Python tkinter bindtag Event handling - how to update which tab is currently selected

Question

I am new to Python and is about to build a GUI using tkinter. The GUI consists of a Notebook and I am trying to create a event handler for when the user click with the mouse on the different tabs. However when I click on a tab, the handlerfunction 'works' but it seems the selected tab is not 'updated' before the function is called.

As a sidenote: So far I have mainly used 'Tkinter 8.5 reference: a GUI for Python (Shipman)'. Please see code below. Grateful for any suggestions!

from tkinter import *
import tkinter.ttk as ttk

root = Tk()
note = ttk.Notebook(root)

tab1 = Frame(note,width = 10)
tab2 = Frame(note,width = 10)
tab3 = Frame(note,width = 10)

note.add(tab1, text = "Tab One")
note.add(tab2, text = "Tab Two")
note.add(tab3, text = "Tab Three")
note.grid()

def personalData(event):
    if event.widget.index("current") == 0:    
       print("One!") 
    else:
       print("Not One!") 

note.bind('<1>',personalData)
root.mainloop()

Answer 1

The reason this is happening is due to the order that events are processed. Custom bindings on widgets are processed before the built-in bindings, meaning that your binding on <1> will fire before the tab actually changes. For more information, research "binding tags" (or sometimes called "bind tags" or "bindtags").

A better solution is to bind on the virtual event <<NotebookTabChanged>> , which the notebook will generate after the tab has changed.

Example:

note.bind("<<NotebookTabChanged>>", personalData)

An additional benefit to binding to the virtual event rather than a mouse click is that the event will fire even if the tab is changed via some other mechanism besides a mouse click. For example, if a tab has the keyboard focus, you can select the next or previous tab using the arrow keys on your keyboard.

Answer 2

You're experiencing this because events of bind are handled before events of bindtags , in other words, you're actually not changing the tab before the callback function to "<1>" event, personalData is handled.

---

One workaround would be to rotate the order of the sequence which the events are being handled with. Add after note.bind(...) :

note.bindtags((note.bindtags()[1:] + note.bindtags()[:1]))

this will put the bind's callback as the last in the sequence and shift the rest to up in the queue.

---

Another way would be to replace the actual event( "<1>" ) to a more appropriate one like in Novel's suggestion .

Answer 3

Note that when you click another tab, the current tab is still the previous tab; if you change your binding to

note.bind('<ButtonRelease-1>',personalData)

it will work right, as now, when the mouse button is released, the current tab is the tab already selected.