okay im stupid i did a rookie mistake and called load_data before _metier_map is init so it was just

class FleetManager:
    def __init__(self, json_path='component/default.json'):
        
        self.current_fleet_index = 0
        self.__metier_map = {
            "Pilote": Pilote,
            "Technicien": Technicien,
            "Armurier": Armurier,
            "Marchand": Marchand,
            "Entretien": Entretien
        }
        self.party = self.load_data(json_path)

instead of :

class FleetManager:
    def __init__(self, json_path='component/default.json'):
        self.party = self.load_data(json_path)
        self.current_fleet_index = 0
        self.__metier_map = {
            "Pilote": Pilote,
            "Technicien": Technicien,
            "Armurier": Armurier,
            "Marchand": Marchand,
            "Entretien": Entretien
        }