title: AttributeError in tkinter
body: <p>I am trying to write a program with tkinter. I have not finished the program but trying to run it just to see how my window will look, I get an error inside of tkinter.</p>
  
  <p>I am not sure what to do now. Anybody know what it wrong? </p>
  
  <p>Here is the message</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;string&gt;", line 420, in run_nodebug
    File "&lt;module1&gt;", line 53, in &lt;module&gt;
    File "&lt;module1&gt;", line 50, in main
    File "&lt;module1&gt;", line 23, in __init__
    File "C:\Python33\lib\tkinter\__init__.py", line 2110, in __init__
      Widget.__init__(self, master, 'button', cnf, kw)
    File "C:\Python33\lib\tkinter\__init__.py", line 2036, in __init__
      classes = [(k, v) for k, v in cnf.items() if isinstance(k, type)]
  AttributeError: 'str' object has no attribute 'items'
  
  import tkinter
  import tkinter.messagebox
  
  #---------------------- define GUI class
  class CalcMPG:
      def __init__(self):
          self.main_window = tkinter.Tk()
  
          #-------------- create 3 frames ---------
          self.uframe= tkinter.Frame(self.main_window) #upper frame
          self.mframe= tkinter.Frame(self.main_window) #middle frame
          self.bframe= tkinter.Frame(self.main_window) #button frame
  
          #------------ create the 3 label widgets ------------
          self.lblgal= tkinter.Label(self.uframe, text="Enter # of gallons used")
          self.lblmiles= tkinter.Label(self.mframe, text="Enter miles travelled")
  
          #------------ create the 2 Entry widgets ------------
          self.entgal= tkinter.Entry(self.uframe, width=10)
          self.entmiles= tkinter.Entry(self.mframe, width=10)
  
          #------------ create the 2 Button widgets -----------
          self.mpgbtn = tkinter.Button(self.bframe, "Calcualte MPG")
          self.extbtn = tkinter.Button(self.bframe, "Exit")
  
  
          #-------- pack upper frame -----------
          self.lblgal.pack(side='left')
          self.entgal.pack(side='right')
  
          #------- pack middle frame ----------
          self.lblmiles.pack(side='left')
          self.entmiles.pack(side='right')
  
          #------- pack bottom frome ----------
          self.mpgbtn.pack(side= 'left')
          self.extbtn.pack(side= 'right')
  
  
          #------- pack frames --------
          self.uframe.pack(side='top')
          self.mframe.pack(side='top')
          self.bframe.pack(side='top')
  
  
          tkinter.mainloop()
  
  #--------------- define main function  ----
  def main():
      calcmpg = CalcMPG()
  
  #--------- invoke main function -------
  main()
  </code></pre>