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()

score 6 · Accepted Answer · edited May 23 '17 at 12:00

6

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:

edited May 23 '17 at 12:00

Community

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answered Nov 04 '16 at 14:37

Bryan Oakley

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Thanks! So I don't need to define a Python class at all; I just need to add to my canvases a unique bindtag, right? – Eduardo Nov 04 '16 at 16:57
@Eduardo: correct. I recommend using a class to create a new type of canvas, but strictly speaking you don't _need_ to. – Bryan Oakley Nov 04 '16 at 17:11
What if I had a custom `Frame` and inside this frame, there is other widgets. How would I make a bind such that the bind works where I click on this custom frame? – Delrius Euphoria Dec 29 '21 at 17:13
@CoolCloud: you can add the `GCanvas` tag to the bindtags of the frame. – Bryan Oakley Dec 29 '21 at 17:17
I did add that, but when `Label` take focus inside the frame, the bindings no longer work – Delrius Euphoria Dec 29 '21 at 17:24
@CoolCloud: that's because the bindings only work for widgets with the bindtag. You asked about clicking on the frame, not on clicking on a label in the frame. – Bryan Oakley Dec 29 '21 at 17:48
Hmmm okay, ill add for all/each widget – Delrius Euphoria Dec 29 '21 at 17:49

score 0 · Answer 2 · edited Feb 22 '23 at 14:48

An express solution I found to resolve a binding conflict between two text() boxes. Listing the bindtags to sort the path of the widget. The first occurrence in list. The binding was triggered on both widgets at the same time.

self.frame2 = Frame(self)
self.frame2.pack()
self.textDomain = Text(self.frame2, width=21, height=10, wrap=WORD, borderwidth=2, relief=GROOVE)
self.textDomain.pack()
self.bindtags = list(self.textDomain.bindtags())
self.textDomain.bind_class(str(self.bindtags[0]), "<Return>", lambda _: self.verifytextBox("textDomain"))
self.textDomain.bind_class(str(self.bindtags[0]), "<Leave>", lambda _: self.verifytextBox("textDomain"))
self.textExtension = Text(self.frame2, width=21, height=10, wrap=WORD, borderwidth=2, relief=GROOVE)
self.textExtension.pack()
self.bindtags = list(self.textExtension.bindtags())
self.textExtension.bind_class(str(self.bindtags[0]), "<Return>", lambda _: self.verifytextBox("textExtension"))
self.textExtension.bind_class(str(self.bindtags[0]), "<Leave>", lambda _: self.verifytextBox("textExtension"))

Confused about Tkinter bind_class

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