file io - Python truncate lines as they are read

Question

I have an application that reads lines from a file and runs its magic on each line as it is read. Once the line is read and properly processed, I would like to delete the line from the file. A backup of the removed line is already being kept. I would like to do something like

file = open('myfile.txt', 'rw+')
for line in file:
   processLine(line)
   file.truncate(line)

This seems like a simple problem, but I would like to do it right rather than a whole lot of complicated seek() and tell() calls.

Maybe all I really want to do is remove a particular line from a file.

After spending far to long on this problem I decided that everyone was probably right and this it just not a good way to do things. It just seemed so elegant solution. What I was looking for was something akin to a FIFO that would just let me pop lines out of a file.

Answer 1

Remove all lines after you've done with them:

with open('myfile.txt', 'r+') as file:
    for line in file:
        processLine(line)
    file.truncate(0)

Remove each line independently:

lines = open('myfile.txt').readlines()

for line in lines[::-1]: # process lines in reverse order
    processLine(line)
    del lines[-1]  # remove the [last] line

open('myfile.txt', 'w').writelines(lines)

You can leave only those lines that cause exceptions:

import fileinput

for line in fileinput.input(['myfile.txt'], inplace=1):
    try: processLine(line)
    except Exception:
         sys.stdout.write(line) # it prints to 'myfile.txt'

In general, as other people already said it is a bad idea what you are trying to do.