Subclasses may override this method.

def _interpolate(self, factor, minInfo, maxInfo, round=True, suppressError=True):
        """
        Subclasses may override this method.
        """
        minInfo = minInfo._toMathInfo()
        maxInfo = maxInfo._toMathInfo()
        result = interpolate(minInfo, maxInfo, factor)
        if result is None and not suppressError:
            raise FontPartsError(("Info from font '%s' and font '%s' could not be "
                                  "interpolated.")
                                 % (minInfo.font.name, maxInfo.font.name))
        if round:
            result = result.round()
        self._fromMathInfo(result)