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Write mode starts every file from empty. That is right for a file that holds one answer, such as today’s score, but it is wrong for a file that is meant to grow: a class list, a log of results, a guest book. For those, each run of the program has to keep what is already there and add to the end of it.

That is what the third mode is for.

name = "Dave"
my_file = open("appending.txt", "a")
my_file.write(name + "\n")
my_file.close()

Append mode

"a" opens a file to append to it: to add on at the end.

If the file does not exist yet, Python creates it, just as in write mode.
If it does exist, everything in it is kept, and whatever you write goes after the last character that is already there.

Say appending.txt holds two names before the program above runs:

Alice
Colin

After the program has run, it holds three:

Alice
Colin
Dave

Run the same program again and there is a second Dave at the end. Append mode never checks what is already in the file; it only adds.

Append mode adds after the very last character in the file. If the last line already in the file does not end with a \n, the new text carries on along that same line: Colin followed by Dave becomes ColinDave. Ending every line you write with "\n" means the next program to append starts on a fresh line.

The three modes side by side

Mode	Used to	If the file does not exist	What is already in the file
`"r"`	read	the program stops with an error	is read, and left alone
`"w"`	write	the file is created	is wiped as soon as the file is opened
`"a"`	append	the file is created	is kept, and new text goes at the end

The question to ask before you pick a mode: should the file still hold its old contents when the program has finished? If not, "w". If so, "a".

Adding to a file until the user is done

A program that appends is often one that keeps going for as long as the user has more to add. You met loops like that in the chapter on while: the loop does not know in advance how many times it will run.

The loop below uses a Boolean variable as a flag to decide when to stop, like words_left in the 2D arrays chapter. repeat holds True or False, and the loop checks it with repeat == True, which works the same as while repeat:.

repeat = True

while repeat == True:
    print("Enter a guest's name")
    guest = input()

    guest_file = open("guests.txt", "a")
    guest_file.write(guest + "\n")
    guest_file.close()

    print("Another guest? y/n")
    answer = input()

    if answer == "n":
        repeat = False

guest_file = open("guests.txt", "r")
for line in guest_file:
    print(line.strip())
guest_file.close()

repeat starts as True, so the condition repeat == True is true and the loop runs at least once.
Every time round, the program asks for a name and appends it to the file.
Only the answer n changes repeat to False. The next time the condition is checked it is false, and the loop ends.
The last four lines read the file back, so you can see what it holds.

True and False have no quotes around them and always start with a capital letter. "True" in quotes is just a piece of text, and repeat == True would never be true for it.

Every name in the loop above opens the file, appends one line and closes it again straight away. That keeps each name safe in the file the moment it is typed, even if the program is stopped halfway. The next exercises work the same way.

Help! My code doesn't work!

Make sure that you check for the following things:

The mode is "a" when the old contents have to stay, and "w" only when they may be thrown away.
Every line you write ends with "\n".
True and False start with a capital letter and have no quotes around them.
The line that changes the flag to False is inside the loop, or the loop never ends.
Every open has a matching close.