Not really, unless it is provided by the OS.
In order to do something like this, I would think you would have to have access to the input stream, to monitor it before anything is actually flushed, which Python doesn't really give you.
woooee, In another thread you indicated to use termios. This is the only way I've been able to successfully capture the tab key, and then use my own logic to perform auto-completion. This was for a command-line program, I'm not working with the gui, otherwise I'd use your Tkinter example. Just wanted to say thanks.