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<p>收集于 <a href="https://dbader.org/" target="_blank" rel="external">Dan Bader 博客</a></p>
<a id="more"></a>
<h1 id="一行代码合并字典-dict"><a href="#一行代码合并字典-dict" class="headerlink" title="一行代码合并字典(dict)"></a>一行代码合并字典(dict)</h1><p>实际开发中，经常需要合并字典，例如：<code>default={'name':'Tom','age':18,'gender':'man'}</code> 默认配置，用户的配置 <code>user={'name':'Jak','age':20}</code>，这时就需要合并两个字典，获取总的配置。</p>
<p>假设有两个字典(dict)： x 和 y ，合并成一个新的 dict ，不改变 x 和 y 的值：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">>>> x = {'a': 1, 'b': 2}</div><div class="line">>>> y = {'b': 3, 'c': 4}</div></pre></td></tr></table></figure>
<p>期待得到一个新的结果 z ，如果 key 相同，则 y 会覆盖 x ：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">>>> z</div><div class="line">{'a': 1, 'b': 3, 'c': 4}</div></pre></td></tr></table></figure>
<h2 id="python3-5"><a href="#python3-5" class="headerlink" title="python3.5+"></a>python3.5+</h2><p>在 PEP448 中，有个新语法可以实现，并且在 python3.5中支持该语法：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">>>> z = {**x,**y}</div><div class="line">>>> z</div><div class="line">{'a': 1, 'b': 3, 'c': 4}</div></pre></td></tr></table></figure>
<p>一行代码搞定</p>
<p>这种语法还支持一次合并多个字典（三个及三个以上）：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">>>> u = {'c':5,'d':6}</div><div class="line">>>> {**x,**y,**u}</div><div class="line">{'a': 1, 'b': 3, 'c': 5, 'd': 6}</div></pre></td></tr></table></figure>
<h2 id="python2-x"><a href="#python2-x" class="headerlink" title="python2.x"></a>python2.x</h2><p>对于 python2.x 和 python3.0-3.4 的版本，有比较优雅的合并，但需要两行代码：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">>>> z = x.copy()</div><div class="line">>>> z.update(y)</div></pre></td></tr></table></figure>
<p>当然，还有个一行代码合并的方法：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">>>> z = dict(x,**y)</div><div class="line">>>> z</div><div class="line">{'a': 1, 'b': 3, 'c': 4}</div></pre></td></tr></table></figure>
<h1 id="一次测试多个标记的不同方法"><a href="#一次测试多个标记的不同方法" class="headerlink" title="一次测试多个标记的不同方法"></a>一次测试多个标记的不同方法</h1><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div></pre></td><td class="code"><pre><div class="line"># 一次测试多个标记</div><div class="line">x, y, z = 0, 1, 0</div><div class="line"></div><div class="line">if x == 1 or y == 1 or z == 1:</div><div class="line"> print('passed')</div><div class="line"></div><div class="line">if 1 in (x, y, z):</div><div class="line"> print('passed')</div><div class="line"></div><div class="line"># 只测试是否为真</div><div class="line">if x or y or z:</div><div class="line"> print('passed')</div><div class="line"></div><div class="line">if any((x, y, z)):</div><div class="line"> print('passed')</div></pre></td></tr></table></figure>
<h1 id="对字典按key排序和按value排序"><a href="#对字典按key排序和按value排序" class="headerlink" title="对字典按key排序和按value排序"></a>对字典按key排序和按value排序</h1><p>字典类型是无序的，因而对字典排序的原理是，将字典转变为列表或者元组。</p>
<p>对字典排序，需要借助函数 <code>sorted()</code>，这是python内置的一个排序函数，它会从一个迭代器返回一个排好序的新列表。</p>
<blockquote>
<p><code>sorted(iterable,key,reverse)</code> 共有三个参数。<br><code>iterable</code> 表示可迭代的对象，如 dict.items( )、dict.key( )等。<br><code>key</code> 是一个函数，用来选取参与排序的元素。<code>reverse</code> 用来指定排序是倒序还是顺序。<code>true</code>是倒序，<code>false</code>是顺序，默认 <code>reverse=false</code>。</p>
</blockquote>
<h2 id="按value排序"><a href="#按value排序" class="headerlink" title="按value排序"></a>按value排序</h2><p>借助 <code>lambda</code> 表达式选取参与排序的元素：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">>>> xs = {'a': 4, 'b': 3, 'c': 2, 'd': 1}</div><div class="line">>>> sorted(xs.items(), key=lambda x: x[1])</div><div class="line">[('d', 1), ('c', 2), ('b', 3), ('a', 4)]</div></pre></td></tr></table></figure>
<blockquote>
<p>items() : 将字典转为可迭代对象，会将字典元素转为元组<br>lambda x: x[1] : 选取元组中第二个元素作为排序参数并输出</p>
</blockquote>
<p>或者可以借助 <code>operator</code> 模块的 <code>itemgetter( )</code> 函数，该函数用于获取对象的哪些维的数据，参数为一些序号（即需要获取的数据在对象中的序号）：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">>>> import operator</div><div class="line">>>> sorted(xs.items(), key=operator.itemgetter(1))</div><div class="line">[('d', 1), ('c', 2), ('b', 3), ('a', 4)]</div></pre></td></tr></table></figure>
<p>注意，<code>operator.itemgetter</code> 函数获取的不是值，而是定义了一个函数，通过该函数作用到对象上才能获取值。</p>
<h2 id="按key排序"><a href="#按key排序" class="headerlink" title="按key排序"></a>按key排序</h2><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line">>>> xs = {'c': 4, 'a': 3, 'd': 2, 'b': 1}</div><div class="line"></div><div class="line">>>> sorted(xs.items(),key=lambda x:x[0])</div><div class="line">[('a', 3), ('b', 1), ('c', 4), ('d', 2)]</div><div class="line"></div><div class="line">>>> sorted(xs.keys())</div><div class="line">['a', 'b', 'c', 'd']</div></pre></td></tr></table></figure>
<p>sorted函数的key参数选默认时，字典默认是按 key 排序</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">>>> sorted(xs.items())</div><div class="line">[('a', 3), ('b', 1), ('c', 4), ('d', 2)]</div></pre></td></tr></table></figure>
<h1 id="巧用dict的get-方法返回默认值"><a href="#巧用dict的get-方法返回默认值" class="headerlink" title="巧用dict的get()方法返回默认值"></a>巧用dict的get()方法返回默认值</h1><blockquote>
<p>python 的 dict 自带有 <code>get(key)</code> 方法，根据指定的key，返回对应的value</p>
</blockquote>
<p>想象下，我们有个字典结构的数据，user ID 为 key，user name 为 value。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line">name_for_userid = {</div><div class="line"> 382: "Alice",</div><div class="line"> 950: "Bob",</div><div class="line"> 590: "Dilbert",</div><div class="line">}</div></pre></td></tr></table></figure>
<p>现在想写一个函数 <code>greeting()</code> ，可以根据 user ID 返回对应的 user name 。</p>
<p>第一个版本如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">def greeting(userid):</div><div class="line"> return "Hi %s!" % name_for_userid[userid]</div></pre></td></tr></table></figure>
<p>然而当输入非法的key时，会抛出 exception。这样非常不好，会直接导致运行中断。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line">>>> greeting(382)</div><div class="line">'Hi Alice!'</div><div class="line">>>> greeting(23333)</div><div class="line">Traceback (most recent call last):</div><div class="line"> File "<stdin>", line 1, in <module></div><div class="line"> File "<stdin>", line 2, in greeting</div><div class="line">KeyError: 23333</div></pre></td></tr></table></figure>
<p>所以，我们需要考虑输入的 user ID 找不到的情况，可以设置个默认返回值。</p>
<p>第二个版本如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line">def greeting(userid):</div><div class="line"> if userid in name_for_userid:</div><div class="line"> return "Hi %s!" % name_for_userid[userid]</div><div class="line"> else:</div><div class="line"> return "Hi there!"</div><div class="line"></div><div class="line">>>> greeting(382)</div><div class="line">"Hi Alice!"</div><div class="line"></div><div class="line">>>> greeting(23333)</div><div class="line">"Hi there!"</div></pre></td></tr></table></figure>
<p>然而这种方法很不好：</p>
<ul>
<li>这是低效的，因为查询了两次字典。</li>
<li>代码冗长的，不简洁。</li>
<li>没有[pythonic]的味道。</li>
</ul>
<p>因而，一个更好的方法是用 <code>try ... except</code> 语句来捕捉并处理异常。</p>
<p>第三个版本如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line">def greeting(userid):</div><div class="line"> try:</div><div class="line"> return "Hi %s!" % name_for_userid[userid]</div><div class="line"> except KeyError:</div><div class="line"> return "Hi there"</div></pre></td></tr></table></figure>
<p>以上方法的处理，虽然正确，但仍然不够简洁。python 的 dict 自带 <code>get()</code> 方法，支持设置一个默认返回值。</p>
<p>第四个版本：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">def greeting(userid):</div><div class="line"> return "Hi %s!" % name_for_userid.get(userid, "there")</div><div class="line"> </div><div class="line">>>> greeting(950)</div><div class="line">"Hi Bob!"</div><div class="line"></div><div class="line">>>> greeting(23333)</div><div class="line">"Hi there!"</div></pre></td></tr></table></figure>
<p>相比这四种方法，最后一种更加的简洁、干净、只靠使用pyhton标准库的特性就能处理完成。</p>
<h1 id="更方便的两个变量换值的方法"><a href="#更方便的两个变量换值的方法" class="headerlink" title="更方便的两个变量换值的方法"></a>更方便的两个变量换值的方法</h1><p>通常我们编程，都是通过借用一个临时变量来交换两个变量的值：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line">>>> a = 23</div><div class="line">>>> b = 56</div><div class="line">>>> tmp = a</div><div class="line">>>> a = b</div><div class="line">>>> b = tmp</div></pre></td></tr></table></figure>
<p>但在 Python 中，有更方便、更简洁的方法：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">>>> a,b = b,a</div></pre></td></tr></table></figure>
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