How to erase line from text file in Python?

Question

I'm trying to make a code to rewrite a specific line from a .txt file. I can get to write in the line i want, but i can't erase the previous text on the line.

Here is my code:
(i'm trying a couple of things)

def writeline(file,n_line, text):
    f=open(file,'r+')
    count=0
    for line in f:
        count=count+1
        if count==n_line :
            f.write(line.replace(str(line),text))
            #f.write('\r'+text)

You can use this code to make a test file for testing:

with open('writetest.txt','w') as f:
    f.write('1 \n2 \n3 \n4 \n5')

writeline('writetest.txt',4,'This is the fourth line')

Edit: For Some reason, if i use 'if count==5:' the code compiles ok (even if it doen't erase the previous text), but if i do 'if count==n_line: ', the file ends up with a lot of garbage.

The Answers work, but i would like to know what are the problems with my code, and why i can't read and write. Thanks!

Answer 1

You are reading from the file and also writing to it. Don't do that. Instead, you should write to a NamedTemporaryFile and then rename it over the original file after you finish writing and close it.

Or if the size of the file is guaranteed to be small, you can use readlines() to read all of it, then close the file, modify the line you want, and write it back out:

def editline(file,n_line,text):
    with open(file) as infile:
        lines = infile.readlines()
    lines[n_line] = text+' \n'
    with open(file, 'w') as outfile:
        outfile.writelines(lines)

Answer 2

Use temporary file:

import os
import shutil


def writeline(filename, n_line, text):
    tmp_filename = filename + ".tmp"

    count = 0
    with open(tmp_filename, 'wt') as tmp:
        with open(filename, 'rt') as src:
            for line in src:
                count += 1
                if count == n_line:
                    line = line.replace(str(line), text + '\n')
                tmp.write(line)
    shutil.copy(tmp_filename, filename)
    os.remove(tmp_filename)


def create_test(fname):
    with open(fname,'w') as f:
        f.write('1 \n2 \n3 \n4 \n5')

if __name__ == "__main__":
    create_test('writetest.txt')
    writeline('writetest.txt', 4, 'This is the fourth line')