我正在尝试用tkinter编写程序.我还没有完成程序,但试图运行它只是为了看看我的窗口看起来如何,我在tkinter中得到一个错误.

我不知道现在该怎么办.谁知道错了什么？

这是消息

Traceback (most recent call last):
  File "Box

#---------------------- 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()