class Material:
    def __init__(self, ID, density):
        self.ID = ID
        self.density = density
        
    DB = []
    
    @staticmethod
    def GetMaterialIDs():
        result = []
        for m in Material.DB:
            result.append(m.ID)
        return result
    
    @staticmethod
    def Get(ID):
        for m in Material.DB:
            if m.ID == ID: return m
        return None
    
Material.DB.append(Material('Stainless steel', density = 8.00))
Material.DB.append(Material('Aluminium', density = 2.71))