擅长:python、mysql、java
<p>这是因为您没有调用<code>lower()</code>,而是将<code>menu_select</code>设置为<em>函数本身</em>:</p>
<pre><code>>>> menu_select = input("? ").lower
?
>>> menu_select
<built-in method lower of str object at 0x10ca5eab0>
</code></pre>
<p>将您的线路改为:</p>
<pre><code>menu_select = input("A/B: ").lower()
</code></pre>