from Tkinter import *

class MyFrame(Frame):
    def __init__(self, parent):
        Frame.__init__(self, parent)
        self.text = Label(self, width=40, height=2, text='Name')
        self.text.pack(side="top", fill="both", expand=True)

        self.namevar = StringVar()
        self.namevar.trace('w', self.validate)
        self.name = Entry(self, textvariable=self.namevar)
        self.name.pack()
        self.name.focus_set()

        self.enter = Button(self, text='Enter', width=10,
                            state=DISABLED,command=self.callback)
        self.enter.pack()

        self.entered = Label(self, text='You entered: ')
        self.entered.pack()

    def callback(self):
        self.entered.config(text='You entered: ' + self.name.get())
        self.name.delete(0, END)

    def validate(self, name, index, mode):
        self.enter.config(state=(NORMAL if self.namevar.get() else DISABLED))
        return True

root = Tk()
frame = MyFrame(root)
frame.pack()
root.mainloop()