code: def _start_selecting(self, event):
          
          self._selecting = True
          canvas = self._canvas
          x = canvas.canvasx(event.x)
          y = canvas.canvasy(event.y)
          self._sstart = (x, y)
          if not self._sobject:
              self._sobject = canvas.create_rectangle(
                  self._sstart[0], self._sstart[1], x, y,
                  dash=(3,5), outline='#0000ff'
              )
          canvas.itemconfigure(self._sobject, state=tk.NORMAL)
docs: Comienza con el proceso de seleccion.