class Player:

    def __init__(self):
        self.skills = []

    def execute_actions(self, **event_args):
        for skill in self.skills:
            skill.execute_actions(player=self, **event_args)


class Skill:

    def __init__(self, type_object, level):
        self.type_object = type_object
        self.level = level

    @property
    def name(self):
        return self.type_object.name

    def execute_actions(self, **event_args):
        if self.level > 0:
            self.type_object.execute_actions(skill=self, **event_args)

    # more type_object properties


class SkillType:

    def __init__(self, name, max_level):
        self.name = name
        self.max_level = max_level
        self.actions = []

    def execute_actions(self, **event_args):
        for action in self.actions:
            if action.event == event_args['event_name']:
                action(**event_args)


class Action:

    def __init__(self, event, callback, group=None, cooldown=None):
        self.event = event
        self.callback = callback
        self.group = group
        self._cooldown = cooldown

   @property
   def cooldown(self):
       if self._cooldown is not None:
           return self._cooldown
       if self.group is not None:
           return self.group.get('cooldown', None)
       return None

# Usage

player = Player()

fireball = SkillType('Fireball', 5)

def fireball_deal_damage(skill, player, target, **event_args):
    player.damage(target, skill.level)

fireball.actions.append(
    Action('player_attack', fireball_deal_damage, cooldown=5)
)

player.skills.append(Skill(fireball, 1))

// skills.json
[
  {
    "name": "Fireball",
    "max_level": 5,
    "actions": [
      {
        "event": "player_attack",
        "action": "deal_damage",
        "data": {"amount_base": 3, "amount_per_level": 1},
        "cooldown": 5
      }
    ]
  }
]

// skills.json
[
  {
    "name": "Fireball",
    "max_level": 5,
    "actions": [
      {
        "event": "player_attack",
        "action": "deal_damage",
        "data": {"amount_base": 3, "amount_per_level": 1},
        "group": 0
      },
      {
        "event": "player_attack",
        "action": "ignite",
        "data": {"duration_base": 0, "duration_per_level": 1},
        "group": 0
      }
    ],
    "groups": [
      {  // Group 0
        "cooldown": 5
      }
  }
]