code: def clicked(self, px, py):
          '''see if the polygon has been clicked on.
          Consider it clicked if the pixel is within 6 of the point
          '''
          if self.hidden:
              return None
          for i in range(len(self._pix_points)):
              if self._pix_points[i] is None:
                  continue
              (pixx,pixy) = self._pix_points[i]
              if abs(px - pixx) < 6 and abs(py - pixy) < 6:
                  self._selected_vertex = i
                  return math.sqrt((px - pixx)**2 + (py - pixy)**2)
          return Nonf clicked(self, px, py):
          '''see if the polygon has been clicked on.
          Consider it clicked if the pixel is within 6 of the point
          '''
          if self.hidden:
              return None
          for i in range(len(self._pix_points)):
              if self._pix_points[i] is None:
                  continue
              (pixx,pixy) = self._pix_points[i]
              if abs(px - pixx) < 6 and abs(py - pixy) < 6:
                  self._selected_vertex = i
                  return math.sqrt((px - pixx)**2 + (py - pixy)**2)
          return None
docs: see if the polygon has been clicked on.
          Consider it clicked if the pixel is within 6 of the point