This must return an int or float.
        If the glyph has no outlines, this must return `None`.

        Subclasses may override this method.

def _get_leftMargin(self):
        """
        This must return an int or float.
        If the glyph has no outlines, this must return `None`.

        Subclasses may override this method.
        """
        bounds = self.bounds
        if bounds is None:
            return None
        xMin, yMin, xMax, yMax = bounds
        return xMin