def to_float(self):
if self.points_possible == <NUM_LIT:0>:<EOL><INDENT>return None<EOL><DEDENT>return float(self.points_earned) / self.points_possible<EOL>
Calculate (points earned) / (points possible).
If points possible is None (e.g. this is a "hidden" score)
then return None.

Returns:
    float or None