code: def callback(self, event):
          
          self.init_width()
  
          if len(self.initial) > 0:
              for cell in self.initial:
                  self.color_square(cell[0], cell[1], True)
              self.initial = []
          self.begin_drag = event
          self.color_square(event.x, event.y)
docs: Selects cells on click.