AttributeError：''对象没有属性'tk'？

Question

这样做的目的是让一个窗口从用户那里获取一个zipcode并保存。保存邮政编码后，它将打开一个新窗口，使用该邮政编码显示各种天气信息。

class EntryW(tk.Frame):
    def __init__( self, master ):
      self.master = master
      self.frame = tk.Frame(self.master)     
      self.master.title("Weather by Josh")    

      self.label =  Label(self, text = "Enter Zipcode")
      self.label.pack(side = LEFT)
      global zipcodeEntryVar
      zipcodeEntryVar = tk.StringVar()

      self.entry = Entry(self,textvariable=zipcodeEntryVar)
      self.entry.pack(side = RIGHT)

      self.button1 = Button( self, text = "Submit", 
                           command +self.new_window )
      self.button1.pack(side = BOTTOM )
      self.frame.pack()

   def new_window(self):

     self.newWindow = tk.Toplevel(self.master)
     self.app = WeatherApp(self.newWindow)

class WeatherApp(tk.Frame):     
      def __init__(self, master):

         self.master = master
         self.frame = tk.Frame(self.master)
         self.quitButton = tk.Button(self.frame, text = 'Quit', 
                                    width = 25, command = self.close_windows)
         self.quitButton.pack()
         self.frame.pack()

      def close_windows(self):
         self.master.destroy()

def main(): 
    root = tk.Tk()
    app = EntryW(root)
    root.mainloop()

if __name__ == '__main__':
    main()