how to take only str as input with entry in tkinter code example

Example 1: read value from entry tkinter

#ent is the name of the Entry
s = ent.get()

Example 2: write in entry() in tkinter

from Tkinter import *

master = Tk()

e = Entry(master)
e.pack()

e.focus_set()

def callback():
    print e.get()

b = Button(master, text="get", width=10, command=callback)
b.pack()

mainloop()