Python - Delete the last line of a txt file while appending it

Question

I would like to specify a raw_input command to delete the last line of the txt file while appending the txt file.

Simple code:

while True:
    userInput = raw_input("Data > ")
    DB = open('Database.txt', 'a')
    if userInput == "Undo":
        ""Delete last line command here""
    else:
        DB.write(userInput)
        DB.close()

cyphar · Accepted Answer · 2015-09-03 01:13:19Z

1

You can't open a file in append mode and read from it / modify the previous lines in the file. You'll have to do something like this:

import os

def peek(f):
        off = f.tell()
        byte = f.read(1)
        f.seek(off, os.SEEK_SET)

        return byte

with open("database.txt", "r+") as DB:
        # Go to the end of file.
        DB.seek(0, 2)

        while True:
                action = raw_input("Data > ")

                if action == "undo":
                        # Skip over the very last "\n" because it's the end of the last action.
                        DB.seek(-2, os.SEEK_END)

                        # Go backwards through the file to find the last "\n".
                        while peek(DB) != "\n":
                                DB.seek(-1, os.SEEK_CUR)

                        # Remove the last entry from the store.
                        DB.seek(1, os.SEEK_CUR)
                        DB.truncate()
                else:
                        # Add the action as a new entry.
                        DB.write(action + "\n")

EDIT: Thanks to Steve Jessop for suggesting doing a backwards search through the file rather than storing the file state and serialising it.

You should note that this code is very racy if you have more than one of this process running (since writes to the file while seeking backwards will break the file). However, it should be noted that you can't really fix this (because the act of removing the last line in a file is a fundamentally racy thing to do).

edited Sep 3, 2015 at 1:13

answered Sep 3, 2015 at 0:40

cyphar

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4 Comments

Steve Jessop Over a year ago

If you can be bothered, you can seek to the correct place in the file and truncate there, rather than truncating at 0 and re-writing the whole file. Let us suppose it to be 200GB in size ;-). Searching backwards through a file for a linebreak is kind of tedious, but if the tail Unix command can do it, so can we.

cyphar Over a year ago

Yes, but tail is broken by design: youtu.be/vm1GJMp0QN4?t=2475. And I'd really hope nobody uses a system like this for a 200GB file (do you have 200GB of virtual memory on tap?) :P

Steve Jessop Over a year ago

you already say that there shouldn't be two things truncating at once, this merely says there shouldn't be a tail -f on it either :-)

cyphar Over a year ago

There, I've changed it so we do a backwards seek through the file.

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