Confused about Tkinter bind_class

Question

I define GCanvas, an extension of Canvas. My intention is to bind to GCanvas at the class level. It isn't working.

I also tried to bind to tk.Canvas and it doesn't work either. Binding to root or to the GCanvas instance works fine. (Neither of this alternatives is useful to me, but I just tried them to see what happened.). Running OS X, El Capitan.

import Tkinter as tk

class GCanvas(tk.Canvas, object):

    def __init__(self, master, **kwargs):
        tk.Canvas.__init__(self, master, kwargs)

    @staticmethod
    def enter(e):
        print "enter", e.widget, e.x, e.y

    @staticmethod
    def leave(e):
        print "leave", e.widget

    @staticmethod
    def motion(e):
        print "motion", e.widget, e.x, e.y

approach = "bindinstance"

root = tk.Tk()
gc = GCanvas(root, width=400, height=300)
print "root is", root, "gc is", gc
gc.pack()

if approach == "bindGCanvas":
    print "binding to GCanvas"
    root.bind_class(GCanvas, '<Enter>', GCanvas.enter)
    root.bind_class(GCanvas, '<Leave>', GCanvas.leave)
    #root.bind_class(GCanvas, '<Motion>', GCanvas.motion)
elif approach == "bindCanvas":
    print "binding to Canvas"
    root.bind_class(tk.Canvas, '<Enter>', GCanvas.enter)
    root.bind_class(tk.Canvas, '<Leave>', GCanvas.leave)
    #root.bind_class(tk.Canvas, '<Motion>', GCanvas.motion)
elif approach == "bindinstance":
    print "binding to instance"
    gc.bind('<Enter>', GCanvas.enter)
    gc.bind('<Leave>', GCanvas.leave)
    #gc.bind('<Motion>', GCanvas.motion)
else:
    print "binding to root"
    root.bind('<Enter>', GCanvas.enter)
    root.bind('<Leave>', GCanvas.leave)
    #root.bind('<Motion>', GCanvas.motion)

root.mainloop()

Answer 1

The "class" in bind_class refers to the internal class name used by the tk library, not the python class name. More precisely, in this context it refers to a bind tag, which happens to be the same name as the tk class, which also happens to be the same name as one of the core Tkinter classes (eg: Toplevel, Canvas, etc).

To bind to GCanvas at the class level, the simplest thing would be to add a bind tag named GCanvas to your canvas, as in the following example:

class GCanvas(tk.Canvas, object):
    def __init__(self, master, **kwargs):
        ...
        # get the current bind tags
        bindtags = list(self.bindtags())

        # add our custom bind tag before the Canvas bind tag
        index = bindtags.index("Canvas")
        bindtags.insert(index, "GCanvas")

        # save the bind tags back to the widget
        self.bindtags(tuple(bindtags))

You can then use bind_class like so:

root.bind_class("GCanvas", "<Enter>", GCanvas.enter)
root.bind_class("GCanvas", "<Leave>", GCanvas.leave)

For more information about bind tags, see these answers to some other tkinter questions:

• https://stackoverflow.com/a/11542200/7432
• https://stackoverflow.com/a/3513906/7432