todos = []

def add(task):
    todos.append({"task": task, "done": False})

def complete(index):
    todos[index]["done"] = True

add("牛乳を買う")
add("本を読む")
complete(0)

for i, t in enumerate(todos):
    mark = "[x]" if t["done"] else "[ ]"
    print(f"{mark} {t['task']}")