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<li><a class="reference internal" href="#">8. 错误和异常</a><ul>
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<li><a class="reference internal" href="#syntax-errors">8.1. 语法错误</a></li>
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<section id="errors-and-exceptions">
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<span id="tut-errors"></span><h1><span class="section-number">8. </span>错误和异常<a class="headerlink" href="#errors-and-exceptions" title="永久链接至标题">¶</a></h1>
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<p>至此，本教程还未深入介绍错误信息，但如果您尝试过本教程前文中的例子，应该已经看到过一些错误信息。错误可（至少）被分为两种：<em>语法错误</em> 和 <em>异常</em>。</p>
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<section id="syntax-errors">
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<span id="tut-syntaxerrors"></span><h2><span class="section-number">8.1. </span>语法错误<a class="headerlink" href="#syntax-errors" title="永久链接至标题">¶</a></h2>
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<p>语法错误又称解析错误，是学习 Python 时最常见的错误：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">while</span> <span class="kc">True</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Hello world'</span><span class="p">)</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>
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<span class="w"> </span><span class="k">while</span> <span class="kc">True</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Hello world'</span><span class="p">)</span>
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<span class="w"> </span><span class="pm">^</span>
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<span class="gr">SyntaxError</span>: <span class="n">invalid syntax</span>
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</pre></div>
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</div>
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<p>解析器会复现出现句法错误的代码行，并用小“箭头”指向行里检测到的第一个错误。错误是由箭头 <em>上方</em> 的 token 触发的（至少是在这里检测出的）：本例中，在 <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a> 函数中检测到错误，因为，在它前面缺少冒号（<code class="docutils literal notranslate"><span class="pre">':'</span></code>） 。错误信息还输出文件名与行号，在使用脚本文件时，就可以知道去哪里查错。</p>
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</section>
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<section id="exceptions">
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<span id="tut-exceptions"></span><h2><span class="section-number">8.2. </span>异常<a class="headerlink" href="#exceptions" title="永久链接至标题">¶</a></h2>
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<p>即使语句或表达式使用了正确的语法，执行时仍可能触发错误。执行时检测到的错误称为 <em>异常</em>，异常不一定导致严重的后果：很快我们就能学会如何处理 Python 的异常。大多数异常不会被程序处理，而是显示下列错误信息：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">10</span> <span class="o">*</span> <span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="mi">0</span><span class="p">)</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
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<span class="gr">ZeroDivisionError</span>: <span class="n">division by zero</span>
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<span class="gp">>>> </span><span class="mi">4</span> <span class="o">+</span> <span class="n">spam</span><span class="o">*</span><span class="mi">3</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
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<span class="gr">NameError</span>: <span class="n">name 'spam' is not defined</span>
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<span class="gp">>>> </span><span class="s1">'2'</span> <span class="o">+</span> <span class="mi">2</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
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<span class="gr">TypeError</span>: <span class="n">Can't convert 'int' object to str implicitly</span>
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</pre></div>
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</div>
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<p>错误信息的最后一行说明程序遇到了什么类型的错误。异常有不同的类型，而类型名称会作为错误信息的一部分中打印出来：上述示例中的异常类型依次是：<a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a>， <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a> 和 <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>。作为异常类型打印的字符串是发生的内置异常的名称。对于所有内置异常都是如此，但对于用户定义的异常则不一定如此（虽然这种规范很有用）。标准的异常类型是内置的标识符（不是保留关键字）。</p>
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<p>此行其余部分根据异常类型，结合出错原因，说明错误细节。</p>
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<p>错误信息的前一部分以堆栈回溯的形式显示发生异常时的上下文。通常它包含列出源代码行的堆栈回溯；但是它不会显示从标准输入中读取的行。</p>
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<p><a class="reference internal" href="../library/exceptions.html#bltin-exceptions"><span class="std std-ref">内置异常</span></a> 列出了内置异常及其含义。</p>
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</section>
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<section id="handling-exceptions">
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<span id="tut-handling"></span><h2><span class="section-number">8.3. </span>异常的处理<a class="headerlink" href="#handling-exceptions" title="永久链接至标题">¶</a></h2>
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<p>可以编写程序处理选定的异常。下例会要求用户一直输入内容，直到输入有效的整数，但允许用户中断程序（使用 <kbd class="kbd docutils literal notranslate">Control-C</kbd> 或操作系统支持的其他操作）；注意，用户中断程序会触发 <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> 异常。</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">try</span><span class="p">:</span>
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<span class="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="nb">input</span><span class="p">(</span><span class="s2">"Please enter a number: "</span><span class="p">))</span>
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<span class="gp">... </span> <span class="k">break</span>
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<span class="gp">... </span> <span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
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<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Oops! That was no valid number. Try again..."</span><span class="p">)</span>
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<span class="gp">...</span>
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</pre></div>
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</div>
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<p><a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句的工作原理如下：</p>
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<ul class="simple">
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<li><p>首先，执行 <em>try 子句</em> （<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 和 <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> 关键字之间的（多行）语句）。</p></li>
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<li><p>如果没有触发异常，则跳过 <em>except 子句</em>，<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句执行完毕。</p></li>
|
||
<li><p>如果执行 try 子句时发生了异常，则跳过该子句中剩下的部分。如果异常的类型与 <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> 关键字后面的异常匹配，则执行 except 子句，然后，继续执行 <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句之后的代码。</p></li>
|
||
<li><p>如果发生的异常不是 except 子句中列示的异常，则将其传递到外部的 <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句中；如果没有找到处理程序，则它是一个 <em>未处理异常</em>，语句执行将终止，并显示如上所示的消息。</p></li>
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</ul>
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<p><a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句可以有多个 except 子句，可为不同异常指定相应的处理程序。但最多只会执行一个处理程序。处理程序只处理对应的 try 子句中发生的异常，而不处理同一 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> 语句内其他处理程序中的异常。except 子句可以用元组命名多个异常，例如：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="o">...</span> <span class="k">except</span> <span class="p">(</span><span class="ne">RuntimeError</span><span class="p">,</span> <span class="ne">TypeError</span><span class="p">,</span> <span class="ne">NameError</span><span class="p">):</span>
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<span class="o">...</span> <span class="k">pass</span>
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</pre></div>
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</div>
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<p>发生的异常与 <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> 子句中的类是同一个类或是它的基类时，异常与 except 子句中的类兼容（反之则不成立 --- 列出派生类的 except 子句与基类不兼容）。例如，下面的代码依次输出 B, C, D：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">B</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span>
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<span class="k">pass</span>
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<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">B</span><span class="p">):</span>
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<span class="k">pass</span>
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<span class="k">class</span> <span class="nc">D</span><span class="p">(</span><span class="n">C</span><span class="p">):</span>
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<span class="k">pass</span>
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|
||
<span class="k">for</span> <span class="bp">cls</span> <span class="ow">in</span> <span class="p">[</span><span class="n">B</span><span class="p">,</span> <span class="n">C</span><span class="p">,</span> <span class="n">D</span><span class="p">]:</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="k">raise</span> <span class="bp">cls</span><span class="p">()</span>
|
||
<span class="k">except</span> <span class="n">D</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"D"</span><span class="p">)</span>
|
||
<span class="k">except</span> <span class="n">C</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"C"</span><span class="p">)</span>
|
||
<span class="k">except</span> <span class="n">B</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"B"</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>注意，如果颠倒 except 子句的顺序（把 <code class="docutils literal notranslate"><span class="pre">except</span> <span class="pre">B</span></code> 放到第一个），则输出 B，B，B --- 即触发第一个匹配的 except 子句。</p>
|
||
<p>最后的 except 子句可以省略异常名，以用作通配符。但应谨慎使用，这种方式很容易掩盖真正的编程错误！它还可用于输出错误消息，然后重新触发异常（同样允许调用者处理异常）：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">sys</span>
|
||
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'myfile.txt'</span><span class="p">)</span>
|
||
<span class="n">s</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
|
||
<span class="n">i</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">s</span><span class="o">.</span><span class="n">strip</span><span class="p">())</span>
|
||
<span class="k">except</span> <span class="ne">OSError</span> <span class="k">as</span> <span class="n">err</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"OS error: </span><span class="si">{0}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">err</span><span class="p">))</span>
|
||
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"Could not convert data to an integer."</span><span class="p">)</span>
|
||
<span class="k">except</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"Unexpected error:"</span><span class="p">,</span> <span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">()[</span><span class="mi">0</span><span class="p">])</span>
|
||
<span class="k">raise</span>
|
||
</pre></div>
|
||
</div>
|
||
<p><a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> ... <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> 语句支持可选的 <em>else 子句</em>，该子句必须放在所有 except 子句之后。如果 try 子句没有触发异常，但又必须执行一些代码时，这个子句很有用。例如：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">arg</span> <span class="ow">in</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">:]:</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">arg</span><span class="p">,</span> <span class="s1">'r'</span><span class="p">)</span>
|
||
<span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s1">'cannot open'</span><span class="p">,</span> <span class="n">arg</span><span class="p">)</span>
|
||
<span class="k">else</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">arg</span><span class="p">,</span> <span class="s1">'has'</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">f</span><span class="o">.</span><span class="n">readlines</span><span class="p">()),</span> <span class="s1">'lines'</span><span class="p">)</span>
|
||
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>使用 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code> 子句比向 <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 子句添加额外的代码要好，可以避免意外捕获非 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> ... <code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code> 语句保护的代码触发的异常。</p>
|
||
<p>发生异常时，它可能具有关联值，即异常 <em>参数</em> 。是否需要参数，以及参数的类型取决于异常的类型。</p>
|
||
<p>except 子句可以在异常名称后面指定一个变量。这个变量和一个异常实例绑定，它的参数存储在 <code class="docutils literal notranslate"><span class="pre">instance.args</span></code> 中。为了方便起见，异常实例定义了 <a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__str__()</span></code></a> ，因此可以直接打印参数而无需引用 <code class="docutils literal notranslate"><span class="pre">.args</span></code> 。也可以在抛出之前首先实例化异常，并根据需要向其添加任何属性。:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">)</span>
|
||
<span class="gp">... </span><span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">inst</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">inst</span><span class="p">))</span> <span class="c1"># the exception instance</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">inst</span><span class="o">.</span><span class="n">args</span><span class="p">)</span> <span class="c1"># arguments stored in .args</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">inst</span><span class="p">)</span> <span class="c1"># __str__ allows args to be printed directly,</span>
|
||
<span class="gp">... </span> <span class="c1"># but may be overridden in exception subclasses</span>
|
||
<span class="gp">... </span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">inst</span><span class="o">.</span><span class="n">args</span> <span class="c1"># unpack args</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'x ='</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'y ='</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go"><class 'Exception'></span>
|
||
<span class="go">('spam', 'eggs')</span>
|
||
<span class="go">('spam', 'eggs')</span>
|
||
<span class="go">x = spam</span>
|
||
<span class="go">y = eggs</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>如果异常有参数，则它们将作为未处理异常的消息的最后一部分（'详细信息'）打印。</p>
|
||
<p>异常处理程序不仅处理 try 子句中遇到的异常，还处理 try 子句中调用（即使是间接地）的函数内部发生的异常。例如:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">this_fails</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">this_fails</span><span class="p">()</span>
|
||
<span class="gp">... </span><span class="k">except</span> <span class="ne">ZeroDivisionError</span> <span class="k">as</span> <span class="n">err</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Handling run-time error:'</span><span class="p">,</span> <span class="n">err</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">Handling run-time error: division by zero</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="raising-exceptions">
|
||
<span id="tut-raising"></span><h2><span class="section-number">8.4. </span>触发异常<a class="headerlink" href="#raising-exceptions" title="永久链接至标题">¶</a></h2>
|
||
<p><a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> 语句支持强制触发指定的异常。例如：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">raise</span> <span class="ne">NameError</span><span class="p">(</span><span class="s1">'HiThere'</span><span class="p">)</span>
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
|
||
<span class="gr">NameError</span>: <span class="n">HiThere</span>
|
||
</pre></div>
|
||
</div>
|
||
<p><a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> 唯一的参数就是要触发的异常。这个参数必须是异常实例或异常类（派生自 <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate"><span class="pre">Exception</span></code></a> 类）。如果传递的是异常类，将通过调用没有参数的构造函数来隐式实例化：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="ne">ValueError</span> <span class="c1"># shorthand for 'raise ValueError()'</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>如果只想判断是否触发了异常，但并不打算处理该异常，则可以使用更简单的 <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> 语句重新触发异常：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">NameError</span><span class="p">(</span><span class="s1">'HiThere'</span><span class="p">)</span>
|
||
<span class="gp">... </span><span class="k">except</span> <span class="ne">NameError</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'An exception flew by!'</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="k">raise</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">An exception flew by!</span>
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span>
|
||
<span class="gr">NameError</span>: <span class="n">HiThere</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="user-defined-exceptions">
|
||
<span id="tut-userexceptions"></span><h2><span class="section-number">8.5. </span>用户自定义异常<a class="headerlink" href="#user-defined-exceptions" title="永久链接至标题">¶</a></h2>
|
||
<p>程序可以通过创建新的异常类命名自己的异常（Python 类的内容详见 <a class="reference internal" href="classes.html#tut-classes"><span class="std std-ref">类</span></a>）。不论是以直接还是间接的方式，异常都应从 <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a> 类派生。</p>
|
||
<p>异常类和其他类一样，可以执行任何操作。但通常会比较简单，只提供让处理异常的程序提取错误信息的一些属性。创建能触发多个不同错误的模块时，一般只为该模块定义异常基类，然后再根据不同的错误条件，创建指定异常类的子类：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Error</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span>
|
||
<span class="w"> </span><span class="sd">"""Base class for exceptions in this module."""</span>
|
||
<span class="k">pass</span>
|
||
|
||
<span class="k">class</span> <span class="nc">InputError</span><span class="p">(</span><span class="n">Error</span><span class="p">):</span>
|
||
<span class="w"> </span><span class="sd">"""Exception raised for errors in the input.</span>
|
||
|
||
<span class="sd"> Attributes:</span>
|
||
<span class="sd"> expression -- input expression in which the error occurred</span>
|
||
<span class="sd"> message -- explanation of the error</span>
|
||
<span class="sd"> """</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">expression</span><span class="p">,</span> <span class="n">message</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">expression</span> <span class="o">=</span> <span class="n">expression</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span>
|
||
|
||
<span class="k">class</span> <span class="nc">TransitionError</span><span class="p">(</span><span class="n">Error</span><span class="p">):</span>
|
||
<span class="w"> </span><span class="sd">"""Raised when an operation attempts a state transition that's not</span>
|
||
<span class="sd"> allowed.</span>
|
||
|
||
<span class="sd"> Attributes:</span>
|
||
<span class="sd"> previous -- state at beginning of transition</span>
|
||
<span class="sd"> next -- attempted new state</span>
|
||
<span class="sd"> message -- explanation of why the specific transition is not allowed</span>
|
||
<span class="sd"> """</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">previous</span><span class="p">,</span> <span class="nb">next</span><span class="p">,</span> <span class="n">message</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">previous</span> <span class="o">=</span> <span class="n">previous</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">next</span> <span class="o">=</span> <span class="nb">next</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>大多数异常命名都以 “Error” 结尾，类似标准异常的命名。</p>
|
||
<p>许多标准模块都需要自定义异常，以报告由其定义的函数中出现的错误。有关类的说明，详见 <a class="reference internal" href="classes.html#tut-classes"><span class="std std-ref">类</span></a>。</p>
|
||
</section>
|
||
<section id="defining-clean-up-actions">
|
||
<span id="tut-cleanup"></span><h2><span class="section-number">8.6. </span>定义清理操作<a class="headerlink" href="#defining-clean-up-actions" title="永久链接至标题">¶</a></h2>
|
||
<p><a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句还有一个可选子句，用于定义在所有情况下都必须要执行的清理操作。例如：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">KeyboardInterrupt</span>
|
||
<span class="gp">... </span><span class="k">finally</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Goodbye, world!'</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">Goodbye, world!</span>
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span>
|
||
<span class="nc">KeyboardInterrupt</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>如果存在 <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> 子句，则 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句是 <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 语句结束前执行的最后一项任务。不论 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> 语句是否触发异常，都会执行 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句。以下内容介绍了几种比较复杂的触发异常情景：</p>
|
||
<ul class="simple">
|
||
<li><p>如果执行 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> 子句期间触发了某个异常，则某个 <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> 子句应处理该异常。如果该异常没有 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code> 子句处理，在 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句执行后会被重新触发。</p></li>
|
||
<li><p><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code> 或 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code> 子句执行期间也会触发异常。 同样，该异常会在 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句执行之后被重新触发。</p></li>
|
||
<li><p>如果执行 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> 语句时遇到 <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a>,、<a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> 或 <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> 语句，则 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句在执行 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code>、<code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code> 或 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> 语句之前执行。</p></li>
|
||
<li><p>如果 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句中包含 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> 语句，则返回值来自 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句的某个 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> 语句的返回值，而不是来自 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> 子句的 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> 语句的返回值。</p></li>
|
||
</ul>
|
||
<p>例如：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">bool_return</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">return</span> <span class="kc">True</span>
|
||
<span class="gp">... </span> <span class="k">finally</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">return</span> <span class="kc">False</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">bool_return</span><span class="p">()</span>
|
||
<span class="go">False</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>这是一个比较复杂的例子：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">divide</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">result</span> <span class="o">=</span> <span class="n">x</span> <span class="o">/</span> <span class="n">y</span>
|
||
<span class="gp">... </span> <span class="k">except</span> <span class="ne">ZeroDivisionError</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"division by zero!"</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="k">else</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"result is"</span><span class="p">,</span> <span class="n">result</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="k">finally</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"executing finally clause"</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">divide</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
|
||
<span class="go">result is 2.0</span>
|
||
<span class="go">executing finally clause</span>
|
||
<span class="gp">>>> </span><span class="n">divide</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
|
||
<span class="go">division by zero!</span>
|
||
<span class="go">executing finally clause</span>
|
||
<span class="gp">>>> </span><span class="n">divide</span><span class="p">(</span><span class="s2">"2"</span><span class="p">,</span> <span class="s2">"1"</span><span class="p">)</span>
|
||
<span class="go">executing finally clause</span>
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">3</span>, in <span class="n">divide</span>
|
||
<span class="gr">TypeError</span>: <span class="n">unsupported operand type(s) for /: 'str' and 'str'</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>如上所示，任何情况下都会执行 <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> 子句。<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> 子句不处理两个字符串相除触发的 <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>，因此会在 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> 子句执行后被重新触发。</p>
|
||
<p>在实际应用程序中，<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> 子句对于释放外部资源（例如文件或者网络连接）非常有用，无论是否成功使用资源。</p>
|
||
</section>
|
||
<section id="predefined-clean-up-actions">
|
||
<span id="tut-cleanup-with"></span><h2><span class="section-number">8.7. </span>预定义的清理操作<a class="headerlink" href="#predefined-clean-up-actions" title="永久链接至标题">¶</a></h2>
|
||
<p>某些对象定义了不需要该对象时要执行的标准清理操作。无论使用该对象的操作是否成功，都会执行清理操作。比如，下例要打开一个文件，并输出文件内容：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"myfile.txt"</span><span class="p">):</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">""</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>这个代码的问题在于，执行完代码后，文件在一段不确定的时间内处于打开状态。在简单脚本中这没有问题，但对于较大的应用程序来说可能会出问题。<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> 语句支持以及时、正确的清理的方式使用文件对象：</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"myfile.txt"</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
|
||
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">f</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">""</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>语句执行完毕后，即使在处理行时遇到问题，都会关闭文件 <em>f</em>。和文件一样，支持预定义清理操作的对象会在文档中指出这一点。</p>
|
||
</section>
|
||
</section>
|
||
|
||
|
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</div>
|
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</div>
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</div>
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<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
|
||
<div class="sphinxsidebarwrapper">
|
||
<h3><a href="../contents.html">目录</a></h3>
|
||
<ul>
|
||
<li><a class="reference internal" href="#">8. 错误和异常</a><ul>
|
||
<li><a class="reference internal" href="#syntax-errors">8.1. 语法错误</a></li>
|
||
<li><a class="reference internal" href="#exceptions">8.2. 异常</a></li>
|
||
<li><a class="reference internal" href="#handling-exceptions">8.3. 异常的处理</a></li>
|
||
<li><a class="reference internal" href="#raising-exceptions">8.4. 触发异常</a></li>
|
||
<li><a class="reference internal" href="#user-defined-exceptions">8.5. 用户自定义异常</a></li>
|
||
<li><a class="reference internal" href="#defining-clean-up-actions">8.6. 定义清理操作</a></li>
|
||
<li><a class="reference internal" href="#predefined-clean-up-actions">8.7. 预定义的清理操作</a></li>
|
||
</ul>
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</li>
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<h4>上一个主题</h4>
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<p class="topless"><a href="inputoutput.html"
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title="上一章"><span class="section-number">7. </span>输入与输出</a></p>
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