python - Turtle - screen.update() not working with copy method? - Stack Overflow

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I have a list with several turtle instances. I want to create a copy of one object and let this one move forward.

This works fine. But when I disable screen udpates and do it manually in my code it gives me a different result and I don't see the turtle on its new position. But I do see that it has moved there.

Here is an example. This produces the expected output. But as soon as I disable tracer and do a manual update it behaves weirdly.

from turtle import Screen, Turtle
from copy import copy


screen = Screen()
screen.setup(width=250, height=250)
#screen.tracer(0)

turtles = [Turtle(), Turtle()]

for i in range(10):
    #screen.update()

    new_turtle = copy(turtles[0])

    new_turtle.forward(10)
    turtles.append(new_turtle)

screen.exitonclick()

I have a list with several turtle instances. I want to create a copy of one object and let this one move forward.

This works fine. But when I disable screen udpates and do it manually in my code it gives me a different result and I don't see the turtle on its new position. But I do see that it has moved there.

Here is an example. This produces the expected output. But as soon as I disable tracer and do a manual update it behaves weirdly.

from turtle import Screen, Turtle
from copy import copy


screen = Screen()
screen.setup(width=250, height=250)
#screen.tracer(0)

turtles = [Turtle(), Turtle()]

for i in range(10):
    #screen.update()

    new_turtle = copy(turtles[0])

    new_turtle.forward(10)
    turtles.append(new_turtle)

screen.exitonclick()

• python
• turtle-graphics
• python-turtle

Share Improve this question Follow edited Feb 24 at 20:27 mkrieger1 23.2k77 gold badges6464 silver badges8080 bronze badges asked Feb 24 at 20:21 Denny CraneDenny Crane 65911 gold badge77 silver badges1919 bronze badges 7

• Is it intentional that you create 10 copies, not one copy? – mkrieger1 Commented Feb 24 at 20:29
• It doesn't matter if 1, 10 or 100 copies. The code is just an example there. But it is supposed to work that way to use this approach in a more complex way. I boiled it down to the problem. :) – Denny Crane Commented Feb 24 at 21:11
• Try using the turtle's .clone() method, rather than the generic copy.copy() - it's more likely to know how to properly do the job. – jasonharper Commented Feb 24 at 21:15
• on Linux it doesn't work at all. It needs [-1] instead of [0]. And with copy() it moves all turtles to the same position - it would need deepcopy but it raise error. Only clone() gives expected result - I see may turtles (before using screen.update()) – furas Commented Feb 24 at 22:53
• screen.update() should happen at the end of the loop, not the start, otherwise the last turtle in the loop will not be updated. Taking a step back, what feature are you trying to implement at a high level? There may be a better way than this if you don't mind providing context. – ggorlen Commented Feb 24 at 23:00

 |  Show 2 more comments

1 Answer 1

Sorted by: Reset to default 1

First: I had to use [-1] instead of [0] to see turtles in one line.

---

On my Linux it never works correctly with copy(). With trace(0) and without trace(0) it shows two turtles on screen (or both in the same place so I see only one). And screen.turtles() also shows only two turtles.

for turtle in screen.turtles():
    print(f'turtle: {turtle} : {turtle.pos()}')

turtle: <turtle.Turtle object at 0x72963525dfd0> : (0.00,0.00)
turtle: <turtle.Turtle object at 0x729635013d90> : (0.00,0.00)

I had to assign new _TurtleImage to turtle and append it to screen to see it on screen.
I found this in source code of .clone()

for i in range(10):

    new_turtle = copy(turtles[-1])
    new_turtle.turtle = _TurtleImage(screen, turtles[-1].turtle.shapeIndex)
    screen._turtles.append(new_turtle)

    new_turtle.forward(10)
    
    turtles.append(new_turtle)

    screen.update()

But in source code of .clone() you can see it makes other things.
And code always works for me when I use .clone() instead of copy().
And code is cleaner and shorter with .clone().

for i in range(10):

    new_turtle = turtles[-1].clone()    # works

    new_turtle.forward(10)
    
    turtles.append(new_turtle)

    screen.update()

---

I don't know why original code works for you without trace(0)
but I think standard method (without trace(0)) may do something more than only run update()
but I didn't find it in source code.

---

My full code which I used for test - with few other ideas in comments

from turtle import Screen, Turtle, _TurtleImage
from copy import copy, deepcopy


screen = Screen()
#screen.setup(width=250, height=250)
screen.tracer(0)

turtles = [Turtle(), Turtle()]

for i in range(10):

    #new_turtle = copy(turtles[-1])       # all turtles use the same position
    #new_turtle.screen = turtles[-1].screen
    #new_turtle.turtle = _TurtleImage(screen, turtles[-1].turtle.shapeIndex)
    #screen._turtles.append(new_turtle)

    #new_turtle = deepcopy(turtles[-1])  # raise error
    new_turtle = turtles[-1].clone()    # works

    new_turtle.forward(10)
    #new_turtle._update_data()
    #new_turtle._drawturtle()
    
    turtles.append(new_turtle)

    screen.update()

for index, t in enumerate(turtles, 1):
    print(f'{index:2}: {t.getscreen()} : {t.pos()}')
    
for turtle in screen.turtles():
    print(f'turtle: {turtle} : {turtle.pos()}')
        
screen.exitonclick()

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