Python Turtle Multiple Click Events

Question

I'm trying to make a turtle version of the 8-Queens Puzzle using Python Turtle.

I've make a start, but have hit a block with the fact that the click event on a custom Turtle object seems to only fire once. I know screen click events fire multiple times, so is this a feature of Turtle instances? What am I missing please?

import turtle

screen = turtle.Screen()
screen.reset()
SIZE = 40
screen.register_shape('box', ((-SIZE/2, SIZE/2), (SIZE/2, SIZE/2), (SIZE/2, -SIZE/2), (-SIZE/2, -SIZE/2)))
screen.register_shape('images/queenlogo40x40.gif')

class Box(turtle.Turtle):
    def __init__(self, x=0, y=0, place_color='green'):
        super(Box, self).__init__()
        self.place_color = place_color
        self.speed(0)
        self.penup()
        self.shape("box")
        self.color(place_color)
        self.setpos(x, y)
        self.has_queen = False
        self.onclick(self.click_handler)

    def click_handler(self, x, y):
        print("start:" , self.has_queen)

        if self.has_queen:
            self.shape('box')
            self.has_queen = False
        else:
            self.shape('images/queenlogo40x40.gif')
            self.has_queen = True

        print("end:" , self.has_queen)


    def __str__(self):
        """ Print piece details """
        return "({0}, {1}), {2}".format(self.xcor(), self.ycor(), self.place_color())

Edit: I can fix this by adding self.onclick(self.click_handler) to the click handler, but that just seems wrong. I'm sure I've seen similar functionality without needing to rebind the event each time it's used.

cdlane · Accepted Answer · 2018-04-25 19:47:07Z

Your example should function correctly, I don't see a conceptual problem.

But there's a glitch in turtle. The information about onclick is stored on the turtle's _item property:

self.screen._onclick(self.turtle._item, fun, btn, add)

But when you change a turtle's shape from an image to a polygon, or vice versa, it destroys the _item property:

if self._type in ["image", "polygon"]:
        screen._delete(self._item)

So your binding is lost. Note that if you change the line:

self.shape('images/queenlogo40x40.gif')

to instead be:

self.shape('turtle')

the code works fine, as you're going from polygon to polygon and _item is preserved. So adding self.onclick(self.click_handler) after the shape change is necessary when going between polygon and image.

I've reworked your code slightly below to address a couple of unrelated issues (e.g. fixed super() call for Python 3; removed incorrect parens in __str__() code.)

from turtle import Turtle, Screen

SIZE = 40

class Box(Turtle):
    def __init__(self, x=0, y=0, place_color='green'):
        super().__init__('box')

        self.speed('fastest')
        self.color(place_color)
        self.place_color = place_color
        self.has_queen = False
        self.penup()
        self.setpos(x, y)

        self.onclick(self.click_handler)

    def click_handler(self, x, y):

        print("start:", self.has_queen)

        if self.has_queen:
            self.shape('box')
        # else:
            # self.shape('turtle')
        else:
            self.shape('queenlogo40x40.gif')

        self.has_queen = not self.has_queen
        self.onclick(self.click_handler)  # redo since self.shape() may undo this

        print("end:", self.has_queen)

    def __str__(self):
        """ Print piece details """
        return "({0}, {1}), {2}".format(self.xcor(), self.ycor(), self.place_color)

screen = Screen()
screen.register_shape('box', ((-SIZE/2, SIZE/2), (SIZE/2, SIZE/2), (SIZE/2, -SIZE/2), (-SIZE/2, -SIZE/2)))
screen.register_shape('queenlogo40x40.gif')

tortoise = Box()

screen.mainloop()

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