Btukfyl

I'm writing a program that asks a user for an integer > 0 which is used as the complexity factor for drawing a Koch curve. The program executes fine, once. When running a second time the Terminator is invoked and the traceback points to what I think is the aTurtle variable not clearing it's last state. If I restart the kernel, and clear all outputs, it works fine again once, then the issue repeats. What am I overlooking?

This was constructed and executed in Jupyter and tested in qtconsole as well. Code is listed below:

import turtle

aTurtle = turtle.Turtle()



#Functions

def drawLS(aTurtle, instructions):

    for cmd in instructions:

        if cmd == 'F':

            aTurtle.forward(5)        

        elif cmd == '+':

            aTurtle.right(70)

        elif cmd == '-':

            aTurtle.left(70)

        else :

            print('Error : %s is an unknown command '%cmd)



def applyProduction():

    axiom = 'F'

    myRules = {'F': 'F-F++F-F'}

    for i in range(n):

        newString = ""

        for ch in axiom :

            newString = newString + myRules.get(ch, ch)

        axiom = newString

    return axiom



def lsystem():

    win = turtle.Screen()

    aTurtle.up()

    aTurtle.setposition(-200, 0)

    aTurtle.down()

    aTurtle.setheading(0)

    newRules = applyProduction()

    drawLS (aTurtle, newRules)

    win.exitonclick()



#Main

while True:

    try:

        n = int(input("Enter an integer greater than 0: "))

        break

    except:

        print ("Error, input was not an integer, please try again.")

if n < 1:

       print ("Error, input was not an integer greater than 0.")

else:

    lsystem()

Generally, you shouldn't plan to survive a win.exitonclick() or any of the other turtle commands that turn control over to the tkinter event loop. There are tricks for doing so but not worth the trouble when it's easier just to avoid this situation. What you want is win.clear() or possibly win.reset(). I've incorporated that below and rewrote your prompting logic to use it's own click event, instead of exitonclick(), and to use the graphic numinput():

from turtle import Screen, Turtle



def drawLS(turtle, instructions):

    for cmd in instructions:

        if cmd == 'F':

            turtle.forward(5)

        elif cmd == '+':

            turtle.right(70)

        elif cmd == '-':

            turtle.left(70)

        else:

            print('Error : %s is an unknown command '%cmd)



def applyProduction(n):

    axiom = 'F'

    myRules = {'F': 'F-F++F-F'}



    for _ in range(n):

        newString = ""

        for ch in axiom:

            newString += myRules.get(ch, ch)

        axiom = newString



    return axiom



def lsystem(n):

    aTurtle.up()

    aTurtle.setposition(-200, 0)

    aTurtle.down()

    aTurtle.setheading(0)

    newRules = applyProduction(n)

    drawLS(aTurtle, newRules)



def prompt(x=None, y=None):  # dummy arguments for onclick(), ignore them

    screen.onclick(None)



    n = screen.numinput("Complexity Factor", "Enter an integer greater than 0", minval=1, maxval=100)



    if n is not None:

        screen.clear()

        lsystem(int(n))



    screen.onclick(prompt)



screen = Screen()



aTurtle = Turtle()

aTurtle.speed('fastest')  # because I have no patience



prompt()



screen.mainloop()

When the drawing finishes, click on the screen to get a new prompt for a different complexity factor and a new drawing.