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()

Answer 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).