def flatten_list(ls=[]):
    flattened_list = []
    
    def aux(ls):
        for elem in ls:
            if not isinstance(elem, list):
                print("ADDING NO-LIST ELEMENT...")
                flattened_list.append(elem)
            else:
                print("RECURSION...")
                aux(elem)
    
    aux(ls)
    
    return flattened_list
            
     
       
print(flatten_list([[1,2,3,[4,5]],[6,7,8,9],10]))