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<a class="post-title-link" href="/2018/04/10/类与对象-1-改变对象实例的字符串显示/" itemprop="url">类与对象-1-改变对象实例的字符串显示</a></h1>
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<h2 id="问题描述"><a href="#问题描述" class="headerlink" title="问题描述"></a>问题描述</h2><p>如果我们想改变对象实例的打印或显示输出，让它们更具可读性，应该怎么做呢？</p>
<h2 id="解决方案"><a href="#解决方案" class="headerlink" title="解决方案"></a>解决方案</h2><p>想要改变一个对象实例的字符串显示，可重新定义它的 <code>__str__()</code> 和 <code>__repr__()</code> 方法。</p>
<p>代码演示：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Pair</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__repr__</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> <span class="string">'Pair({0.x!r}, {0.y!r})'</span>.format(self)</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__str__</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> <span class="string">'({0.x!s}, {0.y!s})'</span>.format(self)</span><br></pre></td></tr></table></figure>
<p><code>__repr__()</code> 方法返回一个<strong>实例的代码表示形式</strong>，通常<strong>用来重新构造这个实例</strong>。 内置的 <code>repr()</code> 函数返回这个字符串，跟我们使用<strong>交互式解释器显示的值是一样的</strong>。</p>
<p> <code>__str__()</code> 方法将实例转换为一个字符串，使用 <code>str()</code> 或 <code>print()</code> 函数会输出这个字符串。</p>
<blockquote>
<p>简而言之：</p>
<p>如果要把一个实例通过 print 显示成字符串，就要实现 <code>__str__()</code>，如果要在交互命令行下直接通过调用实例显示，就要实现 <code>__repr__()</code> 。</p>
</blockquote>
<p>代码示例：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">></span>>> p = Pair(3, 4)</span><br><span class="line"><span class="meta">></span>>> p</span><br><span class="line">Pair(3, 4) # 输出的是 __repr__() </span><br><span class="line"><span class="meta">></span>>> print(p)</span><br><span class="line">(3, 4) # 输出的是 __str__() </span><br><span class="line"><span class="meta">></span>>></span><br></pre></td></tr></table></figure>
<h2 id="扩展讨论"><a href="#扩展讨论" class="headerlink" title="扩展讨论"></a>扩展讨论</h2><h3 id="为何要自定义-repr-和-str-？"><a href="#为何要自定义-repr-和-str-？" class="headerlink" title="为何要自定义 __repr__() 和 __str__() ？"></a>为何要自定义 <code>__repr__()</code> 和 <code>__str__()</code> ？</h3><p>自定义 <code>__repr__()</code> 和 <code>__str__()</code> 通常是很好的习惯，因为它能<strong>简化调试和实例输出</strong>。 </p>
<p>例如，对于某个对象的实例，如果我们仅仅只是打印输出或日志输出，那么我们就会看到更加详细与有用的实例信息。</p>
<blockquote>
<p>简而言之：</p>
<p><code>__repr__()</code> 是给程序员看的。</p>
<p><code>__str__()</code> 是给用户看的。</p>
</blockquote>
<h3 id="几种格式化方法"><a href="#几种格式化方法" class="headerlink" title="几种格式化方法"></a>几种格式化方法</h3><p>上面的例子中我们还演示了<strong>在格式化的时候怎样使用不同的字符串表现形式。</strong> </p>
<h4 id="r-格式化代码"><a href="#r-格式化代码" class="headerlink" title="!r 格式化代码"></a><code>!r</code> 格式化代码</h4><p><code>!r</code> 格式化代码指明输出使用 <code>__repr__()</code> 来代替默认的 <code>__str__()</code> 。 </p>
<p>用前面的类来测试一下：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">></span>>> p = Pair(3, 4)</span><br><span class="line"><span class="meta">></span>>> print('p is {0!r}'.format(p))</span><br><span class="line">p is Pair(3, 4) # 使用 !r 格式化后输出的是 __repr__()</span><br><span class="line"><span class="meta">></span>>> print('p is {0}'.format(p))</span><br><span class="line">p is (3, 4) # 没有格式化的默认输出是 __str__()</span><br><span class="line"><span class="meta">></span>>></span><br></pre></td></tr></table></figure>
<h4 id="0-x-格式化代码"><a href="#0-x-格式化代码" class="headerlink" title="{0.x} 格式化代码"></a><code>{0.x}</code> 格式化代码</h4><p>上面的 <code>format()</code> 方法的使用看上去很有趣，格式化代码 <code>{0.x}</code> 对应的是<strong>第 1 个参数的 x 属性</strong>。 </p>
<p>因此，在下面的方法中，0 实际上指的就是 <code>self</code> 本身。</p>
<p>示例代码：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">def __repr__(self):</span><br><span class="line"> return 'Pair({0.x!r}, {0.y!r})'.format(self)</span><br></pre></td></tr></table></figure>
<h4 id="格式化代码"><a href="#格式化代码" class="headerlink" title="% 格式化代码"></a><code>%</code> 格式化代码</h4><p>作为上面这种实现的一个替代，我们也可以使用 <code>%</code> 操作符。</p>
<p>示例代码：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">def __repr__(self):</span><br><span class="line"> return 'Pair(%r, %r)' % (self.x, self.y)</span><br></pre></td></tr></table></figure>
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<h2 id="问题描述"><a href="#问题描述" class="headerlink" title="问题描述"></a>问题描述</h2><p>我们想把<strong>多个字典或者映射合并</strong>，然后再执行某些操作（如：查找或校验某些 key 是否存在）。</p>
<p>应该如何做呢？</p>
<h2 id="解决方案"><a href="#解决方案" class="headerlink" title="解决方案"></a>解决方案</h2><p>使用 <code>collections</code> 模块中的 <code>ChainMap</code> 类。</p>
<p>示例代码：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> collections <span class="keyword">import</span> ChainMap</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">test_chainmap</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="string">"""合并多个字典"""</span></span><br><span class="line"></span><br><span class="line"> a_dict = {<span class="string">'x'</span>: <span class="number">1</span>, <span class="string">'z'</span>: <span class="number">3</span>}</span><br><span class="line"> b_dict = {<span class="string">'y'</span>: <span class="number">2</span>, <span class="string">'z'</span>: <span class="number">4</span>}</span><br><span class="line"> <span class="comment"># 合并两个字典(映射)，从逻辑上变成了一个</span></span><br><span class="line"> cm = ChainMap(a_dict, b_dict)</span><br><span class="line"> <span class="keyword">assert</span> cm[<span class="string">'x'</span>] == <span class="number">1</span></span><br><span class="line"> <span class="keyword">assert</span> cm[<span class="string">'y'</span>] == <span class="number">2</span></span><br><span class="line"> <span class="keyword">assert</span> cm[<span class="string">'z'</span>] == <span class="number">3</span></span><br><span class="line"> <span class="comment"># 遇到重复的 key，总是返回第一次出现的 value</span></span><br><span class="line"> <span class="comment"># 因此不管访问几次，cm['z'] 总是返回字典 a 中的 value。</span></span><br><span class="line"> <span class="keyword">assert</span> cm[<span class="string">'z'</span>] == <span class="number">3</span></span><br></pre></td></tr></table></figure>
<h2 id="扩展讨论"><a href="#扩展讨论" class="headerlink" title="扩展讨论"></a>扩展讨论</h2><p><strong>一个 <code>ChainMap</code> 接受多个字典并将它们在逻辑上变为一个字典。</strong> </p>
<p>然后，</p>
<p>这些字典并不是真的合并在一起了， <code>ChainMap</code> 类只是在内部创建了一个<strong>容纳这些字典的列表</strong>并<strong>重新定义了一些常见的字典操作</strong>来遍历这个列表。</p>
<h3 id="ChainMap的字典操作"><a href="#ChainMap的字典操作" class="headerlink" title="ChainMap的字典操作"></a>ChainMap的字典操作</h3><p>使用 ChainMap 合并后，大部分字典操作都是可以正常使用的。</p>
<p>示例代码：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">test_chainmap_operate</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="string">"""ChainMap的字典操作"""</span></span><br><span class="line"></span><br><span class="line"> a_dict = {<span class="string">'x'</span>: <span class="number">1</span>, <span class="string">'z'</span>: <span class="number">3</span>}</span><br><span class="line"> b_dict = {<span class="string">'y'</span>: <span class="number">2</span>, <span class="string">'z'</span>: <span class="number">4</span>}</span><br><span class="line"> cm = ChainMap(a_dict, b_dict)</span><br><span class="line"></span><br><span class="line"> <span class="keyword">assert</span> len(cm) == <span class="number">3</span></span><br><span class="line"> <span class="keyword">assert</span> list(cm.keys()) == [<span class="string">'x'</span>, <span class="string">'y'</span>, <span class="string">'z'</span>]</span><br><span class="line"> <span class="keyword">assert</span> list(cm.values()) == [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]</span><br><span class="line"></span><br><span class="line"> <span class="comment"># ChainMap的更新操作</span></span><br><span class="line"> cm[<span class="string">'z'</span>] = <span class="number">10</span></span><br><span class="line"> cm[<span class="string">'w'</span>] = <span class="number">40</span></span><br><span class="line"> <span class="comment"># 结果只影响列表中第一个字典，也就是 a_dict</span></span><br><span class="line"> <span class="keyword">assert</span> a_dict == {<span class="string">'w'</span>: <span class="number">40</span>, <span class="string">'x'</span>: <span class="number">1</span>, <span class="string">'z'</span>: <span class="number">10</span>}</span><br><span class="line"> <span class="comment"># ChainMap的删除操作</span></span><br><span class="line"> <span class="keyword">del</span> cm[<span class="string">'x'</span>]</span><br><span class="line"> <span class="comment"># 结果也只影响列表中第一个字典，也就是 a_dict</span></span><br><span class="line"> <span class="keyword">assert</span> a_dict == {<span class="string">'w'</span>: <span class="number">40</span>, <span class="string">'z'</span>: <span class="number">10</span>}</span><br></pre></td></tr></table></figure>
<p><code>ChainMap</code> 对于编程语言中的作用范围变量（比如 <code>globals</code> , <code>locals</code> 等）是非常有用的。 </p>
<h3 id="ChainMap更多的用法"><a href="#ChainMap更多的用法" class="headerlink" title="ChainMap更多的用法"></a>ChainMap更多的用法</h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>values = ChainMap()</span><br><span class="line"><span class="meta">>>> </span>values[<span class="string">'x'</span>] = <span class="number">1</span></span><br><span class="line"><span class="meta">>>> </span><span class="comment"># 添加一个新的映射</span></span><br><span class="line"><span class="meta">>>> </span>values = values.new_child()</span><br><span class="line"><span class="meta">>>> </span>values[<span class="string">'x'</span>] = <span class="number">2</span></span><br><span class="line"><span class="meta">>>> </span><span class="comment"># 添加一个新的映射</span></span><br><span class="line"><span class="meta">>>> </span>values = values.new_child()</span><br><span class="line"><span class="meta">>>> </span>values[<span class="string">'x'</span>] = <span class="number">3</span></span><br><span class="line"><span class="meta">>>> </span>values</span><br><span class="line">ChainMap({<span class="string">'x'</span>: <span class="number">3</span>}, {<span class="string">'x'</span>: <span class="number">2</span>}, {<span class="string">'x'</span>: <span class="number">1</span>})</span><br><span class="line"><span class="meta">>>> </span>values[<span class="string">'x'</span>]</span><br><span class="line"><span class="number">3</span></span><br><span class="line"><span class="meta">>>> </span><span class="comment"># 放弃最后的映射</span></span><br><span class="line"><span class="meta">>>> </span>values = values.parents</span><br><span class="line"><span class="meta">>>> </span>values[<span class="string">'x'</span>]</span><br><span class="line"><span class="number">2</span></span><br><span class="line"><span class="meta">>>> </span><span class="comment">#放弃最后的映射</span></span><br><span class="line"><span class="meta">>>> </span>values = values.parents</span><br><span class="line"><span class="meta">>>> </span>values[<span class="string">'x'</span>]</span><br><span class="line"><span class="number">1</span></span><br><span class="line"><span class="meta">>>> </span>values</span><br><span class="line">ChainMap({<span class="string">'x'</span>: <span class="number">1</span>})</span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
<h3 id="update-vs-ChainMap"><a href="#update-vs-ChainMap" class="headerlink" title="update vs ChainMap"></a>update vs ChainMap</h3><p>也许你可能想使用 <code>update()</code> 合并两个字典。</p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>a = {<span class="string">'x'</span>: <span class="number">1</span>, <span class="string">'z'</span>: <span class="number">3</span> }</span><br><span class="line"><span class="meta">>>> </span>b = {<span class="string">'y'</span>: <span class="number">2</span>, <span class="string">'z'</span>: <span class="number">4</span> }</span><br><span class="line"><span class="meta">>>> </span>merged = dict(b)</span><br><span class="line"><span class="meta">>>> </span>merged.update(a)</span><br><span class="line"><span class="meta">>>> </span>merged[<span class="string">'x'</span>]</span><br><span class="line"><span class="number">1</span></span><br><span class="line"><span class="meta">>>> </span>merged[<span class="string">'y'</span>]</span><br><span class="line"><span class="number">2</span></span><br><span class="line"><span class="meta">>>> </span>merged[<span class="string">'z'</span>]</span><br><span class="line"><span class="number">3</span></span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
<p>这样也OK，但它需要你创建一个完全不同的字典对象（或者是破坏现有字典结构）。而且，如原字典做了更新，合并后的新字典并不会同步更新。</p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>a[<span class="string">'x'</span>] = <span class="number">13</span></span><br><span class="line"><span class="meta">>>> </span>merged[<span class="string">'x'</span>]</span><br><span class="line"><span class="number">1</span></span><br></pre></td></tr></table></figure>
<p>而<code>ChainMap</code> 使用的是原来的字典，它不是创建新的字典，所以避免了上面所说的结果。</p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>a = {<span class="string">'x'</span>: <span class="number">1</span>, <span class="string">'z'</span>: <span class="number">3</span> }</span><br><span class="line"><span class="meta">>>> </span>b = {<span class="string">'y'</span>: <span class="number">2</span>, <span class="string">'z'</span>: <span class="number">4</span> }</span><br><span class="line"><span class="meta">>>> </span>merged = ChainMap(a, b)</span><br><span class="line"><span class="meta">>>> </span>merged[<span class="string">'x'</span>]</span><br><span class="line"><span class="number">1</span></span><br><span class="line"><span class="meta">>>> </span>a[<span class="string">'x'</span>] = <span class="number">42</span></span><br><span class="line"><span class="meta">>>> </span>merged[<span class="string">'x'</span>] <span class="comment"># Notice change to merged dicts</span></span><br><span class="line"><span class="number">42</span></span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
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<h2 id="问题描述"><a href="#问题描述" class="headerlink" title="问题描述"></a>问题描述</h2><p>如果我们需要在数据序列上执行聚集函数（比如 <code>sum()</code> , <code>min()</code> , <code>max()</code> ）， 首先需要转换或者过滤数据，如何做更好呢？</p>
<h2 id="解决方案"><a href="#解决方案" class="headerlink" title="解决方案"></a>解决方案</h2><p>一个非常优雅的方式就是使用一个生成器表达式参数去结合数据计算与转换。</p>
<p> 比如，如果计算平方和，可以像下面这样做：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">nums = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]</span><br><span class="line">s = sum(x * x <span class="keyword">for</span> x <span class="keyword">in</span> nums)</span><br></pre></td></tr></table></figure>
<p>下面是更多的例子：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># Determine if any .py files exist in a directory</span></span><br><span class="line"><span class="keyword">import</span> os</span><br><span class="line">files = os.listdir(<span class="string">'dirname'</span>)</span><br><span class="line"><span class="keyword">if</span> any(name.endswith(<span class="string">'.py'</span>) <span class="keyword">for</span> name <span class="keyword">in</span> files):</span><br><span class="line"> print(<span class="string">'There be python!'</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> print(<span class="string">'Sorry, no python.'</span>)</span><br><span class="line"><span class="comment"># Output a tuple as CSV</span></span><br><span class="line">s = (<span class="string">'ACME'</span>, <span class="number">50</span>, <span class="number">123.45</span>)</span><br><span class="line">print(<span class="string">','</span>.join(str(x) <span class="keyword">for</span> x <span class="keyword">in</span> s))</span><br><span class="line"><span class="comment"># Data reduction across fields of a data structure</span></span><br><span class="line">portfolio = [</span><br><span class="line"> {<span class="string">'name'</span>:<span class="string">'GOOG'</span>, <span class="string">'shares'</span>: <span class="number">50</span>},</span><br><span class="line"> {<span class="string">'name'</span>:<span class="string">'YHOO'</span>, <span class="string">'shares'</span>: <span class="number">75</span>},</span><br><span class="line"> {<span class="string">'name'</span>:<span class="string">'AOL'</span>, <span class="string">'shares'</span>: <span class="number">20</span>},</span><br><span class="line"> {<span class="string">'name'</span>:<span class="string">'SCOX'</span>, <span class="string">'shares'</span>: <span class="number">65</span>}</span><br><span class="line">]</span><br><span class="line">min_shares = min(s[<span class="string">'shares'</span>] <span class="keyword">for</span> s <span class="keyword">in</span> portfolio)</span><br></pre></td></tr></table></figure>
<h2 id="扩展讨论"><a href="#扩展讨论" class="headerlink" title="扩展讨论"></a>扩展讨论</h2><p>上面的示例演示了当生成器表达式作为一个单独参数传递给函数时候的巧妙语法（你并不需要多加一个括号）。 比如，下面这些语句是等效的：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">s = sum((x * x <span class="keyword">for</span> x <span class="keyword">in</span> nums)) <span class="comment"># 显示的传递一个生成器表达式对象</span></span><br><span class="line">s = sum(x * x <span class="keyword">for</span> x <span class="keyword">in</span> nums) <span class="comment"># 更加优雅的实现方式，省略了括号</span></span><br></pre></td></tr></table></figure>
<p><strong>使用一个生成器表达式作为参数会比先创建一个临时列表更加高效和优雅。</strong><br>比如，如果不使用生成器表达式的话，可能会考虑使用下面的实现方式：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">nums = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]</span><br><span class="line">s = sum([x * x <span class="keyword">for</span> x <span class="keyword">in</span> nums])</span><br></pre></td></tr></table></figure>
<p>这种方式同样可以达到想要的效果，但是它会多一个步骤，先创建一个额外的列表。 对于小型列表可能没什么关系，但是如果元素数量非常大的时候， 它会创建一个巨大的仅仅被使用一次就被丢弃的临时数据结构。而生成器方案会以迭代的方式转换数据，因此更省内存。</p>
<p>在使用一些聚集函数比如 <code>min()</code> 和 <code>max()</code> 的时候你可能更加倾向于使用生成器版本， 它们接受的一个 key 关键字参数或许对你很有帮助。 </p>
<p>比如，在上面的证券例子中，你可能会考虑下面的实现版本：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># Original: Returns 20</span></span><br><span class="line">min_shares = min(s[<span class="string">'shares'</span>] <span class="keyword">for</span> s <span class="keyword">in</span> portfolio)</span><br><span class="line"><span class="comment"># Alternative: Returns {'name': 'AOL', 'shares': 20}</span></span><br><span class="line">min_shares = min(portfolio, key=<span class="keyword">lambda</span> s: s[<span class="string">'shares'</span>])</span><br></pre></td></tr></table></figure>
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<h2 id="问题描述"><a href="#问题描述" class="headerlink" title="问题描述"></a>问题描述</h2><p>假设我们有一段<strong>通过下标访问列表或元组中元素</strong>的代码，有时这样的代码会难以阅读 怎么办呢？如何通过名称来访问这些元素呢？</p>
<p>换句话说：如何给序列起个名呢？</p>
<h2 id="解决方案"><a href="#解决方案" class="headerlink" title="解决方案"></a>解决方案</h2><p>使用 <code>collections.namedtuple()</code> 能解决这个问题。</p>
<p>这个函数实际上是一个返回 Python 中<strong>标准元组类型</strong>子类的一个工厂方法。 </p>
<p>我们需要向 namedtuple 传递一个「类型名」和需要的「字段」给它，然后它就会返回一个「类」，我们可以初始化这个类，为定义的字段传递值。 </p>
<p>代码示例：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span><span class="keyword">from</span> collections <span class="keyword">import</span> namedtuple</span><br><span class="line"><span class="meta">>>> </span>Subscriber = namedtuple(<span class="string">'Subscriber'</span>, [<span class="string">'addr'</span>, <span class="string">'joined'</span>])</span><br><span class="line"><span class="meta">>>> </span>sub = Subscriber(<span class="string">'[email protected]'</span>, <span class="string">'2012-10-19'</span>)</span><br><span class="line"><span class="meta">>>> </span>sub</span><br><span class="line">Subscriber(addr=<span class="string">'[email protected]'</span>, joined=<span class="string">'2012-10-19'</span>)</span><br><span class="line"><span class="meta">>>> </span>sub.addr</span><br><span class="line"><span class="string">'[email protected]'</span></span><br><span class="line"><span class="meta">>>> </span>sub.joined</span><br><span class="line"><span class="string">'2012-10-19'</span></span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
<p>尽管 <code>namedtuple</code> 的实例看起来像一个普通的类实例，但是它跟元组类型是可交换的，支持所有的普通元组操作，比如索引和解压。 </p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>len(sub)</span><br><span class="line"><span class="number">2</span></span><br><span class="line"><span class="meta">>>> </span>addr, joined = sub</span><br><span class="line"><span class="meta">>>> </span>addr</span><br><span class="line"><span class="string">'[email protected]'</span></span><br><span class="line"><span class="meta">>>> </span>joined</span><br><span class="line"><span class="string">'2012-10-19'</span></span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
<p>命名元组的一个主要用途是<strong>将代码从下标操作中解脱出来</strong>。</p>
<p>因此，如果从数据库调用中返回了一个很大的元组列表，通过下标去操作其中的元素， 当在表中添加了新的列的时候代码可能就会出错了。</p>
<p>但是如果使用了命名元组，那么就不会有这样的顾虑。</p>
<p>为了说明清楚，下面是使用普通元组的代码：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">compute_cost</span><span class="params">(records)</span>:</span></span><br><span class="line"> total = <span class="number">0.0</span></span><br><span class="line"> <span class="keyword">for</span> rec <span class="keyword">in</span> records:</span><br><span class="line"> total += rec[<span class="number">1</span>] * rec[<span class="number">2</span>]</span><br><span class="line"> <span class="keyword">return</span> total</span><br></pre></td></tr></table></figure>
<p>下标操作通常会让代码表意不清晰，并且非常依赖 records 的结构。 </p>
<p>下面是使用命名元组的版本：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> collections <span class="keyword">import</span> namedtuple</span><br><span class="line"></span><br><span class="line">Stock = namedtuple(<span class="string">'Stock'</span>, [<span class="string">'name'</span>, <span class="string">'shares'</span>, <span class="string">'price'</span>])</span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">compute_cost</span><span class="params">(records)</span>:</span></span><br><span class="line"> total = <span class="number">0.0</span></span><br><span class="line"> <span class="keyword">for</span> rec <span class="keyword">in</span> records:</span><br><span class="line"> s = Stock(*rec)</span><br><span class="line"> total += s.shares * s.price</span><br><span class="line"> <span class="keyword">return</span> total</span><br></pre></td></tr></table></figure>
<h2 id="扩展讨论"><a href="#扩展讨论" class="headerlink" title="扩展讨论"></a>扩展讨论</h2><p>命名元组另一个用途就是<strong>作为字典的替代</strong>，<strong>因为字典存储需要更多的内存空间</strong>。 如果你需要构建一个非常大的包含字典的数据结构，那么使用命名元组会更加高效。 </p>
<p>但是需要注意的是，不像字典那样，一个命名元组是不可更改的。</p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>s = Stock(<span class="string">'ACME'</span>, <span class="number">100</span>, <span class="number">123.45</span>)</span><br><span class="line"><span class="meta">>>> </span>s</span><br><span class="line">Stock(name=<span class="string">'ACME'</span>, shares=<span class="number">100</span>, price=<span class="number">123.45</span>)</span><br><span class="line"><span class="meta">>>> </span>s.shares = <span class="number">75</span></span><br><span class="line">Traceback (most recent call last):</span><br><span class="line">File <span class="string">"<stdin>"</span>, line <span class="number">1</span>, <span class="keyword">in</span> <module></span><br><span class="line">AttributeError: can<span class="string">'t set attribute</span></span><br><span class="line"><span class="string">>>></span></span><br></pre></td></tr></table></figure>
<p>如果真的需要改变属性的值，那么可以使用命名元组实例的 <code>_replace()</code> 方法， 它会创建一个全新的命名元组并将对应的字段用新的值取代。</p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>s = s._replace(shares=<span class="number">75</span>)</span><br><span class="line"><span class="meta">>>> </span>s</span><br><span class="line">Stock(name=<span class="string">'ACME'</span>, shares=<span class="number">75</span>, price=<span class="number">123.45</span>)</span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
<p><code>_replace()</code> 方法还有一个很有用的特性就是<strong>当你的命名元组拥有可选或者缺失字段时候</strong>， 它是一个非常方便的填充数据的方法。 </p>
<p>你可以先创建一个包含缺省值的原型元组，然后使用 <code>_replace()</code> 方法创建新的值被更新过的实例。</p>
<p>比如：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> collections <span class="keyword">import</span> namedtuple</span><br><span class="line"></span><br><span class="line">Stock = namedtuple(<span class="string">'Stock'</span>, [<span class="string">'name'</span>, <span class="string">'shares'</span>, <span class="string">'price'</span>, <span class="string">'date'</span>, <span class="string">'time'</span>])</span><br><span class="line"></span><br><span class="line"><span class="comment"># Create a prototype instance</span></span><br><span class="line">stock_prototype = Stock(<span class="string">''</span>, <span class="number">0</span>, <span class="number">0.0</span>, <span class="keyword">None</span>, <span class="keyword">None</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment"># Function to convert a dictionary to a Stock</span></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">dict_to_stock</span><span class="params">(s)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> stock_prototype._replace(**s)</span><br></pre></td></tr></table></figure>
<p>下面是它的使用方法：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span>a = {<span class="string">'name'</span>: <span class="string">'ACME'</span>, <span class="string">'shares'</span>: <span class="number">100</span>, <span class="string">'price'</span>: <span class="number">123.45</span>}</span><br><span class="line"><span class="meta">>>> </span>dict_to_stock(a)</span><br><span class="line">Stock(name=<span class="string">'ACME'</span>, shares=<span class="number">100</span>, price=<span class="number">123.45</span>, date=<span class="keyword">None</span>, time=<span class="keyword">None</span>)</span><br><span class="line"><span class="meta">>>> </span>b = {<span class="string">'name'</span>: <span class="string">'ACME'</span>, <span class="string">'shares'</span>: <span class="number">100</span>, <span class="string">'price'</span>: <span class="number">123.45</span>, <span class="string">'date'</span>: <span class="string">'12/17/2012'</span>}</span><br><span class="line"><span class="meta">>>> </span>dict_to_stock(b)</span><br><span class="line">Stock(name=<span class="string">'ACME'</span>, shares=<span class="number">100</span>, price=<span class="number">123.45</span>, date=<span class="string">'12/17/2012'</span>, time=<span class="keyword">None</span>)</span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
<blockquote>
<p>最后要说的是:</p>
<p>如果要定义一个需要更新很多实例属性的高效数据结构，那么命名元组并不是最佳选择。 这时候我们应该考虑定义一个包含 <code>__slots__</code> 方法的类。</p>
</blockquote>
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