Quote - does that make a difference? i use sleep in the cycle, too.

It does make a difference.

You need to create a Tk dialog (to give you a button) and then use a timer to trigger the next render if the button hasn't been pressed, or abort the script if the button was pressed.

You may be able to use a Python Timer from the threading module, but I'd suggest looking up the Tk after and after_cancel methods as these might be easier to use (and in case the threading Timer class doesn't work).