[Python] Why isn't there a screen that pops up to show the snake game?

Why isn't there a screen that pops up to show the snake game? I don't see the issue with my code. Here is my code if it can help.

from turtle import Screen, Turtle

from snake import Snake

import time


screen = Screen()

screen.setup(width=600, height=600)

screen.bgcolor("black")

screen.title("My Snake Game")

screen.tracer(0)

snake = Snake()

screen.listen()

screen.onkey(snake.move_up, "Up")

screen.onkey(snake.move_down,"Down")

screen.onkey(snake.move_left, "Left")

screen.onkey(snake.move_right,"Right")

game_is_on = True

while game_is_on:

screen.update()

time.sleep(0.1)

snake.move()

screen.exitonclick()


Also there is a side file that I've created called snake.py that I use in the main.py file, here it is :

from turtle import Turtle

STARTING_POSITIONS = [(0, 0), (-20, 0), (-40, 0)]

MOVE_DISTANCE = 20

UP = 90

DOWN = 270

LEFT = 180

RIGHT = 0

class Snake:

def __init__(self):
self.segments = []
self.create_snake()
self.head = self.segments[0]
def create_snake(self):
for position in STARTING_POSITIONS:
new_segment = Turtle("square")
new_segment.color("white")
new_segment.penup()
new_segment.goto(position)
self.segments.append(new_segment)


def move(self):
for seg_num in range(len(self.segments) - 1, 0, -1):
new_x = self.segments[seg_num - 1].xcor()
new_y = self.segments[seg_num - 1].ycor()
self.segments[seg_num].goto(new_x, new_y)
self.head.forward(MOVE_DISTANCE)

def move_left(self):
if self.head.heading() != RIGHT:
self.head.setheading(LEFT)

def move_right(self):
if self.head.heading() != LEFT:
self.head.setheading(RIGHT)

def move_up(self):
if self.head.heading() != DOWN:
self.head.setheading(UP)

def move_down(self):
if self.head.heading() != UP:
self.head.setheading(DOWN)

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