title: object has no attribute get
body: <p>I am working with the tkinter module in python 3.3 I am relatively new to this and am working with entry boxes. for some reason when I run the following code I get an error message saying AttributeError: 'NoneType' object has no attribute 'get'. Could someone explain to me why? I did a similar program with a single entry that workded just fine.</p>
  
  <pre><code>from tkinter import *
  master =Tk()
  class quad(object):
  def __init__(self, ae, be, ce):
      self.ae = ae
      self.be = be
      self.ce = ce
  
  def calculate(self):
      a = self.ae.get()
      b = self.be.get()
      c = self.ce.get()
      A = float(a)
      B = float(b)
      C = float(c)
      D = (-B)/(2*A)
      E = ((B**2 -4*A*C)**(.5))/(2*A)
      first = D + E
      second = D - E
      print(first, "\n", second)
  Label(master, text='A=').grid(row=0, column=0)
  Label(master, text='B=').grid(row=1, column=0)
  Label(master, text='C=').grid(row=2, column=0)      
  ae = Entry(master).grid(row=0, column=1)
  be = Entry(master).grid(row=1, column=1)
  ce = Entry(master).grid(row=2, column=1)
  model =quad(ae, be, ce)
  Button(master, text='submit', width=10, command=model.calculate).grid(row=3, column=1, sticky=W)
  mainloop()
  </code></pre>