import tkinter as tk

class Lotfi(tk.Entry):
    def __init__(self, master=None, **kwargs):
        self.var = tk.StringVar()
        tk.Entry.__init__(self, master, textvariable=self.var, **kwargs)
        self.old_value = ''
        self.var.trace('w', self.check)
        self.get, self.set = self.var.get, self.var.set

    def check(self, *args):
        if self.get().isdigit():
            # the current value is only digits; allow this
            self.old_value = self.get()
        else:
            # there's non-digit characters in the input; reject this
            self.set(self.old_value)

#demo:
window = tk.Tk()
From_entry=Lotfi(window, width=25)
From_entry.grid(column=1,row=2,padx=5)
window.mainloop()