Python Tkinter canvas class inheritance

Question

Using Tkinter 8.6, Python 3.7.3:

A friendly user here instructed me on how to have an image act like a button by the way of creating an Imgbutton class that is a subclass of Tkinter Canvas.

I have some questions regarding this code, here is the simplified version of it:

#!/usr/local/bin/python3

import tkinter as tk
from PIL import Image, ImageTk

class Imgbutton(tk.Canvas):

    def __init__(self, master=None, image=None, command=None, **kw):

        super(Imgbutton, self).__init__(master=master, **kw)

        self.set_img = self.create_image(0, 0, anchor='nw', image=image)

        self.bind_class( self, '<Button-1>',
                    lambda _: self.config(relief='sunken'), add="+")

        self.bind_class( self, '<ButtonRelease-1>',
                    lambda _: self.config(relief='groove'), add='+')

        self.bind_class( self, '<Button-1>',
                    lambda _: command() if command else None, add="+")

Questions:

1. When I create an Imgbutton object, the separated line above gets executed but I do not understand why.
2. Does the self.set_img correspond to an object of Imgbuttonor tk.Canvas class?
3. Is there any point here where an actual canvas is created? I believed you need to create a canvas before you can add anything to it.

---

This part might be unneccessary to mention but here I am creating an Imgbuttonobject:

root = tk.Tk()

but_img = tk.PhotoImage(file='button.png')

but = Imgbutton(root, image=but_img, width=but_img.width(),
            height=but_img.height(), borderwidth=2, highlightthickness=0)
but.pack()

root.mainloop()

Answer 1

When I create an Imgbutton object, the separated line above gets executed but I do not understand why.

It's executed because it's part of the code. I'm not sure why you think it wouldn't be called. If you don't want it to be called, move it outside of the __init__ method.

Does the self.set_img correspond to an object of Imgbuttonor tk.Canvas class?

self refers to the instance of the Imgbutton class. set_img will be the identifier returned by the canvas when it creates the object on the canvas.

Is there any point here where an actual canvas is created?

Yes. Imgbutton is the canvas. That is how inheritance works: Imgbutton is a Canvas, with some enhancements. It gets created when you do but = Imgbutton(...). Though, perhaps a bit more accurately the actual canvas is created when you call super, which tells tkinter to create the object.