title: Python Tkinter can't select option in optionmenu
body: <p>I am having the problem where I am able to populate two optionmenus using Tkinter, (the second one dynamically based off the first), but I notice when I try to select one of the values in the second optionmenu, it does not allow me to select. I have noticed on the same GUI, sometimes when running another function, it will have this affect on a different option menu that worked fine prior to running the function. The options will show correctly, and the mouse can scan over them, but when you click on one it doesn't show that it has been selected, or execute the command set to it. Has anyone had this problem before? </p>
  
  <p><img src="https://i.stack.imgur.com/dSmLn.png" alt="enter image description here">
  Ok, so my hopes of someone else having the same issue are going down, and I'll include more code by request of some of the responders in case this might shed light on the issue. I'll try and pull everything applicable:</p>
  
  <pre><code>class GUI(Frame):
  
      def __init__(self, parent):
          Frame.__init__(self, parent)
          self.parent = parent
          self.build_gui()
      #reads the tabs on an excel and inputs the tab names as the values in the first optionmenu (this works)
      def read_board_tabs(self):
          #many of these variables in the function may not be defined. Ignore them as they are taken out of context. This should just help visually see the structure.
          filepath = 'Board Control.xlsx'
          wkbk = load_workbook((filepath))
          sheets = wkbk.get_sheet_names()
          print sheets
          return sheets
  
      #reads content of that excel tab chosen to populate the second optionmenu (populating the option menu works, but they can't be selected once populated)
      def read_serials(self, board):
          #many of these variables in the function may not be defined. Ignore them as they are taken out of context. This should just help visually see the structure.
          sheet = board
          if(sheet == ''):
              return ['']
          else:
              filepath = 'Board Control.xlsx'
              workbook = excel_transfer.workbook(filepath)
              wb = workbook.open_existing_workbook()
              ws = workbook.activate_specific_worksheet(wb, sheet)
              row = 1
              current_row = 0
              previous_row = 0
              serials = []
              #read the serials that exist for the board selected
              while current_row != None:
                  previous_row = current_row
                  row = row + 1
                  cell = 'A' + str(row)
                  current_row = workbook.read_new_cell(cell, ws)
                  if(current_row != None):
                      serials.append(current_row)
              if(len(serials) == 0):
                  serials = ['']
              self.BOARD_SERIAL_OPTION['menu'].delete(0, 'end')
              for item in serials:
                  self.BOARD_SERIAL_OPTION['menu'].add_command(label=item)  
  
      #this is the command that won't execute when I try to select the second optionmenu value
      def find_board_info(self, serial):
          #many of these variables in the function may not be defined. Ignore them as they are taken out of context. This should just help visually see the structure.
          self.board_checkout_status_var.set('')
          sheet = (self.boards_var).get()
          new_search = StringFound.StringSearch(serial)
          results = new_search.read_board(sheet)
          if(results == ['']):
              self.boardstatusvar.set('No board was found')
          else:
              self.boardstatusvar.set('Board: %s Serial: %s' %(results[1], serial))
              self.boardstatus_locationvar.set('Found in file: %s' %results[0])
              self.boards_var.set('%s serial %s' %(results[1], serial))
              self.dispositionvar.set(results[3])            
              self.TXvar.set(results[5])  
              self.RXvar.set(results[6])   
              self.lastvar.set(results[10])  
              self.datevar.set(results[9])
              if(results[14] != None):
                  self.currentvar.set(results[10]) 
              self.locationvar.set(results[4])     
              self.imagevar.set(results[8]) 
              self.notesvar.set(results[12]) 
          self.current_board_chosen = [sheet, serial, results[15]]
  
      #creates the gui
      def build_gui(self):
          n = Notebook(self)
          board_process = Tkinter.LabelFrame(self, text="Board Updates")
          n.add(board_process, text='Board Signout')
          n.pack()
          self.boards_var = StringVar()
          self.boards_var.set("")
          self.serials_var = StringVar()
          self.serials_var.set("")
          self.SEARCHBOARDFRAME = Tkinter.LabelFrame(board_process, text='Find Board')
          self.SEARCHBOARDFRAME.grid(row=0, column=0, sticky='WE')
          self.BOARD_SEARCH_LABEL = Label(self.SEARCHBOARDFRAME, text='Type:')
          self.BOARD_SEARCH_LABEL.grid(row=0, column=0, sticky='W', padx=5, pady=2)
          self.BOARD_SEARCH_OPTION = OptionMenu(self.SEARCHBOARDFRAME, self.boards_var, *self.list_of_boards, command=self.read_serials) 
          self.BOARD_SEARCH_OPTION.grid(row=0, column=1, sticky='W', padx=5, pady=2)   
          self.BOARD_SERIAL_LABEL = Label(self.SEARCHBOARDFRAME, text='Serial:')
          self.BOARD_SERIAL_LABEL.grid(row=1, column=0, sticky='W', padx=5, pady=2)
          self.BOARD_SERIAL_OPTION = OptionMenu(self.SEARCHBOARDFRAME, self.serials_var, *self.list_of_serials, command=self.find_board_info)
          self.BOARD_SERIAL_OPTION.grid(row=1, column=1, sticky='W', padx=5, pady=2)
  
  if __name__ == '__main__':
      root = Tk()
      app = GUI(root)
      root.mainloop()
  </code></pre>