328 lines
517 KiB
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328 lines
517 KiB
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<html><head><title>控制语句</title><meta charSet="utf-8"/><meta name="viewport" content="width=device-width, initial-scale=1.0"/><meta property="og:title" content="控制语句"/><meta property="og:description" content="Python 控制语句."/><meta property="og:image" content="https://wiki.7wate.com/static/og-image.png"/><meta property="og:width" content="1200"/><meta property="og:height" content="675"/><link rel="icon" href="../../../../static/icon.png"/><meta name="description" content="Python 控制语句."/><meta name="generator" content="Quartz"/><link rel="preconnect" href="https://fonts.googleapis.com"/><link rel="preconnect" href="https://fonts.gstatic.com"/><script async src="https://umami.7wate.com/script.js" data-website-id="c061efdc-95dd-4d21-9d04-a1ffda0a85b9"></script><script>
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<h2 id="条件语句">条件语句<a aria-hidden="true" tabindex="-1" href="#条件语句" class="internal"> §</a></h2>
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<p>Python 中的条件语句是通过一条或多条语句的执行结果（即 True 或 False）来决定执行的代码块。</p>
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<p>判断的值可以分为：</p>
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<ul>
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<li>假值 ：None、空列表、空集合、空字典，空元组、空字符串、0、False 等。</li>
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<li>真值 ：非空列表、非空集合、非空字典，非空元组、非空字符串、非 0 数值、True 等。</li>
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</ul>
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<h3 id="if">If<a aria-hidden="true" tabindex="-1" href="#if" class="internal"> §</a></h3>
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<p>在 Python 中，要构造分支结构可以使用 <code>if</code>、<code>elif</code> 和 <code>else</code> 关键字。<code>elif</code> 和 <code>else</code> 都是可选的，可以根据需要进行使用。</p>
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<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);"># 示例</span></span>
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<span data-line><span style="color:var(--shiki-token-keyword);">>>></span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">int</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-function);">input</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"Please enter an integer: "</span><span style="color:var(--shiki-token-punctuation);">))</span></span>
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<span data-line><span style="color:var(--shiki-color-text);">Please enter an integer</span><span style="color:var(--shiki-token-punctuation);">:</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">42</span></span>
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<span data-line><span style="color:var(--shiki-token-keyword);">>>></span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">if</span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);"><</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">0</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">0</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'Negative changed to zero'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">elif</span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">==</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">0</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'Zero'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">elif</span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">==</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">1</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'Single'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">else</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'More'</span><span style="color:var(--shiki-token-punctuation);">)</span></span></code></pre></div>
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<p>当然如果要构造出更多的分支，可以使用 <code>if...elif...else...</code> 结构或者嵌套的 <code>if...else...</code> 结构。</p>
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<h3 id="match">Match<a aria-hidden="true" tabindex="-1" href="#match" class="internal"> §</a></h3>
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<p>Python 3.10 引入了新的 <code>match</code> 语句，它是模式匹配的一种形式。<code>match</code> 语句接受一个表达式并将它的值与一系列的模式进行比较。 每个模式都关联到一个代码块，当模式与表达式的值匹配时，该代码块将被执行。这在某种程度上类似于其他语言中的 switch 语句。</p>
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<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">http_error</span><span style="color:var(--shiki-color-text);">(</span><span style="color:var(--shiki-token-parameter);">status</span><span style="color:var(--shiki-color-text);">):</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">match</span><span style="color:var(--shiki-color-text);"> status</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">case</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">400</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-string-expression);">"Bad request"</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">case</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">404</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-string-expression);">"Not found"</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">case</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">418</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-string-expression);">"I'm a teapot"</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);">#"变量名" `_` 被作为 通配符 并必定会匹配成功</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">case</span><span style="color:var(--shiki-color-text);"> _</span><span style="color:var(--shiki-token-punctuation);">:</span><span style="color:var(--shiki-color-text);"> </span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-string-expression);">"Something's wrong with the internet"</span></span></code></pre></div>
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<p>你可以使用 <code>|</code> （“ or ”）在一个模式中组合几个字面值:</p>
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<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-keyword);">case</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">401</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">|</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">403</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">|</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">404</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-string-expression);">"Not allowed"</span></span></code></pre></div>
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<p><em>注意，<code>match</code> 语句和模式匹配的概念是 Python 3.10 中新增的特性，可能在更早版本的 Python 中无法使用。</em></p>
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<h2 id="循环语句">循环语句<a aria-hidden="true" tabindex="-1" href="#循环语句" class="internal"> §</a></h2>
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<h3 id="for">For<a aria-hidden="true" tabindex="-1" href="#for" class="internal"> §</a></h3>
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<p>Python 的 for 语句用于遍历任何序列的项目，如列表或字符串。</p>
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<p>Python 的 for 语句与 C 或 Pascal 中的不同。<strong>Python 的 for 语句不迭代算术递增数值</strong>（如 Pascal），或是给予用户定义迭代步骤和暂停条件的能力（如 C），而是迭代列表或字符串等任意序列，元素的迭代顺序与在序列中出现的顺序一致。</p>
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<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);">"""</span></span>
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<span data-line><span style="color:var(--shiki-token-comment);">用for循环实现1~100求和</span></span>
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<span data-line><span style="color:var(--shiki-token-comment);">"""</span></span>
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<span data-line> </span>
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<span data-line><span style="color:var(--shiki-token-function);">sum</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">0</span></span>
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<span data-line><span style="color:var(--shiki-token-keyword);">for</span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">in</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">range</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">101</span><span style="color:var(--shiki-token-punctuation);">):</span></span>
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<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">sum</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">+=</span><span style="color:var(--shiki-color-text);"> x</span></span>
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<span data-line><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-function);">sum</span><span style="color:var(--shiki-token-punctuation);">)</span></span></code></pre></div>
|
|||
|
|
<p><strong>内置函数 range() 表示不可变的数字序列，通常用于在 for 循环中循环指定的次数。</strong></p>
|
|||
|
|
<p>在 Python 中，<code>range()</code> 是一个内置函数，用于生成一个不可变的数字序列。通常，这个函数在 for 循环中使用，用于指定循环的次数。<code>range(101)</code> 可以用来生成一个包含 0 到 100（不包含 101）的整数序列。</p>
|
|||
|
|
<p>你也可以根据需要更改 <code>range()</code> 函数的参数，例如：</p>
|
|||
|
|
<ul>
|
|||
|
|
<li><code>range(1, 101)</code>：生成一个包含 1 到 100（不包含 101）的整数序列。</li>
|
|||
|
|
<li><code>range(1, 101, 2)</code>：生成一个包含 1 到 100 的奇数序列，其中 2 是步长。</li>
|
|||
|
|
<li><code>range(100, 0, -2)</code>：生成一个包含 100 到 1 的偶数序列，其中 -2 是步长。</li>
|
|||
|
|
</ul>
|
|||
|
|
<h3 id="while">While<a aria-hidden="true" tabindex="-1" href="#while" class="internal"> §</a></h3>
|
|||
|
|
<p><code>while</code> 循环语句用于在条件满足的情况下重复执行一个代码块。条件表达式的结果为 <code>True</code> 时，继续循环；结果为 <code>False</code> 时，结束循环。</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);">"""</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);">猜数字游戏</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);">"""</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">import</span><span style="color:var(--shiki-color-text);"> random</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);">answer </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> random</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-token-function);">randint</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">1</span><span style="color:var(--shiki-token-punctuation);">, </span><span style="color:var(--shiki-token-constant);">100</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);">counter </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">0</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">while</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">True</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> counter </span><span style="color:var(--shiki-token-keyword);">+=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">1</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> number </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">int</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-function);">input</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'请输入: '</span><span style="color:var(--shiki-token-punctuation);">))</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">if</span><span style="color:var(--shiki-color-text);"> number </span><span style="color:var(--shiki-token-keyword);"><</span><span style="color:var(--shiki-color-text);"> answer</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'大一点'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">elif</span><span style="color:var(--shiki-color-text);"> number </span><span style="color:var(--shiki-token-keyword);">></span><span style="color:var(--shiki-color-text);"> answer</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'小一点'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">else</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'恭喜你猜对了!'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">break</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'你总共猜了</span><span style="color:var(--shiki-token-constant);">%d</span><span style="color:var(--shiki-token-string-expression);">次'</span><span style="color:var(--shiki-token-punctuation);"> </span><span style="color:var(--shiki-token-keyword);">%</span><span style="color:var(--shiki-token-punctuation);"> counter)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">if</span><span style="color:var(--shiki-color-text);"> counter </span><span style="color:var(--shiki-token-keyword);">></span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">7</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'你的智商余额明显不足'</span><span style="color:var(--shiki-token-punctuation);">)</span></span></code></pre></div>
|
|||
|
|
<h3 id="break">Break<a aria-hidden="true" tabindex="-1" href="#break" class="internal"> §</a></h3>
|
|||
|
|
<p><code>break</code> 语句可以提前退出循环。具体来说，<strong><code>break</code> 用于完全结束一个循环，并跳出该循环体。</strong></p>
|
|||
|
|
<h3 id="continue">Continue<a aria-hidden="true" tabindex="-1" href="#continue" class="internal"> §</a></h3>
|
|||
|
|
<p><code>continue</code> 语句用于<strong>跳过当前循环的剩余语句，然后继续进行下一轮循环。</strong></p>
|
|||
|
|
<h3 id="else">Else<a aria-hidden="true" tabindex="-1" href="#else" class="internal"> §</a></h3>
|
|||
|
|
<p>在 Python 中，<code>else</code> 子句可以与 <code>for</code> 循环和 <code>while</code> 循环一起使用。当循环正常完成（即没有碰到 <code>break</code> 语句）时，<code>else</code> 块的内容会被执行。这个特性在很多其他语言中都没有，因此对于初学者来说可能会感到有些不熟悉。</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);"># 使用 else 的 for 循环的示例</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">for</span><span style="color:var(--shiki-color-text);"> i </span><span style="color:var(--shiki-token-keyword);">in</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">range</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">5</span><span style="color:var(--shiki-token-punctuation);">):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">if</span><span style="color:var(--shiki-color-text);"> i </span><span style="color:var(--shiki-token-keyword);">==</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">10</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">break</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">else</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"循环正常完成"</span><span style="color:var(--shiki-token-punctuation);">)</span></span></code></pre></div>
|
|||
|
|
<p>在这个示例中，循环会正常完成，因为没有任何一个元素使得 <code>i == 10</code> 为 <code>True</code>，所以 <code>break</code> 语句不会被执行，<code>else</code> 块的内容会被打印出来。</p>
|
|||
|
|
<p>如果我们修改 <code>if</code> 语句的条件使得 <code>break</code> 语句被执行，那么 <code>else</code> 块的内容就不会被打印出来：</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="Python" data-theme="default"><code data-language="Python" data-theme="default"><span data-line><span style="color:var(--shiki-color-text);">for i in range(5):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> if i == 3:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> break</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);">else:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> print("循环正常完成")</span></span></code></pre></div>
|
|||
|
|
<p>在这个示例中，当 <code>i</code> 等于 3 时，<code>break</code> 语句就会被执行，所以循环没有正常完成，<code>else</code> 块的内容不会被打印出来。</p>
|
|||
|
|
<p>同样的，<code>else</code> 也可以与 <code>while</code> 循环一起使用，当 <code>while</code> 循环的条件变为 <code>False</code> 时，<code>else</code> 块的内容会被执行。</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-color-text);">i </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">0</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">while</span><span style="color:var(--shiki-color-text);"> i </span><span style="color:var(--shiki-token-keyword);"><</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">5</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">if</span><span style="color:var(--shiki-color-text);"> i </span><span style="color:var(--shiki-token-keyword);">==</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">3</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">break</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> i </span><span style="color:var(--shiki-token-keyword);">+=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">1</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">else</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"循环正常完成"</span><span style="color:var(--shiki-token-punctuation);">)</span></span></code></pre></div>
|
|||
|
|
<p>在这个示例中，当 <code>i</code> 等于 3 时，<code>break</code> 语句就会被执行，所以循环没有正常完成，<code>else</code> 块的内容不会被打印出来。</p>
|
|||
|
|
<h2 id="异常语句">异常语句<a aria-hidden="true" tabindex="-1" href="#异常语句" class="internal"> §</a></h2>
|
|||
|
|
<h3 id="tryexceptfinally">try、except、finally<a aria-hidden="true" tabindex="-1" href="#tryexceptfinally" class="internal"> §</a></h3>
|
|||
|
|
<pre><code class="mermaid">graph TD
|
|||
|
|
A(开始)
|
|||
|
|
B[try 块]
|
|||
|
|
C[except 块1]
|
|||
|
|
D[except 块2]
|
|||
|
|
E[else 块]
|
|||
|
|
F[finally 块]
|
|||
|
|
G(结束)
|
|||
|
|
A --> B
|
|||
|
|
B -- 异常1 --> C
|
|||
|
|
B -- 异常2 --> D
|
|||
|
|
B -- 无异常 --> E
|
|||
|
|
C --> F
|
|||
|
|
D --> F
|
|||
|
|
E --> F
|
|||
|
|
F --> G
|
|||
|
|
|
|||
|
|
</code></pre>
|
|||
|
|
<ol>
|
|||
|
|
<li><strong>确定需要的异常处理结构</strong>：在开始编写异常处理结构时，首先确定你需要 <code>try</code>, <code>except</code>, <code>else</code> 和/或 <code>finally</code> 语句块。要记住的是，<code>except</code> 和 <code>finally</code> 块至少需要一个，否则 <code>try</code> 将失去其意义。</li>
|
|||
|
|
<li><strong>编写 <code>try</code> 语句块</strong>：将可能抛出异常的代码放入 <code>try</code> 块。如果在 <code>try</code> 块中发生异常，Python 将停止执行 <code>try</code> 块的其余部分，并转到 <code>except</code> 块。</li>
|
|||
|
|
<li><strong>编写 <code>except</code> 语句块</strong>：为每种可能抛出的异常类型编写一个 <code>except</code> 块。Python 会按照它们在代码中出现的顺序来检查这些 <code>except</code> 块。如果匹配到异常，Python 将执行相应的 <code>except</code> 块并停止查找。</li>
|
|||
|
|
<li><strong>处理多个异常</strong>：你可以将多个异常类型放入一个元组中，然后使用一个 <code>except</code> 块来处理它们。例如：<code>except (TypeError, ValueError):</code>。</li>
|
|||
|
|
<li><strong>处理所有异常</strong>：如果 <code>except</code> 块后面没有指定异常类型，那么这个 <code>except</code> 块将处理所有异常。你可以通过 <code>logging</code> 或 <code>sys</code> 模块获取异常的详细信息。</li>
|
|||
|
|
<li><strong>重新抛出异常</strong>：如果你在 <code>except</code> 块中捕获了一个异常，然后想要再次抛出它，你可以使用 <code>raise</code> 语句而不需要附加任何参数或信息。</li>
|
|||
|
|
<li><strong>编写 <code>else</code> 语句块</strong>：<code>else</code> 块中的代码只有在 <code>try</code> 块没有发生任何异常时才会被执行。</li>
|
|||
|
|
<li><strong>编写 <code>finally</code> 语句块</strong>：无论是否发生异常，<code>finally</code> 块中的代码总是会被执行。这对于清理（例如关闭文件或网络连接）非常有用。</li>
|
|||
|
|
<li><strong>重构代码</strong>：考虑使用 <code>with</code> 语句或 <code>getattr()</code> 方法等内置的异常处理语句，而不是 <code>try/except</code>。此外，如果可能，尽量避免在同一个 <code>except</code> 块中捕获和抛出相同的异常。最后，除非你确定需要处理所有可能的异常，否则不应该捕获所有异常，因为这可能会隐藏严重的问题。</li>
|
|||
|
|
</ol>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);"># 示例</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">try</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 这里可能会抛出异常的代码</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">do_something</span><span style="color:var(--shiki-token-punctuation);">()</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">except</span><span style="color:var(--shiki-color-text);"> SomeException </span><span style="color:var(--shiki-token-keyword);">as</span><span style="color:var(--shiki-color-text);"> e</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 当捕获到SomeException时的处理代码</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">handle_exception</span><span style="color:var(--shiki-token-punctuation);">(e)</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 实例</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">div</span><span style="color:var(--shiki-color-text);">(</span><span style="color:var(--shiki-token-parameter);">a</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-parameter);">b</span><span style="color:var(--shiki-color-text);">):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">try</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(a </span><span style="color:var(--shiki-token-keyword);">/</span><span style="color:var(--shiki-token-punctuation);"> b)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">except</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">ZeroDivisionError</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"错误：b 不应为 0 !!"</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">except</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">Exception</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">as</span><span style="color:var(--shiki-color-text);"> e</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"意外错误：</span><span style="color:var(--shiki-token-constant);">{}</span><span style="color:var(--shiki-token-string-expression);">"</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-token-function);">format</span><span style="color:var(--shiki-token-punctuation);">(e))</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">else</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'只有当一切正常时，才会运行 else'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">finally</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'始终运行 finally 块。'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 测试</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-function);">div</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">2</span><span style="color:var(--shiki-token-punctuation);">, </span><span style="color:var(--shiki-token-constant);">0</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-function);">div</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">2</span><span style="color:var(--shiki-token-punctuation);">, </span><span style="color:var(--shiki-token-string-expression);">'错误的类型'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-function);">div</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">1</span><span style="color:var(--shiki-token-punctuation);">, </span><span style="color:var(--shiki-token-constant);">2</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 在一行中捕获多个异常</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">try</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(a </span><span style="color:var(--shiki-token-keyword);">/</span><span style="color:var(--shiki-token-punctuation);"> b)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">except</span><span style="color:var(--shiki-color-text);"> (</span><span style="color:var(--shiki-token-constant);">ZeroDivisionError</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">TypeError</span><span style="color:var(--shiki-color-text);">) </span><span style="color:var(--shiki-token-keyword);">as</span><span style="color:var(--shiki-color-text);"> e</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(e)</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 当存在 finally 时，except 是可选的</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">try</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">open</span><span style="color:var(--shiki-token-punctuation);">(database)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">finally</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">close</span><span style="color:var(--shiki-token-punctuation);">(database)</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 捕获所有错误并记录下来</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">try</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">do_work</span><span style="color:var(--shiki-token-punctuation);">()</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">except</span><span style="color:var(--shiki-token-punctuation);">:</span><span style="color:var(--shiki-color-text);"> </span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 从 logging 模块获取详细信息</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> logging</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-token-function);">exception</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">'捕获到异常!'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 从 sys.exc_info() 方法获取详细信息</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> error_type</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> error_value</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> trace_back </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> sys</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-token-function);">exc_info</span><span style="color:var(--shiki-token-punctuation);">()</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(error_value)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">raise</span></span></code></pre></div>
|
|||
|
|
<h3 id="with">With<a aria-hidden="true" tabindex="-1" href="#with" class="internal"> §</a></h3>
|
|||
|
|
<p>Python 的 with 语句支持通过上下文管理器所定义的运行时上下文这一概念。</p>
|
|||
|
|
<p><code>with</code> 语句是一种处理上下文管理器的语句，上下文管理器通常包含 <code>__enter__</code> 和 <code>__exit__</code> 这两个方法。在 <code>with</code> 语句的代码块被执行前，会首先执行 <code>__enter__</code> 方法，在执行完毕后，会调用 <code>__exit__</code> 方法。这在你需要管理资源，如文件，网络连接或锁定等情况非常有用，因为它可以<strong>保证在任何情况下都会执行必要的清理操作。</strong></p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);"># 在这里，文件已经被关闭，无需再次手动关闭</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">with</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">open</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"file.txt"</span><span style="color:var(--shiki-token-punctuation);">, </span><span style="color:var(--shiki-token-string-expression);">"r"</span><span style="color:var(--shiki-token-punctuation);">)</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">as</span><span style="color:var(--shiki-color-text);"> file</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">for</span><span style="color:var(--shiki-color-text);"> line </span><span style="color:var(--shiki-token-keyword);">in</span><span style="color:var(--shiki-color-text);"> file</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(line)</span></span></code></pre></div>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-comment);"># 基本思想是 with 所求值的对象必须有一个 enter() 方法，一个 exit() 方法。</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 紧跟 with 后面的语句被求值后，返回对象的 enter() 方法被调用，并将返回值赋值给 as 后面的变量。</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 当 with 的代码块全部被执行完之后，将调用前面返回对象的 exit() 方法。</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">class</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">ManagedFile</span><span style="color:var(--shiki-color-text);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">__init__</span><span style="color:var(--shiki-color-text);">(</span><span style="color:var(--shiki-token-parameter);">self</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-parameter);">filename</span><span style="color:var(--shiki-color-text);">):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> self</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-color-text);">filename </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> filename</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">__enter__</span><span style="color:var(--shiki-color-text);">(</span><span style="color:var(--shiki-token-parameter);">self</span><span style="color:var(--shiki-color-text);">):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> self</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-color-text);">file </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">open</span><span style="color:var(--shiki-token-punctuation);">(self.filename, </span><span style="color:var(--shiki-token-string-expression);">'r'</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> self</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-color-text);">file</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">__exit__</span><span style="color:var(--shiki-color-text);">(</span><span style="color:var(--shiki-token-parameter);">self</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-parameter);">exc_type</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-parameter);">exc_val</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-parameter);">exc_tb</span><span style="color:var(--shiki-color-text);">):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">if</span><span style="color:var(--shiki-color-text);"> self</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-color-text);">file</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> self</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-color-text);">file</span><span style="color:var(--shiki-token-punctuation);">.</span><span style="color:var(--shiki-token-function);">close</span><span style="color:var(--shiki-token-punctuation);">()</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 使用自定义的上下文管理器</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">with</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">ManagedFile</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"file.txt"</span><span style="color:var(--shiki-token-punctuation);">)</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">as</span><span style="color:var(--shiki-color-text);"> file</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(file.</span><span style="color:var(--shiki-token-function);">read</span><span style="color:var(--shiki-token-punctuation);">())</span></span></code></pre></div>
|
|||
|
|
<h3 id="raise">Raise<a aria-hidden="true" tabindex="-1" href="#raise" class="internal"> §</a></h3>
|
|||
|
|
<p><code>raise</code> 语句用于引发特定的异常。你可以定义异常类型并附加一个错误消息：</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-keyword);">raise</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">ValueError</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"这是一个无效的值！"</span><span style="color:var(--shiki-token-punctuation);">)</span></span></code></pre></div>
|
|||
|
|
<p>如果你在 <code>except</code> 块中使用 <code>raise</code> 语句，而不提供任何参数，它将默认重新引发最近的异常。</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-keyword);">try</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">5</span><span style="color:var(--shiki-token-keyword);">/</span><span style="color:var(--shiki-token-constant);">0</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">except</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">ZeroDivisionError</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">as</span><span style="color:var(--shiki-color-text);"> e</span><span style="color:var(--shiki-token-punctuation);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-string-expression);">"发生了一个错误！"</span><span style="color:var(--shiki-token-punctuation);">)</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">raise</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 重新引发最近的异常</span></span></code></pre></div>
|
|||
|
|
<h2 id="其他语句">其他语句<a aria-hidden="true" tabindex="-1" href="#其他语句" class="internal"> §</a></h2>
|
|||
|
|
<h3 id="assert">Assert<a aria-hidden="true" tabindex="-1" href="#assert" class="internal"> §</a></h3>
|
|||
|
|
<p><code>assert</code> 语句用于断言某个条件为真，如果条件为假，则会抛出 <code>AssertionError</code> 异常。它常常用于调试代码，确认代码的某些方面满足预期，例如：</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-color-text);">x </span><span style="color:var(--shiki-token-keyword);">=</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">1</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">assert</span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">==</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">1</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 条件为真，没有问题</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-comment);"># 条件为假，抛出 AssertionError，并附带错误信息</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">assert</span><span style="color:var(--shiki-color-text);"> x </span><span style="color:var(--shiki-token-keyword);">==</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">2</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-string-expression);">"x should be 2 but is actually "</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">+</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-constant);">str</span><span style="color:var(--shiki-token-punctuation);">(x)</span><span style="color:var(--shiki-color-text);"> </span></span></code></pre></div>
|
|||
|
|
<p><em>注意，<code>assert</code> 语句在优化模式下（使用 <code>-O</code> 参数启动 Python 时）会被全局禁用。</em></p>
|
|||
|
|
<h3 id="pass">Pass<a aria-hidden="true" tabindex="-1" href="#pass" class="internal"> §</a></h3>
|
|||
|
|
<p><code>pass</code> 语句是 Python 中的空语句，用于在需要语句的地方保持语法的完整性，但是<strong>实际上不做任何事情</strong>。通常，我们使用它作为未完成代码的占位符：</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">my_function</span><span style="color:var(--shiki-color-text);">():</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">pass</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># </span><span style="color:var(--shiki-token-keyword);">TODO</span><span style="color:var(--shiki-token-comment);">: implement this function</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-keyword);">class</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">MyEmptyClass</span><span style="color:var(--shiki-color-text);">:</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">pass</span></span></code></pre></div>
|
|||
|
|
<h3 id="return">Return<a aria-hidden="true" tabindex="-1" href="#return" class="internal"> §</a></h3>
|
|||
|
|
<p><code>return</code> 语句用于从函数返回一个值。所有函数都会返回一个值：如果函数执行到结尾而没有遇到 <code>return</code> 语句，它将返回特殊值 <code>None</code>：</p>
|
|||
|
|
<div data-rehype-pretty-code-fragment><pre style="background-color:var(--shiki-color-background);" tabindex="0" data-language="python" data-theme="default"><code data-language="python" data-theme="default"><span data-line><span style="color:var(--shiki-token-keyword);">def</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-function);">add</span><span style="color:var(--shiki-color-text);">(</span><span style="color:var(--shiki-token-parameter);">a</span><span style="color:var(--shiki-token-punctuation);">,</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-parameter);">b</span><span style="color:var(--shiki-color-text);">):</span></span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-keyword);">return</span><span style="color:var(--shiki-color-text);"> a </span><span style="color:var(--shiki-token-keyword);">+</span><span style="color:var(--shiki-color-text);"> b</span></span>
|
|||
|
|
<span data-line> </span>
|
|||
|
|
<span data-line><span style="color:var(--shiki-token-function);">print</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-function);">add</span><span style="color:var(--shiki-token-punctuation);">(</span><span style="color:var(--shiki-token-constant);">1</span><span style="color:var(--shiki-token-punctuation);">, </span><span style="color:var(--shiki-token-constant);">2</span><span style="color:var(--shiki-token-punctuation);">))</span><span style="color:var(--shiki-color-text);"> </span><span style="color:var(--shiki-token-comment);"># 输出：3</span></span></code></pre></div></article></div><div class="right sidebar"><div class="graph "><h3>Graph View</h3><div class="graph-outer"><div id="graph-container" data-cfg="{"drag":true,"zoom":true,"depth":1,"scale":1.1,"repelForce":0.5,"centerForce":0.3,"linkDistance":30,"fontSize":0.6,"opacityScale":1,"showTags":true,"removeTags":[]}"></div><svg version="1.1" id="global-graph-icon" xmlns="http://www.w3.org/2000/svg" xmlnsXlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewBox="0 0 55 55" fill="currentColor" xmlSpace="preserve"><path d="M49,0c-3.309,0-6,2.691-6,6c0,1.035,0.263,2.009,0.726,2.86l-9.829,9.829C32.542,17.634,30.846,17,29,17
|
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s-3.542,0.634-4.898,1.688l-7.669-7.669C16.785,10.424,17,9.74,17,9c0-2.206-1.794-4-4-4S9,6.794,9,9s1.794,4,4,4
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c0.74,0,1.424-0.215,2.019-0.567l7.669,7.669C21.634,21.458,21,23.154,21,25s0.634,3.542,1.688,4.897L10.024,42.562
|
|||
|
|
C8.958,41.595,7.549,41,6,41c-3.309,0-6,2.691-6,6s2.691,6,6,6s6-2.691,6-6c0-1.035-0.263-2.009-0.726-2.86l12.829-12.829
|
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|
|
c1.106,0.86,2.44,1.436,3.898,1.619v10.16c-2.833,0.478-5,2.942-5,5.91c0,3.309,2.691,6,6,6s6-2.691,6-6c0-2.967-2.167-5.431-5-5.91
|
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v-10.16c1.458-0.183,2.792-0.759,3.898-1.619l7.669,7.669C41.215,39.576,41,40.26,41,41c0,2.206,1.794,4,4,4s4-1.794,4-4
|
|||
|
|
s-1.794-4-4-4c-0.74,0-1.424,0.215-2.019,0.567l-7.669-7.669C36.366,28.542,37,26.846,37,25s-0.634-3.542-1.688-4.897l9.665-9.665
|
|||
|
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C46.042,11.405,47.451,12,49,12c3.309,0,6-2.691,6-6S52.309,0,49,0z M11,9c0-1.103,0.897-2,2-2s2,0.897,2,2s-0.897,2-2,2
|
|||
|
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S11,10.103,11,9z M6,51c-2.206,0-4-1.794-4-4s1.794-4,4-4s4,1.794,4,4S8.206,51,6,51z M33,49c0,2.206-1.794,4-4,4s-4-1.794-4-4
|
|||
|
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s1.794-4,4-4S33,46.794,33,49z M29,31c-3.309,0-6-2.691-6-6s2.691-6,6-6s6,2.691,6,6S32.309,31,29,31z M47,41c0,1.103-0.897,2-2,2
|
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s-2-0.897-2-2s0.897-2,2-2S47,39.897,47,41z M49,10c-2.206,0-4-1.794-4-4s1.794-4,4-4s4,1.794,4,4S51.206,10,49,10z"></path></svg></div><div id="global-graph-outer"><div id="global-graph-container" data-cfg="{"drag":true,"zoom":true,"depth":-1,"scale":0.9,"repelForce":0.5,"centerForce":0.3,"linkDistance":30,"fontSize":0.6,"opacityScale":1,"showTags":true,"removeTags":[]}"></div></div></div><div class="toc desktop-only"><button type="button" id="toc" class><h3>Table of Contents</h3><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="fold"><polyline points="6 9 12 15 18 9"></polyline></svg></button><div id="toc-content"><ul class="overflow"><li class="depth-0"><a href="#条件语句" data-for="条件语句">条件语句</a></li><li class="depth-1"><a href="#if" data-for="if">If</a></li><li class="depth-1"><a href="#match" data-for="match">Match</a></li><li class="depth-0"><a href="#循环语句" data-for="循环语句">循环语句</a></li><li class="depth-1"><a href="#for" data-for="for">For</a></li><li class="depth-1"><a href="#while" data-for="while">While</a></li><li class="depth-1"><a href="#break" data-for="break">Break</a></li><li class="depth-1"><a href="#continue" data-for="continue">Continue</a></li><li class="depth-1"><a href="#else" data-for="else">Else</a></li><li class="depth-0"><a href="#异常语句" data-for="异常语句">异常语句</a></li><li class="depth-1"><a href="#tryexceptfinally" data-for="tryexceptfinally">try、except、finally</a></li><li class="depth-1"><a href="#with" data-for="with">With</a></li><li class="depth-1"><a href="#raise" data-for="raise">Raise</a></li><li class="depth-0"><a href="#其他语句" data-for="其他语句">其他语句</a></li><li class="depth-1"><a href="#assert" data-for="assert">Assert</a></li><li class="depth-1"><a href="#pass" data-for="pass">Pass</a></li><li class="depth-1"><a href="#return" data-for="return">Return</a></li></ul></div></div><div class="backlinks "><h3>Backlinks</h3><ul class="overflow"><li>No backlinks found</li></ul></div><div class="explorer mobile-only"><button type="button" id="explorer" data-behavior="collapse" data-collapsed="collapsed" data-savestate="true" data-tree="[{"path":"Journal","collapsed":true},{"path":"Journal/2024","collapsed":true},{"path":"Journal/2024/W33","collapsed":true},{"path":"Obsidian","collapsed":true},{"path":"Obsidian/Templates","collapsed":true},{"path":"Personal","collapsed":true},{"path":"Personal/Blog","collapsed":true},{"path":"Personal/Blog/2018","collapsed":true},{"path":"Personal/Blog/2020","collapsed":true},{"path":"Personal/Blog/2021","collapsed":true},{"path":"Personal/Blog/2022","collapsed":true},{"path":"Personal/Blog/2023","collapsed":true},{"path":"Personal/Blog/2024","collapsed":true},{"path":"Personal/Book","collapsed":true},{"path":"Personal/Book/个人成长","collapsed":true},{"path":"Personal/Book/医学健康","collapsed":true},{"path":"Personal/Book/历史","collapsed":true},{"path":"Personal/Book/哲学宗教","collapsed":true},{"path":"Personal/Book/心理","collapsed":true},{"path":"Personal/Book/政治军事","collapsed":true},{"path":"Personal/Book/教育学习","collapsed":true},{"path":"Personal/Book/文学"
|
|||
|
|
function toggleCallout() {
|
|||
|
|
const outerBlock = this.parentElement;
|
|||
|
|
outerBlock.classList.toggle(`is-collapsed`);
|
|||
|
|
const collapsed = outerBlock.classList.contains(`is-collapsed`);
|
|||
|
|
const height = collapsed ? this.scrollHeight : outerBlock.scrollHeight;
|
|||
|
|
outerBlock.style.maxHeight = height + `px`;
|
|||
|
|
let current = outerBlock;
|
|||
|
|
let parent = outerBlock.parentElement;
|
|||
|
|
while (parent) {
|
|||
|
|
if (!parent.classList.contains(`callout`)) {
|
|||
|
|
return;
|
|||
|
|
}
|
|||
|
|
const collapsed2 = parent.classList.contains(`is-collapsed`);
|
|||
|
|
const height2 = collapsed2 ? parent.scrollHeight : parent.scrollHeight + current.scrollHeight;
|
|||
|
|
parent.style.maxHeight = height2 + `px`;
|
|||
|
|
current = parent;
|
|||
|
|
parent = parent.parentElement;
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
function setupCallout() {
|
|||
|
|
const collapsible = document.getElementsByClassName(
|
|||
|
|
`callout is-collapsible`
|
|||
|
|
);
|
|||
|
|
for (const div of collapsible) {
|
|||
|
|
const title = div.firstElementChild;
|
|||
|
|
if (title) {
|
|||
|
|
title.removeEventListener(`click`, toggleCallout);
|
|||
|
|
title.addEventListener(`click`, toggleCallout);
|
|||
|
|
const collapsed = div.classList.contains(`is-collapsed`);
|
|||
|
|
const height = collapsed ? title.scrollHeight : div.scrollHeight;
|
|||
|
|
div.style.maxHeight = height + `px`;
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
document.addEventListener(`nav`, setupCallout);
|
|||
|
|
window.addEventListener(`resize`, setupCallout);
|
|||
|
|
</script><script type="module">
|
|||
|
|
import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.esm.min.mjs';
|
|||
|
|
const darkMode = document.documentElement.getAttribute('saved-theme') === 'dark'
|
|||
|
|
mermaid.initialize({
|
|||
|
|
startOnLoad: false,
|
|||
|
|
securityLevel: 'loose',
|
|||
|
|
theme: darkMode ? 'dark' : 'default'
|
|||
|
|
});
|
|||
|
|
document.addEventListener('nav', async () => {
|
|||
|
|
await mermaid.run({
|
|||
|
|
querySelector: '.mermaid'
|
|||
|
|
})
|
|||
|
|
});
|
|||
|
|
</script><script src="https://cdn.jsdelivr.net/npm/katex@0.16.7/dist/contrib/copy-tex.min.js" type="application/javascript"></script><script src="../../../../postscript.js" type="module"></script></html>
|