擅长:python、mysql、java
<p><strong><em>打印</em></strong>与返回</em></strong>不同。根据文档,<a href="https://docs.python.org/3/library/functions.html#exec" rel="nofollow noreferrer">exec</a><strong>始终</strong>返回<code>None</code>:</p>
<blockquote>
<p>[...] The return value is <code>None</code>.</p>
</blockquote>
<p>使用<a href="https://docs.python.org/3/library/functions.html#eval" rel="nofollow noreferrer">eval</a>之类的方法也不会对您有帮助,因为您的行的计算结果也是<code>None</code>(我重复一下:<em>打印</em>与返回</em>不同)</p>
<p>如果您想实际捕获正在打印的内容,请查看<a href="https://stackoverflow.com/questions/3906232/python-get-the-print-output-in-an-exec-statement">this other SO question</a></p>
<hr/>
<p>如果您希望实际返回<em>而不是打印</em>,则相应地更改您的行:</p>
<pre class="lang-py prettyprint-override"><code>line4 = "issubclass(C,A) and issubclass(C,B)"
</code></pre>
<p>然后,使用<a href="https://docs.python.org/3/library/functions.html#eval" rel="nofollow noreferrer">eval</a>:</p>
<pre class="lang-py prettyprint-override"><code>myValue = eval(line4)
# myValue = False
</code></pre>