'returns list of coordinate-pairs of points of item
Example (for insiders):
>>> from turtle import *
>>> getscreen()._pointlist(getturtle().turtle._item)
[(0.0, 9.9999999999999982), (0.0, -9.9999999999999982),
(9.9999999999999982, 0.0)]'

def _pointlist(self, item):

    cl = self.cv.coords(item)
    pl = [(cl[i], (- cl[(i + 1)])) for i in range(0, len(cl), 2)]
    return pl
