擅长:python、mysql、java
<p>测试指向<code>None</code>的名称和现有名称是两个语义不同的操作。</p>
<p>要检查<code>val</code>是否为None:</p>
<pre><code>if val is None:
pass # val exists and is None
</code></pre>
<p>要检查名称是否存在:</p>
<pre><code>try:
val
except NameError:
pass # val does not exist at all
</code></pre>