simple rpg-esque leveling system converted to Python (originally Lua)

level.py

import math

class Stats:
    def __init__(self, base_stats, aptitude):
        self._base_stats = dict(base_stats)
        self._aptitude = dict(aptitude)
        self.level(1)

    def __str__(self):
        stats = self._stats
        ret = "level %d\n" % self._level
        for k, v in stats.items():
            ret += "%s: %d\n" % (k, v)
        return ret

    def level(self, level):
        self._stats = dict(self._base_stats)
        self._level = level
        stats = self._stats
        base = self._base_stats
        aptitude = self._aptitude

        for k,v in stats.items():
            stats[k] = math.floor(base[k] + pow(level-1, 2) * 3 * aptitude[k])

    def diff(self, old_level, new_level):
        # TODO
        # print(v, before[i], after[i], "%+d" % (after[i] - before[i]))
        return

base_stats = { "hp": 17, "sp": 12, "attack": 15, "defense": 14, "exp": 20 }
aptitude = { "hp": 1.2, "sp": 0.9, "attack": 1.2, "defense": 1.1, "exp": 2.0 }

stats = Stats(base_stats, aptitude)
print(stats)
stats.level(level = 50)
print(stats)