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<li><a class="reference internal" href="#">3. Python 速览</a><ul>
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<section id="an-informal-introduction-to-python">
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<span id="tut-informal"></span><h1><span class="section-number">3. </span>Python 速览<a class="headerlink" href="#an-informal-introduction-to-python" title="永久链接至标题">¶</a></h1>
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<p>下面的例子以是否显示提示符（<a class="reference internal" href="../glossary.html#term-0"><span class="xref std std-term">>>></span></a> 与 <a class="reference internal" href="../glossary.html#term-1"><span class="xref std std-term">...</span></a>）区分输入与输出：输入例子中的代码时，要键入以提示符开头的行中提示符后的所有内容；未以提示符开头的行是解释器的输出。注意，例子中的某行出现的第二个提示符是用来结束多行命令的，此时，要键入一个空白行。</p>
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<p id="index-0">本手册中的许多例子，甚至交互式命令都包含注释。Python 注释以 <code class="docutils literal notranslate"><span class="pre">#</span></code> 开头，直到该物理行结束。注释可以在行开头，或空白符与代码之后，但不能在字符串里面。字符串中的 # 号就是 # 号。注释用于阐明代码，Python 不解释注释，键入例子时，可以不输入注释。</p>
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<p>示例如下：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># this is the first comment</span>
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<span class="n">spam</span> <span class="o">=</span> <span class="mi">1</span> <span class="c1"># and this is the second comment</span>
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<span class="c1"># ... and now a third!</span>
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<span class="n">text</span> <span class="o">=</span> <span class="s2">"# This is not a comment because it's inside quotes."</span>
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</pre></div>
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</div>
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<section id="using-python-as-a-calculator">
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<span id="tut-calculator"></span><h2><span class="section-number">3.1. </span>Python 用作计算器<a class="headerlink" href="#using-python-as-a-calculator" title="永久链接至标题">¶</a></h2>
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<p>现在，尝试一些简单的 Python 命令。启动解释器，等待主提示符（<code class="docutils literal notranslate"><span class="pre">>>></span></code> ）出现。</p>
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<section id="numbers">
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<span id="tut-numbers"></span><h3><span class="section-number">3.1.1. </span>数字<a class="headerlink" href="#numbers" title="永久链接至标题">¶</a></h3>
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<p>解释器像一个简单的计算器：输入表达式，就会给出答案。表达式的语法很直接：运算符 <code class="docutils literal notranslate"><span class="pre">+</span></code>、<code class="docutils literal notranslate"><span class="pre">-</span></code>、<code class="docutils literal notranslate"><span class="pre">*</span></code>、<code class="docutils literal notranslate"><span class="pre">/</span></code> 的用法和其他大部分语言一样（比如，Pascal 或 C）；括号 (<code class="docutils literal notranslate"><span class="pre">()</span></code>) 用来分组。例如：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">2</span> <span class="o">+</span> <span class="mi">2</span>
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<span class="go">4</span>
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<span class="gp">>>> </span><span class="mi">50</span> <span class="o">-</span> <span class="mi">5</span><span class="o">*</span><span class="mi">6</span>
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<span class="go">20</span>
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<span class="gp">>>> </span><span class="p">(</span><span class="mi">50</span> <span class="o">-</span> <span class="mi">5</span><span class="o">*</span><span class="mi">6</span><span class="p">)</span> <span class="o">/</span> <span class="mi">4</span>
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<span class="go">5.0</span>
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<span class="gp">>>> </span><span class="mi">8</span> <span class="o">/</span> <span class="mi">5</span> <span class="c1"># division always returns a floating point number</span>
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<span class="go">1.6</span>
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</pre></div>
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</div>
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<p>整数（如，<code class="docutils literal notranslate"><span class="pre">2</span></code>、<code class="docutils literal notranslate"><span class="pre">4</span></code>、<code class="docutils literal notranslate"><span class="pre">20</span></code> ）的类型是 <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>，带小数（如，<code class="docutils literal notranslate"><span class="pre">5.0</span></code>、<code class="docutils literal notranslate"><span class="pre">1.6</span></code> ）的类型是 <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>。本教程后半部分将介绍更多数字类型。</p>
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<p>除法运算（<code class="docutils literal notranslate"><span class="pre">/</span></code>）返回浮点数。用 <code class="docutils literal notranslate"><span class="pre">//</span></code> 运算符执行 <a class="reference internal" href="../glossary.html#term-floor-division"><span class="xref std std-term">floor division</span></a> 的结果是整数（忽略小数）；计算余数用 <code class="docutils literal notranslate"><span class="pre">%</span></code>：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">17</span> <span class="o">/</span> <span class="mi">3</span> <span class="c1"># classic division returns a float</span>
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<span class="go">5.666666666666667</span>
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<span class="gp">>>></span>
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<span class="gp">>>> </span><span class="mi">17</span> <span class="o">//</span> <span class="mi">3</span> <span class="c1"># floor division discards the fractional part</span>
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<span class="go">5</span>
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<span class="gp">>>> </span><span class="mi">17</span> <span class="o">%</span> <span class="mi">3</span> <span class="c1"># the % operator returns the remainder of the division</span>
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<span class="go">2</span>
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<span class="gp">>>> </span><span class="mi">5</span> <span class="o">*</span> <span class="mi">3</span> <span class="o">+</span> <span class="mi">2</span> <span class="c1"># result * divisor + remainder</span>
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<span class="go">17</span>
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</pre></div>
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</div>
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<p>Python 用 <code class="docutils literal notranslate"><span class="pre">**</span></code> 运算符计算乘方 <a class="footnote-reference brackets" href="#id3" id="id1">1</a>：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">5</span> <span class="o">**</span> <span class="mi">2</span> <span class="c1"># 5 squared</span>
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<span class="go">25</span>
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<span class="gp">>>> </span><span class="mi">2</span> <span class="o">**</span> <span class="mi">7</span> <span class="c1"># 2 to the power of 7</span>
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<span class="go">128</span>
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</pre></div>
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</div>
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<p>等号（<code class="docutils literal notranslate"><span class="pre">=</span></code>）用于给变量赋值。赋值后，下一个交互提示符的位置不显示任何结果：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">width</span> <span class="o">=</span> <span class="mi">20</span>
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<span class="gp">>>> </span><span class="n">height</span> <span class="o">=</span> <span class="mi">5</span> <span class="o">*</span> <span class="mi">9</span>
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<span class="gp">>>> </span><span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
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<span class="go">900</span>
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</pre></div>
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</div>
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<p>如果变量未定义（即，未赋值），使用该变量会提示错误：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">n</span> <span class="c1"># try to access an undefined variable</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 'n' is not defined</span>
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</pre></div>
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</div>
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<p>Python 全面支持浮点数；混合类型运算数的运算会把整数转换为浮点数：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">4</span> <span class="o">*</span> <span class="mf">3.75</span> <span class="o">-</span> <span class="mi">1</span>
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||||
<span class="go">14.0</span>
|
||||
</pre></div>
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</div>
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<p>交互模式下，上次输出的表达式会赋给变量 <code class="docutils literal notranslate"><span class="pre">_</span></code>。把 Python 当作计算器时，用该变量实现下一步计算更简单，例如：</p>
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<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">tax</span> <span class="o">=</span> <span class="mf">12.5</span> <span class="o">/</span> <span class="mi">100</span>
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||||
<span class="gp">>>> </span><span class="n">price</span> <span class="o">=</span> <span class="mf">100.50</span>
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||||
<span class="gp">>>> </span><span class="n">price</span> <span class="o">*</span> <span class="n">tax</span>
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||||
<span class="go">12.5625</span>
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||||
<span class="gp">>>> </span><span class="n">price</span> <span class="o">+</span> <span class="n">_</span>
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||||
<span class="go">113.0625</span>
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||||
<span class="gp">>>> </span><span class="nb">round</span><span class="p">(</span><span class="n">_</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
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||||
<span class="go">113.06</span>
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||||
</pre></div>
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||||
</div>
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||||
<p>最好把该变量当作只读类型。不要为它显式赋值，否则会创建一个同名独立局部变量，该变量会用它的魔法行为屏蔽内置变量。</p>
|
||||
<p>除了 <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> 和 <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>，Python 还支持其他数字类型，例如 <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate"><span class="pre">Decimal</span></code></a> 或 <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate"><span class="pre">Fraction</span></code></a>。Python 还内置支持 <a class="reference internal" href="../library/stdtypes.html#typesnumeric"><span class="std std-ref">复数</span></a>，后缀 <code class="docutils literal notranslate"><span class="pre">j</span></code> 或 <code class="docutils literal notranslate"><span class="pre">J</span></code> 用于表示虚数（例如 <code class="docutils literal notranslate"><span class="pre">3+5j</span></code> ）。</p>
|
||||
</section>
|
||||
<section id="strings">
|
||||
<span id="tut-strings"></span><h3><span class="section-number">3.1.2. </span>字符串<a class="headerlink" href="#strings" title="永久链接至标题">¶</a></h3>
|
||||
<p>除了数字，Python 还可以操作字符串。字符串有多种表现形式，用单引号（<code class="docutils literal notranslate"><span class="pre">'……'</span></code>）或双引号（<code class="docutils literal notranslate"><span class="pre">"……"</span></code>）标注的结果相同 <a class="footnote-reference brackets" href="#id4" id="id2">2</a>。反斜杠 <code class="docutils literal notranslate"><span class="pre">\</span></code> 用于转义：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'spam eggs'</span> <span class="c1"># single quotes</span>
|
||||
<span class="go">'spam eggs'</span>
|
||||
<span class="gp">>>> </span><span class="s1">'doesn</span><span class="se">\'</span><span class="s1">t'</span> <span class="c1"># use \' to escape the single quote...</span>
|
||||
<span class="go">"doesn't"</span>
|
||||
<span class="gp">>>> </span><span class="s2">"doesn't"</span> <span class="c1"># ...or use double quotes instead</span>
|
||||
<span class="go">"doesn't"</span>
|
||||
<span class="gp">>>> </span><span class="s1">'"Yes," they said.'</span>
|
||||
<span class="go">'"Yes," they said.'</span>
|
||||
<span class="gp">>>> </span><span class="s2">"</span><span class="se">\"</span><span class="s2">Yes,</span><span class="se">\"</span><span class="s2"> they said."</span>
|
||||
<span class="go">'"Yes," they said.'</span>
|
||||
<span class="gp">>>> </span><span class="s1">'"Isn</span><span class="se">\'</span><span class="s1">t," they said.'</span>
|
||||
<span class="go">'"Isn\'t," they said.'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>交互式解释器会为输出的字符串加注引号，特殊字符使用反斜杠转义。虽然，有时输出的字符串看起来与输入的字符串不一样（外加的引号可能会改变），但两个字符串是相同的。如果字符串中有单引号而没有双引号，该字符串外将加注双引号，反之，则加注单引号。<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> 函数输出的内容更简洁易读，它会省略两边的引号，并输出转义后的特殊字符：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'"Isn</span><span class="se">\'</span><span class="s1">t," they said.'</span>
|
||||
<span class="go">'"Isn\'t," they said.'</span>
|
||||
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'"Isn</span><span class="se">\'</span><span class="s1">t," they said.'</span><span class="p">)</span>
|
||||
<span class="go">"Isn't," they said.</span>
|
||||
<span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'First line.</span><span class="se">\n</span><span class="s1">Second line.'</span> <span class="c1"># \n means newline</span>
|
||||
<span class="gp">>>> </span><span class="n">s</span> <span class="c1"># without print(), \n is included in the output</span>
|
||||
<span class="go">'First line.\nSecond line.'</span>
|
||||
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="c1"># with print(), \n produces a new line</span>
|
||||
<span class="go">First line.</span>
|
||||
<span class="go">Second line.</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>如果不希望前置 <code class="docutils literal notranslate"><span class="pre">\</span></code> 的字符转义成特殊字符，可以使用 <em>原始字符串</em>，在引号前添加 <code class="docutils literal notranslate"><span class="pre">r</span></code> 即可：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'C:\some</span><span class="se">\n</span><span class="s1">ame'</span><span class="p">)</span> <span class="c1"># here \n means newline!</span>
|
||||
<span class="go">C:\some</span>
|
||||
<span class="go">ame</span>
|
||||
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="sa">r</span><span class="s1">'C:\some\name'</span><span class="p">)</span> <span class="c1"># note the r before the quote</span>
|
||||
<span class="go">C:\some\name</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>字符串字面值可以包含多行。 一种实现方式是使用三重引号：<code class="docutils literal notranslate"><span class="pre">"""..."""</span></code> 或 <code class="docutils literal notranslate"><span class="pre">'''...'''</span></code>。 字符串中将自动包括行结束符，但也可以在换行的地方添加一个 <code class="docutils literal notranslate"><span class="pre">\</span></code> 来避免此情况。 参见以下示例：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="s2">"""</span><span class="se">\</span>
|
||||
<span class="s2">Usage: thingy [OPTIONS]</span>
|
||||
<span class="s2"> -h Display this usage message</span>
|
||||
<span class="s2"> -H hostname Hostname to connect to</span>
|
||||
<span class="s2">"""</span><span class="p">)</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>输出如下（请注意开始的换行符没有被包括在内）：</p>
|
||||
<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>Usage: thingy [OPTIONS]
|
||||
-h Display this usage message
|
||||
-H hostname Hostname to connect to
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>字符串可以用 <code class="docutils literal notranslate"><span class="pre">+</span></code> 合并（粘到一起），也可以用 <code class="docutils literal notranslate"><span class="pre">*</span></code> 重复：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># 3 times 'un', followed by 'ium'</span>
|
||||
<span class="gp">>>> </span><span class="mi">3</span> <span class="o">*</span> <span class="s1">'un'</span> <span class="o">+</span> <span class="s1">'ium'</span>
|
||||
<span class="go">'unununium'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>相邻的两个或多个 <em>字符串字面值</em> （引号标注的字符）会自动合并：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'Py'</span> <span class="s1">'thon'</span>
|
||||
<span class="go">'Python'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>拼接分隔开的长字符串时，这个功能特别实用：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">text</span> <span class="o">=</span> <span class="p">(</span><span class="s1">'Put several strings within parentheses '</span>
|
||||
<span class="gp">... </span> <span class="s1">'to have them joined together.'</span><span class="p">)</span>
|
||||
<span class="gp">>>> </span><span class="n">text</span>
|
||||
<span class="go">'Put several strings within parentheses to have them joined together.'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>这项功能只能用于两个字面值，不能用于变量或表达式：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">prefix</span> <span class="o">=</span> <span class="s1">'Py'</span>
|
||||
<span class="gp">>>> </span><span class="n">prefix</span> <span class="s1">'thon'</span> <span class="c1"># can't concatenate a variable and a string literal</span>
|
||||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>
|
||||
<span class="w"> </span><span class="n">prefix</span> <span class="s1">'thon'</span>
|
||||
<span class="w"> </span><span class="pm">^</span>
|
||||
<span class="gr">SyntaxError</span>: <span class="n">invalid syntax</span>
|
||||
<span class="gp">>>> </span><span class="p">(</span><span class="s1">'un'</span> <span class="o">*</span> <span class="mi">3</span><span class="p">)</span> <span class="s1">'ium'</span>
|
||||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>
|
||||
<span class="w"> </span><span class="p">(</span><span class="s1">'un'</span> <span class="o">*</span> <span class="mi">3</span><span class="p">)</span> <span class="s1">'ium'</span>
|
||||
<span class="w"> </span><span class="pm">^</span>
|
||||
<span class="gr">SyntaxError</span>: <span class="n">invalid syntax</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>合并多个变量，或合并变量与字面值，要用 <code class="docutils literal notranslate"><span class="pre">+</span></code>：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">prefix</span> <span class="o">+</span> <span class="s1">'thon'</span>
|
||||
<span class="go">'Python'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>字符串支持 <em>索引</em> （下标访问），第一个字符的索引是 0。单字符没有专用的类型，就是长度为一的字符串：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span> <span class="o">=</span> <span class="s1">'Python'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="c1"># character in position 0</span>
|
||||
<span class="go">'P'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span> <span class="c1"># character in position 5</span>
|
||||
<span class="go">'n'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>索引还支持负数，用负数索引时，从右边开始计数：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="c1"># last character</span>
|
||||
<span class="go">'n'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># second-last character</span>
|
||||
<span class="go">'o'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">6</span><span class="p">]</span>
|
||||
<span class="go">'P'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>注意，-0 和 0 一样，因此，负数索引从 -1 开始。</p>
|
||||
<p>除了索引，字符串还支持 <em>切片</em>。索引可以提取单个字符，<em>切片</em> 则提取子字符串：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># characters from position 0 (included) to 2 (excluded)</span>
|
||||
<span class="go">'Py'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span> <span class="c1"># characters from position 2 (included) to 5 (excluded)</span>
|
||||
<span class="go">'tho'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>注意，输出结果包含切片开始，但不包含切片结束。因此，<code class="docutils literal notranslate"><span class="pre">s[:i]</span> <span class="pre">+</span> <span class="pre">s[i:]</span></code> 总是等于 <code class="docutils literal notranslate"><span class="pre">s</span></code>：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">2</span><span class="p">]</span> <span class="o">+</span> <span class="n">word</span><span class="p">[</span><span class="mi">2</span><span class="p">:]</span>
|
||||
<span class="go">'Python'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">4</span><span class="p">]</span> <span class="o">+</span> <span class="n">word</span><span class="p">[</span><span class="mi">4</span><span class="p">:]</span>
|
||||
<span class="go">'Python'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>切片索引的默认值很有用；省略开始索引时，默认值为 0，省略结束索引时，默认为到字符串的结尾：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># character from the beginning to position 2 (excluded)</span>
|
||||
<span class="go">'Py'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">4</span><span class="p">:]</span> <span class="c1"># characters from position 4 (included) to the end</span>
|
||||
<span class="go">'on'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">2</span><span class="p">:]</span> <span class="c1"># characters from the second-last (included) to the end</span>
|
||||
<span class="go">'on'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>还可以这样理解切片，索引指向的是字符 <em>之间</em> ，第一个字符的左侧标为 0，最后一个字符的右侧标为 <em>n</em> ，<em>n</em> 是字符串长度。例如：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span> <span class="o">+---+---+---+---+---+---+</span>
|
||||
<span class="o">|</span> <span class="n">P</span> <span class="o">|</span> <span class="n">y</span> <span class="o">|</span> <span class="n">t</span> <span class="o">|</span> <span class="n">h</span> <span class="o">|</span> <span class="n">o</span> <span class="o">|</span> <span class="n">n</span> <span class="o">|</span>
|
||||
<span class="o">+---+---+---+---+---+---+</span>
|
||||
<span class="mi">0</span> <span class="mi">1</span> <span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span> <span class="mi">5</span> <span class="mi">6</span>
|
||||
<span class="o">-</span><span class="mi">6</span> <span class="o">-</span><span class="mi">5</span> <span class="o">-</span><span class="mi">4</span> <span class="o">-</span><span class="mi">3</span> <span class="o">-</span><span class="mi">2</span> <span class="o">-</span><span class="mi">1</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>第一行数字是字符串中索引 0...6 的位置，第二行数字是对应的负数索引位置。<em>i</em> 到 <em>j</em> 的切片由 <em>i</em> 和 <em>j</em> 之间所有对应的字符组成。</p>
|
||||
<p>对于使用非负索引的切片，如果两个索引都不越界，切片长度就是起止索引之差。例如， <code class="docutils literal notranslate"><span class="pre">word[1:3]</span></code> 的长度是 2。</p>
|
||||
<p>索引越界会报错：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">42</span><span class="p">]</span> <span class="c1"># the word only has 6 characters</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">IndexError</span>: <span class="n">string index out of range</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>但是，切片会自动处理越界索引：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">4</span><span class="p">:</span><span class="mi">42</span><span class="p">]</span>
|
||||
<span class="go">'on'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">42</span><span class="p">:]</span>
|
||||
<span class="go">''</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>Python 字符串不能修改，是 <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a> 的。因此，为字符串中某个索引位置赋值会报错：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'J'</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">TypeError</span>: <span class="n">'str' object does not support item assignment</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">2</span><span class="p">:]</span> <span class="o">=</span> <span class="s1">'py'</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">TypeError</span>: <span class="n">'str' object does not support item assignment</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>要生成不同的字符串，应新建一个字符串：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'J'</span> <span class="o">+</span> <span class="n">word</span><span class="p">[</span><span class="mi">1</span><span class="p">:]</span>
|
||||
<span class="go">'Jython'</span>
|
||||
<span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">2</span><span class="p">]</span> <span class="o">+</span> <span class="s1">'py'</span>
|
||||
<span class="go">'Pypy'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>内置函数 <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> 返回字符串的长度：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'supercalifragilisticexpialidocious'</span>
|
||||
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
|
||||
<span class="go">34</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<div class="admonition seealso">
|
||||
<p class="admonition-title">参见</p>
|
||||
<dl class="simple">
|
||||
<dt><a class="reference internal" href="../library/stdtypes.html#textseq"><span class="std std-ref">文本序列类型 --- str</span></a></dt><dd><p>字符串是 <em>序列类型</em> ，支持序列类型的各种操作。</p>
|
||||
</dd>
|
||||
<dt><a class="reference internal" href="../library/stdtypes.html#string-methods"><span class="std std-ref">字符串的方法</span></a></dt><dd><p>字符串支持很多变形与查找方法。</p>
|
||||
</dd>
|
||||
<dt><a class="reference internal" href="../reference/lexical_analysis.html#f-strings"><span class="std std-ref">格式字符串字面值</span></a></dt><dd><p>内嵌表达式的字符串字面值。</p>
|
||||
</dd>
|
||||
<dt><a class="reference internal" href="../library/string.html#formatstrings"><span class="std std-ref">格式字符串语法</span></a></dt><dd><p>使用 <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.format()</span></code></a> 格式化字符串。</p>
|
||||
</dd>
|
||||
<dt><a class="reference internal" href="../library/stdtypes.html#old-string-formatting"><span class="std std-ref">printf 风格的字符串格式化</span></a></dt><dd><p>这里详述了用 <code class="docutils literal notranslate"><span class="pre">%</span></code> 运算符格式化字符串的操作。</p>
|
||||
</dd>
|
||||
</dl>
|
||||
</div>
|
||||
</section>
|
||||
<section id="lists">
|
||||
<span id="tut-lists"></span><h3><span class="section-number">3.1.3. </span>列表<a class="headerlink" href="#lists" title="永久链接至标题">¶</a></h3>
|
||||
<p>Python 支持多种 <em>复合</em> 数据类型，可将不同值组合在一起。最常用的 <em>列表</em> ，是用方括号标注，逗号分隔的一组值。<em>列表</em> 可以包含不同类型的元素，但一般情况下，各个元素的类型相同：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">9</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">25</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="n">squares</span>
|
||||
<span class="go">[1, 4, 9, 16, 25]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>和字符串（及其他内置 <a class="reference internal" href="../glossary.html#term-sequence"><span class="xref std std-term">sequence</span></a> 类型）一样，列表也支持索引和切片：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="c1"># indexing returns the item</span>
|
||||
<span class="go">1</span>
|
||||
<span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||||
<span class="go">25</span>
|
||||
<span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="o">-</span><span class="mi">3</span><span class="p">:]</span> <span class="c1"># slicing returns a new list</span>
|
||||
<span class="go">[9, 16, 25]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>切片操作返回包含请求元素的新列表。以下切片操作会返回列表的 <a class="reference internal" href="../library/copy.html#shallow-vs-deep-copy"><span class="std std-ref">浅拷贝</span></a>：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span><span class="p">[:]</span>
|
||||
<span class="go">[1, 4, 9, 16, 25]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>列表还支持合并操作：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">+</span> <span class="p">[</span><span class="mi">36</span><span class="p">,</span> <span class="mi">49</span><span class="p">,</span> <span class="mi">64</span><span class="p">,</span> <span class="mi">81</span><span class="p">,</span> <span class="mi">100</span><span class="p">]</span>
|
||||
<span class="go">[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>与 <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a> 字符串不同, 列表是 <a class="reference internal" href="../glossary.html#term-mutable"><span class="xref std std-term">mutable</span></a> 类型，其内容可以改变：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">cubes</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">27</span><span class="p">,</span> <span class="mi">65</span><span class="p">,</span> <span class="mi">125</span><span class="p">]</span> <span class="c1"># something's wrong here</span>
|
||||
<span class="gp">>>> </span><span class="mi">4</span> <span class="o">**</span> <span class="mi">3</span> <span class="c1"># the cube of 4 is 64, not 65!</span>
|
||||
<span class="go">64</span>
|
||||
<span class="gp">>>> </span><span class="n">cubes</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="mi">64</span> <span class="c1"># replace the wrong value</span>
|
||||
<span class="gp">>>> </span><span class="n">cubes</span>
|
||||
<span class="go">[1, 8, 27, 64, 125]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p><code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> <em>方法</em> 可以在列表末尾添加新元素（后面我们会看到更多例子）：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">cubes</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">216</span><span class="p">)</span> <span class="c1"># add the cube of 6</span>
|
||||
<span class="gp">>>> </span><span class="n">cubes</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">7</span> <span class="o">**</span> <span class="mi">3</span><span class="p">)</span> <span class="c1"># and the cube of 7</span>
|
||||
<span class="gp">>>> </span><span class="n">cubes</span>
|
||||
<span class="go">[1, 8, 27, 64, 125, 216, 343]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>为切片赋值可以改变列表大小，甚至清空整个列表：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">letters</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">,</span> <span class="s1">'d'</span><span class="p">,</span> <span class="s1">'e'</span><span class="p">,</span> <span class="s1">'f'</span><span class="p">,</span> <span class="s1">'g'</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span>
|
||||
<span class="go">['a', 'b', 'c', 'd', 'e', 'f', 'g']</span>
|
||||
<span class="gp">>>> </span><span class="c1"># replace some values</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'C'</span><span class="p">,</span> <span class="s1">'D'</span><span class="p">,</span> <span class="s1">'E'</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span>
|
||||
<span class="go">['a', 'b', 'C', 'D', 'E', 'f', 'g']</span>
|
||||
<span class="gp">>>> </span><span class="c1"># now remove them</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span> <span class="o">=</span> <span class="p">[]</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span>
|
||||
<span class="go">['a', 'b', 'f', 'g']</span>
|
||||
<span class="gp">>>> </span><span class="c1"># clear the list by replacing all the elements with an empty list</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span><span class="p">[:]</span> <span class="o">=</span> <span class="p">[]</span>
|
||||
<span class="gp">>>> </span><span class="n">letters</span>
|
||||
<span class="go">[]</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>内置函数 <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> 也支持列表：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">letters</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">,</span> <span class="s1">'d'</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">letters</span><span class="p">)</span>
|
||||
<span class="go">4</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>还可以嵌套列表（创建包含其他列表的列表），例如：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="n">n</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="p">[</span><span class="n">a</span><span class="p">,</span> <span class="n">n</span><span class="p">]</span>
|
||||
<span class="gp">>>> </span><span class="n">x</span>
|
||||
<span class="go">[['a', 'b', 'c'], [1, 2, 3]]</span>
|
||||
<span class="gp">>>> </span><span class="n">x</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||||
<span class="go">['a', 'b', 'c']</span>
|
||||
<span class="gp">>>> </span><span class="n">x</span><span class="p">[</span><span class="mi">0</span><span class="p">][</span><span class="mi">1</span><span class="p">]</span>
|
||||
<span class="go">'b'</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</section>
|
||||
</section>
|
||||
<section id="first-steps-towards-programming">
|
||||
<span id="tut-firststeps"></span><h2><span class="section-number">3.2. </span>走向编程的第一步<a class="headerlink" href="#first-steps-towards-programming" title="永久链接至标题">¶</a></h2>
|
||||
<p>当然，Python 还可以完成比二加二更复杂的任务。 例如，可以编写 <a class="reference external" href="https://en.wikipedia.org/wiki/Fibonacci_number">斐波那契数列</a> 的初始子序列，如下所示：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># Fibonacci series:</span>
|
||||
<span class="gp">... </span><span class="c1"># the sum of two elements defines the next</span>
|
||||
<span class="gp">... </span><span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
|
||||
<span class="gp">>>> </span><span class="k">while</span> <span class="n">a</span> <span class="o"><</span> <span class="mi">10</span><span class="p">:</span>
|
||||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
|
||||
<span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
|
||||
<span class="gp">...</span>
|
||||
<span class="go">0</span>
|
||||
<span class="go">1</span>
|
||||
<span class="go">1</span>
|
||||
<span class="go">2</span>
|
||||
<span class="go">3</span>
|
||||
<span class="go">5</span>
|
||||
<span class="go">8</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>本例引入了几个新功能。</p>
|
||||
<ul>
|
||||
<li><p>第一行中的 <em>多重赋值</em> ：变量 <code class="docutils literal notranslate"><span class="pre">a</span></code> 和 <code class="docutils literal notranslate"><span class="pre">b</span></code> 同时获得新值 0 和 1 。最后一行又用了一次多重赋值，体现了，等号右边的所有表达式的值，都是在这一语句对任何变量赋新值之前求出来的——求值顺序为从左到右。</p></li>
|
||||
<li><p><a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> 循环只要条件（这里是 <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">10</span></code>）为真就会一直执行。Python 和 C 一样，任何非零整数都为真，零为假。这个条件也可以是字符串或列表类型的值，事实上，任何序列都可以：长度非零就为真，空序列则为假。示例中的判断只是最简单的比较。比较操作符的写法和 C 语言一样： <code class="docutils literal notranslate"><span class="pre"><</span></code> （小于）、 <code class="docutils literal notranslate"><span class="pre">></span></code> （大于）、 <code class="docutils literal notranslate"><span class="pre">==</span></code> （等于）、 <code class="docutils literal notranslate"><span class="pre"><=</span></code> （小于等于)、 <code class="docutils literal notranslate"><span class="pre">>=</span></code> （大于等于）及 <code class="docutils literal notranslate"><span class="pre">!=</span></code> （不等于）。</p></li>
|
||||
<li><p><em>循环体</em> 是 <em>缩进的</em> ：缩进是 Python 组织语句的方式。在交互式命令行里，得为每个缩进的行输入空格（或制表符）。使用文本编辑器可以实现更复杂的输入方式；所有像样的文本编辑器都支持自动缩进。交互式输入复合语句时，要在最后输入空白行表示完成（因为解析器不知道哪一行代码是代码块的最后一行）。注意，同一块语句的每一行的缩进相同。</p></li>
|
||||
<li><p><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> 函数输出给定参数的值。除了可以以单一的表达式作为参数（比如，前面的计算器的例子），它还能处理多个参数，包括浮点数与字符串。它输出的字符串不带引号，且各参数项之间会插入一个空格，这样可以实现更好的格式化操作：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">i</span> <span class="o">=</span> <span class="mi">256</span><span class="o">*</span><span class="mi">256</span>
|
||||
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'The value of i is'</span><span class="p">,</span> <span class="n">i</span><span class="p">)</span>
|
||||
<span class="go">The value of i is 65536</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<p>关键字参数 <em>end</em> 可以取消输出后面的换行, 或用另一个字符串结尾：</p>
|
||||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
|
||||
<span class="gp">>>> </span><span class="k">while</span> <span class="n">a</span> <span class="o"><</span> <span class="mi">1000</span><span class="p">:</span>
|
||||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">','</span><span class="p">)</span>
|
||||
<span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
|
||||
<span class="gp">...</span>
|
||||
<span class="go">0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</li>
|
||||
</ul>
|
||||
<p class="rubric">备注</p>
|
||||
<dl class="footnote brackets">
|
||||
<dt class="label" id="id3"><span class="brackets"><a class="fn-backref" href="#id1">1</a></span></dt>
|
||||
<dd><p><code class="docutils literal notranslate"><span class="pre">**</span></code> 比 <code class="docutils literal notranslate"><span class="pre">-</span></code> 的优先级更高, 所以 <code class="docutils literal notranslate"><span class="pre">-3**2</span></code> 会被解释成 <code class="docutils literal notranslate"><span class="pre">-(3**2)</span></code> ，因此，结果是 <code class="docutils literal notranslate"><span class="pre">-9</span></code>。要避免这个问题，并且得到 <code class="docutils literal notranslate"><span class="pre">9</span></code>, 可以用 <code class="docutils literal notranslate"><span class="pre">(-3)**2</span></code>。</p>
|
||||
</dd>
|
||||
<dt class="label" id="id4"><span class="brackets"><a class="fn-backref" href="#id2">2</a></span></dt>
|
||||
<dd><p>与其他语言不同，特殊字符如 <code class="docutils literal notranslate"><span class="pre">\n</span></code> 在单引号（<code class="docutils literal notranslate"><span class="pre">'...'</span></code> ）和双引号（<code class="docutils literal notranslate"><span class="pre">"..."</span></code> ）里的意义一样。这两种引号唯一的区别是，不需要在单引号里转义双引号 <code class="docutils literal notranslate"><span class="pre">"</span></code> （但此时必须把单引号转义成 <code class="docutils literal notranslate"><span class="pre">\'</span></code> ），反之亦然。</p>
|
||||
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||||
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