Subclasses may override this method.

def _interpolate(self, factor, minGlyph, maxGlyph,
                     round=True, suppressError=True):
        """
        Subclasses may override this method.
        """
        minGlyph = minGlyph._toMathGlyph()
        maxGlyph = maxGlyph._toMathGlyph()
        try:
            result = interpolate(minGlyph, maxGlyph, factor)
        except IndexError:
            result = None
        if result is None and not suppressError:
            raise FontPartsError(("Glyphs '%s' and '%s' could not be "
                                  "interpolated.")
                                 % (minGlyph.name, maxGlyph.name))
        if result is not None:
            if round:
                result = result.round()
            self._fromMathGlyph(result, toThisGlyph=True)