擅长:python、mysql、java
<p><code>while</code>循环。你知道吗</p>
<pre><code>valid_classes = (1, 2, 3)
class_num = 0
while class_num not in valid_classes:
try:
class_num = int(input("Please enter a class number from {}:".format(valid_classes)))
except ValueError:
pass
</code></pre>