python3.4 curses module: bug reappearance code

# python3.4 curses module: bug reappearance code
import curses

def callback(stdscr, *args, **kwargs):
    for y in range(10):
        stdscr.addch(y, 0, b'y')
    for x in range(10):
        stdscr.addch(0, x, b'x')
    stdscr.refresh()
    while 1:
        curses.napms(100)

if __name__ == '__main__':
    curses.wrapper(callback)