Subclasses may override this method.

def _set_bcpIn(self, value):
        """
        Subclasses may override this method.
        """
        x, y = absoluteBCPIn(self.anchor, value)
        segment = self._segment
        if segment.type == "move" and value != (0, 0):
            raise FontPartsError(("Cannot set the bcpIn for the first "
                                  "point in an open contour.")
                                 )
        else:
            offCurves = segment.offCurve
            if offCurves:
                # if the two off curves are located at the anchor
                # coordinates we can switch to a line segment type.
                if value == (0, 0) and self.bcpOut == (0, 0):
                    segment.type = "line"
                    segment.smooth = False
                else:
                    offCurves[-1].x = x
                    offCurves[-1].y = y
            elif value != (0, 0):
                segment.type = "curve"
                offCurves = segment.offCurve
                offCurves[-1].x = x
                offCurves[-1].y = y