Combobox enable ctrl+a to select text

I would like to enable ctrl+a to select the text within a combobox. Instead of selecting all it does <end> (more or less at least).

minimal example:

#!/usr/bin/env python3
import tkinter as tk
from tkinter import ttk
from tkinter.messagebox import showinfo

root = tk.Tk()


def month_changed(event):
    msg = f'You selected {month_cb.get()}!'
    showinfo(title='Result', message=msg)


# month of year
months = ['Jan', 'Feb']

# create a combobox
selected_month = tk.StringVar()

month_cb = ttk.Combobox(root, textvariable=selected_month)
month_cb['values'] = months
month_cb.pack(fill='x', padx=5, pady=5)

month_cb.bind('<<ComboboxSelected>>', month_changed)
month_cb.bind('<Control-a>', doSomeSelecting) #doSomeSelcting tbd
root.mainloop()

I stole the example and minimized it from here to get a quick example: https://www.pythontutorial.net/tkinter/tkinter-combobox/

Solution

So what you are doing is overriding the default bindings for your platform. On X11 Tk sets up a default binding for Control-Key-slash to generate the <<SelectAll>> virtual event. On Win32 this is extended with Control-Key-a as well. On X11 Control-a is bound to <<LineStart>>.

So the platfrom correct thing is to leave it alone and learn to use Control-slash to select all. To override this you need to bind Control-a to a function that generates the SelectAll virtual event and also prevents the default event handler from then moving the insertion point to the start of the line. For that:

def selall(ev):
    ev.widget.event_generate('<<SelectAll>>')
    return 'break'

month_cb.bind('<Control-a>', selall)

The return 'break' is important here otherwise the event handlers will continue being called and our selection will be undone when something generates the <<LineStart>> event after our <<SelectAll>>.

This can be investigated in IDLE using month_cb.bindtags() to find that it's class bindings are TCombobox. Then month_cb.bind_class('TCombobox') to see all the events that are bound to this class. For the virtual events, root.event_info('<<SelectAll>>') shows the set of events that cause this virtual event to be raised.