Python 3 Turtle - Screen Events and Coords

[peteralien]

This program moves a Turtle in 4 directions using keys. I want to control the Turtle collision with the Screen boundaries too. But I'm having a very weird problem!

For example, when I move the Turtle to the right side it works OK, but when I turn the Turtle to the left side, graphically it turns OK but the coordinate values that Print command gives me, instead decreasing the X-coordinate value, increase one time it and then start to decrease!
Of course this messes up with the collision control I'm trying to create!

I just tested everything I could think of but so far got no luck!

I'm using Python 3.7.7 with Turtle graphics, in Thonny 3.2.7. I tested it in Repl.it and the result was the same!


Here is the code:

import turtle

def turtleUp():
  t1.setheading(90)
  if not colisao():
    t1.sety(t1.ycor() + 10)

def turtleDown():
  t1.setheading(270)
  if not colisao():
    t1.sety(t1.ycor() - 10)

def turtleRight():
  t1.setheading(0)
  if not colisao():
    t1.setx(t1.xcor() + 10)

def turtleLeft():
  t1.setheading(180)
  if not colisao():
    t1.setx(t1.xcor() - 10)
  
def colisao():
  print(t1.xcor(), t1.ycor())
  if t1.xcor() < -470 or t1.xcor() > 460 or t1.ycor() < -370 or t1.ycor() > 360:
    return True
  else:
    return False

screen_width = 1000
screen_height = 800

s = turtle.Screen()
s.setup(screen_width, screen_height)
s.bgcolor("lime")

t1 = turtle.Turtle()
t1.speed(0)
t1.shape("turtle")
t1.color("black")
t1.up()
t1.goto(0, 0)

s.onkeypress(turtleUp, "w")
s.onkeypress(turtleDown, "s")
s.onkeypress(turtleRight, "p")
s.onkeypress(turtleLeft, "o")

s.listen()

s.mainloop()