Question

I’m writing a todo list program. It keeps a file with a thing to do per line, and lets the user add or delete items. The problem is that for some reason, I end up with a lot of zero bytes at the start of the file, even though the item is correctly deleted. I’ll show you a couple of screenshots to make sure I’m making myself clear.

This is the file in Notepad++ before running the program:

This is the file after deleting item 3 (counting from 1):

This is the relevant code. The actual program is bigger, but running just this part triggers the error.

import os
TODO_FILE = r"E:\javi\code\Python\todo-list\src\todo.txt"

def del_elems(f, delete):
    """Takes an open file and either a number or a list of numbers, and deletes the
    lines corresponding to those numbers (counting from 1)."""
    if isinstance(delete, int):
        delete = [delete]
    lines = f.readlines()
    f.truncate(0)
    counter = 1
    for line in lines:
        if counter not in delete:
            f.write(line)
        counter += 1

f = open(TODO_FILE, "r+")
del_elems(f, 3)
f.close()

Could you please point out where’s the mistake?

Answer 1

It looks to me like you’re forgetting to rewind your file stream. After f.truncate(0), add f.seek(0). Otherwise, I think your next write will try to start at the position from which you left off, filling in null bytes on its way there.

(Notice that the number of null characters in your example equals the number of characters in your deleted lines plus a carriage-return and line-feed character for each one.)