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Question

I'm trying to make a simple switch that changes a variable (in this case switchvalue) when I hit a key. My approach doesn't seem to be working, the key detection is working as far as I can tell.

import turtle
from turtle import Turtle, Screen

screen = Screen()

jack = Turtle("turtle")
jack.color("red", "green")
jack.pensize(10)
jack.speed(0)
switchvalue = 1


def switch():
    global switchvalue
    if switchvalue == 1:
        switchvalue = 0
    if switchvalue == 0:
        switchvalue = 1




turtle.listen()

turtle.onkey(switch,"s")

screen.mainloop()

if switchvalue == 0:
    jack.forward(100)

Answer 1

You got your logic wrong at the function switch() . See what happens in the beggining when switchvalue is 1

def switch():
    global switchvalue
    if switchvalue == 1: # True
        switchvalue = 0 # change it to 0
    if switchvalue == 0: # Whoops, True again, because you switched it to 0 before
        switchvalue = 1

As you can see you are changing switchvalue to 0 then checking if it is 0 and then it gets changed back to 1 , in other words both if statements are executed. You should instead use elif or else so that if one succeds the " if loop" (metaphorically speaking) will break aka the other if s will not be checked.

def switch():
    global switchvalue
    # IF one if succeds all the others will not be accounted
    if switchvalue == 1:
        switchvalue = 0
    elif switchvalue == 0:
        switchvalue = 1

Answer 2

Even if you fix the elif issue that @Countour-Integral points out, this code isn't going to work. Code after the mainloop() call isn't executed until after turtle shuts down, at which time forward() is meaningless:

screen.mainloop()

if switchvalue == 0:
    jack.forward(100)

You're mixing the functional API of turtle with its object-oriented API which is why you need your double import of turtle:

import turtle
from turtle import Turtle, Screen

This only leads to trouble. Let's rewrite the program to use just the object-oriented API and actually work:

from turtle import Turtle, Screen

switchvalue = True

def switch():
    global switchvalue

    switchvalue = not switchvalue

def move():
    delay = 100  # if not moving, slow for human intervention

    if switchvalue:
        jack.forward(1)
        delay = 0  # moving, check back right away

    screen.ontimer(move, delay)  # delay in milliseconds

screen = Screen()

jack = Turtle('turtle')
jack.color('red', 'green')
jack.speed('fastest')
jack.pensize(10)

screen.onkey(switch, 's')
screen.listen()

move()

screen.mainloop()

When the program starts, the turtle will be wandering off to the right. By pressing 's', you can stop and restart this motion.