Update label of tkinter menubar item?

I do not know if that used to be different on 2.7, but it does not work on 3.4 anymore.

On python 3.4 you should start counting entries with 0 and use entryconfig.

menu.entryconfig(0, label = "Clicked!")

http://effbot.org/tkinterbook/menu.htm

Check this dynamic menu example. The main feature here is that you don't need to care about a serial number (index) of your menu item. No index (place) of your menu is needed to track. Menu item could be the first or the last, it doesn't matter. So you could add new menus without index tracking (position) of your menus.

The code is on Python 3.6.

# Using lambda keyword and refresh function to create a dynamic menu.
import tkinter as tk

def show(x):
    """ Show your choice """
    global label
    new_label = 'Choice is: ' + x
    menubar.entryconfigure(label, label=new_label)  # change menu text
    label = new_label  # update menu label to find it next time
    choice.set(x)

def refresh():
    """ Refresh menu contents """
    global label, l
    if l[0] == 'one':
        l = ['four', 'five', 'six', 'seven']
    else:
        l = ['one', 'two', 'three']
    choice.set('')
    menu.delete(0, 'end')  # delete previous contents of the menu
    menubar.entryconfigure(label, label=const_str)  # change menu text
    label = const_str  # update menu label to find it next time
    for i in l:
        menu.add_command(label=i, command=lambda x=i: show(x))

root = tk.Tk()
# Set some variables
choice = tk.StringVar()
const_str = 'Choice'
label = const_str
l = ['dummy']
# Create some widgets
menubar = tk.Menu(root)
root.configure(menu=menubar)
menu = tk.Menu(menubar, tearoff=False)
menubar.add_cascade(label=label, menu=menu)
b = tk.Button(root, text='Refresh menu', command=refresh)
b.pack()
b.invoke()
tk.Label(root, textvariable=choice).pack()
root.mainloop()

I found the solution myself in the Tcl manpages:

Use the entryconfigure() method like so, which changes the value after it has been clicked:

The first parameter 1 has to be the index of the item you want to change, starting from 1.

from tkinter import *

root = Tk()
menu_bar = Menu(root)

def clicked(menu):
    menu.entryconfigure(1, label="Clicked!")

file_menu = Menu(menu_bar, tearoff=False)
file_menu.add_command(label="An example item", command=lambda: clicked(file_menu))
menu_bar.add_cascade(label="File", menu=file_menu)

root.config(menu=menu_bar)
root.mainloop()