[Tkinter] Update value in Entry widget

[Barrowman]

A metulburr said you code is updating so fast the eye cannot register the changes so you only see the last one.
If you import time and changed your example code to this 

def start():
    for n in range(1000000):
        if n%100000 == 0:
            v.set(str(n))
            time.sleep(0.5)

you would see the values changing.