Tkinter get（）不返回值

Question

def updatehw():
   markIntHw = tkinter.DoubleVar()
   dateSrtHw = tkinter.StringVar()

   mark = ttk.Entry(englishGraphsRoot, textvariable=markIntHw, text="Mark")
   dat = ttk.Entry(englishGraphsRoot, textvariable=dateSrtHw, text="Date")

   def getr():
      xv = markIntHw.get()
      xt = dateSrtHw.get()
      cursor.execute('INSERT INTO graph_englishhw(xticks, xvalues) VALUES(?, ?)', (xt, xv))
      connection.commit()
      mark.destroy()
      dat.destroy()
      buttonUpdateHw.destroy()

   buttonUpdateHw = ttk.Button(command=getr, text="Submit")
   buttonUpdateHw.pack()

除了xt和xv不会获取输入的值，而是将0.0和None添加到表中。