rouse Feb 20 '17 at 23:49
Okay, that's a different problem for another time. – Peter Wood Feb 20 '17 at 23:50

Python: finding and deleting strings in a text file

2 Answers2