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, sys      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.