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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=gb2312"><style type="text/css"><!--@import url(stylesheet/text.css);@import url(stylesheet/box.css);--></style><title>简明 Python 教程 / 异常 / 引发异常 </title></head><body><table width="100%"><tr><th colspan="3" align="center"><span class="header">简明 Python 教程</span></th></tr><th colspan="3" align="center">第13章 异常</th><tr><th width="20%" align="left"><a href="ch13s02.html">上一页</a></th><th width="60%" align="center"><span class="header2">引发异常</span></th><th align="right"><a href="ch13s04.html">下一页</a></th></tr></table><hr noshade><h1>引发异常</h1><p>你可以使用<code>raise</code>语句 <dfn>引发</dfn> 异常。你还得指明错误/异常的名称和伴随异常 <dfn>触发的</dfn> 异常对象。你可以引发的错误或异常应该分别是一个<code>Error</code>或<code>Exception</code>类的直接或间接导出类。</p><h2><a name="how">如何引发异常</a></h2><p class="exampletitle"><a name="e132">例13.2 如何引发异常</a></p><p class="filebox"><code class="comment">#!/usr/bin/python<br># Filename: raising.py</code><br><br><code class="key">class </code><code class="func">ShortInputException</code><code>(Exception):</code><br><code class="cite"> '''A user-defined exception class.'''</code><br><code class="key"> def </code><code class="func">__init__</code><code>(self, length, atleast):<br> Exception.__init__(self)<br> self.length = length<br> self.atleast = atleast</code><br><br><code class="key">try</code><code>:<br> s = </code><code class="func">raw_input</code><code>(</code><code class="cite">'Enter something --> '</code><code>)</code><br><code class="key"> if </code><code class="func">len</code><code>(s) < </code><code class="cite">3</code><code>:<br> raise ShortInputException(</code><code class="func">len</code><code>(s), </code><code class="cite">3</code><code>)</code><br><code class="comment"> # Other work can continue as usual here</code><br><code class="key">except </code><code>EOFError:</code><br><code class="key"> print </code><code class="cite">'\nWhy did you do an EOF on me?'</code><br><code class="key">except </code><code>ShortInputException, x:</code><br><code class="key"> print </code><code class="cite">'ShortInputException: The input was of length %d, \<br> was expecting at least %d' </code><code>% (x.length, x.atleast)</code><br><code class="key">else</code><code>:</code><br><code class="key"> print </code><code class="cite">'No exception was raised.'</code></p><p>源文件（<a href="code/raising.py">code/raising.py</a>）</p><h2>输出</h2><p class="codebox"><code>$ python raising.py<br>Enter something --><br>Why did you do an EOF on me?<br><br>$ python raising.py<br>Enter something --> ab<br>ShortInputException: The input was of length 2, was expecting at least 3<br><br>$ python raising.py<br>Enter something --> abc<br>No exception was raised.</code></p><h2>它如何工作</h2><p>这里，我们创建了我们自己的异常类型，其实我们可以使用任何预定义的异常/错误。这个新的异常类型是<code>ShortInputException</code>类。它有两个域——<code>length</code>是给定输入的长度，<code>atleast</code>则是程序期望的最小长度。</p><p>在<code>except</code>从句中，我们提供了错误类和用来表示错误/异常对象的变量。这与函数调用中的形参和实参概念类似。在这个特别的<code>except</code>从句中，我们使用异常对象的<code>length</code>和<code>atleast</code>域来为用户打印一个恰当的消息。</p><hr noshade><table width="100%"><tr><th width="20%" align="left"><a href="ch13s02.html">上一页</a></th><th width="60%" align="center"><a href="ch13.html">上一级</a></th><th width="20%" align="right"><a href="ch13s04.html">下一页</a></th></tr><tr><th width="20%" align="left">try..except</th><th width="60%" align="center"><a href="index.html">首页</a></th><th align="right">try..finally</th></tr></table></body></html>⌨️ 快捷键说明
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