Trying to write to a .txt file using "a" says file is not open for writing

I'm new to python and so I'm trying to make an ATM (simulator I guess). One of the functions is to register, and so I'm trying to save the usernames in a .txt file for reference (passwords will come later of course), but for some reason it won't work.

def atm_register():
    print "To register, please choose a username."

    username = raw_input('> ')

    with open("users.txt") as f:
        f.write(username, "a")

It tells me, that the file is not open for writing. Btw, the users.txt file is in the same directory as the program.

Given that you're calling f.write(username, "a") and file write() only takes one parameter - the text to write - I guess you intend the "a" to append to the file?

That goes in the open() call; other answers telling you to use open(name, "w") are bad advice, as they will overwrite the existing file. And you might want to write a newline after the username, too, to keep the file tidy. e.g.

with open("users.txt", "a") as f:
    f.write(username + "\n")