tkinter command callback not called when modifieng varaible code example

Example 1: calling a function in python upon entry content changing tkinter

from tkinter import *

root = Tk()
sv = StringVar()

def callback():
    print(sv.get())
    return True

e = Entry(root, textvariable=sv, validate="focusout", validatecommand=callback)
e.grid()
e = Entry(root)
e.grid()
root.mainloop()

Example 2: calling a function in python upon entry content changing tkinter

from Tkinter import *

def callback(sv):
    print sv.get()

root = Tk()
sv = StringVar()
sv.trace("w", lambda name, index, mode, sv=sv: callback(sv))
e = Entry(root, textvariable=sv)
e.pack()
root.mainloop()