title: tkinter button limit command to not repeat
body: <p>Hello I'm trying to make a countdown timer app in tkinter and I'm using this class to make the button then to start the countdown. I'm quite new to python and also on tkinter.</p>
  
  <p>The problem is after I press the button more than once it executes refresh_label more so it count downs faster. </p>
  
  <p>How can I stop this from happening?</p>
  
  <pre><code>    class Countdown():
      def __init__(self,minutes, parent):
  
          self.minutes = minutes
          self.parent = parent
          self.label = Label(parent,text='00:00',font=("Dense", 30),width=10,
                  bg= '#e74c3c')
          self.label.place(x= 245, y= 330)
          self.my_time = dt.time(0, self.minutes, 0)
          self.var = 0
          self.start = Button(self.parent,font=('Dense',15),text="Start", height = 4,
                  width = 51, fg = "#a1dbcd", bg="#e74c3c", command = self.refresh_label)
          self.start.grid(row= 1,column=0, pady=415)
  
      def refresh_label(self):
          # This is the method that starts the countdown. I convert my_time 
          # from datetime.time object to datetime.datetime then i subtract
          # a second in each 1000 ms and I refresh the text of the button 
          self.var += 1
          second = (dt.datetime.combine(dt.date(1, 1, 1), self.my_time)- 
          dt.timedelta(seconds = self.var)).time()
          self.label.configure(text=second)
          self.label.after(1000, self.refresh_label)
  </code></pre>