'Bind fun to mouse-move-event (with pressed mouse button) on turtle.
fun must be a function with two arguments, the coordinates of the
actual mouse position on the canvas.
num, the number of the mouse-button defaults to 1
Every sequence of mouse-move-events on a turtle is preceded by a
mouse-click event on that turtle.'

def _ondrag(self, item, fun, num=1, add=None):

    if (fun is None):
        self.cv.tag_unbind(item, ('<Button%s-Motion>' % num))
    else:
        def eventfun(event):
            try:
                (x, y) = ((self.cv.canvasx(event.x) / self.xscale), ((- self.cv.canvasy(event.y)) / self.yscale))
                fun(x, y)
            except:
                pass
        self.cv.tag_bind(item, ('<Button%s-Motion>' % num), eventfun, add)
