Turtle graphics - How do I control when the window closes?

import turtle

turtle.forward(100)
turtle.left(90)
turtle.forward(100)
# etc.

turtle.getscreen()._root.mainloop()  # <-- run the Tkinter main loop

(edit: turtle.done() as suggested by hua below is less ugly.)

Just use turtle.done() or turtle.Screen().exitonclick() as a last command of your turtle program.