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<h2 class="wrapper title">Python 3.8新特性——赋值表达式</h2>
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<span>Author：</span><span>flypython</span> | <span>Date: </span><span>2019-04-01</span> | <span>Category：</span><span><a href="/fly/Python新特性/" title="Python新特性">Python新特性</a></span>
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<p>上周，Python3.8版本发布，到底带来了哪些新的特性呢？我们应该在哪些场景中使用这些特性呢？本周，我们通过几篇文章来告诉你答案。</p>
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<p><img src="https://tva1.sinaimg.cn/large/006tNbRwly1gai72bypmej30u00u9wl1.jpg" alt></p>
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<h4 id="安装"><a href="#安装" class="headerlink" title="安装"></a>安装</h4><p>首先，我们并不推荐安装最新版本到常用的开发环境中，你可以使用虚拟环境或者docker来尝鲜。</p>
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<p>官方安装包</p>
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<p>网址：<code>https://www.python.org/downloads/release/python-380/</code></p>
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<p>也可以使用docker</p>
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<p>拉取镜像命令<br><code>docker pull python:3.8</code></p>
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<h2 id="赋值表达式"><a href="#赋值表达式" class="headerlink" title="赋值表达式"></a>赋值表达式</h2><p><img src="https://tva1.sinaimg.cn/large/006tNbRwly1gai72nu5otj30jg0atdfq.jpg" alt></p>
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<p>赋值表达式被叫做海象运算符，因为它的形状像海象。如果熟悉go语言的话，会对这个表达式会熟悉。</p>
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<p>赋值表达式的语法是 </p>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">name := expression</span><br></pre></td></tr></table></figure>
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<p>和赋值语句 <code>=</code> 作用差不多，非必不可少，但可以简化代码。</p>
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<h4 id="官方示例"><a href="#官方示例" class="headerlink" title="官方示例"></a>官方示例</h4><figure class="highlight plain"><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">>>> a = False</span><br><span class="line">>>> print(a)</span><br><span class="line">False</span><br><span class="line">>>> print(a := True)</span><br><span class="line">True</span><br></pre></td></tr></table></figure>
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<p>此例子赋值之后，后续还需要使用变量。赋值表达可用于简化代码，提高可读性。</p>
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<figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">>>> inputs = list()</span><br><span class="line">>>> while True:</span><br><span class="line">... current = input("your input:")</span><br><span class="line">... if current == "quit":</span><br><span class="line">... break</span><br><span class="line">... inputs.append(current)</span><br><span class="line">...</span><br><span class="line">your input:a</span><br><span class="line">your input:b</span><br><span class="line">your input:test</span><br><span class="line">your input:quit</span><br><span class="line">>>> inputs</span><br><span class="line">['a', 'b', 'test']</span><br></pre></td></tr></table></figure>
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<p>使用赋值操作符时：</p>
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<figure class="highlight plain"><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">>>> inputs = list()</span><br><span class="line">>>> while (current := input("your input:")) != "quit":</span><br><span class="line">... inputs.append(current)</span><br><span class="line">...</span><br><span class="line">your input:a</span><br><span class="line">your input:b</span><br><span class="line">your input:test</span><br><span class="line">your input:quit</span><br><span class="line">>>> inputs</span><br><span class="line">['a', 'b', 'test']</span><br></pre></td></tr></table></figure>
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<p>此例子，省略了一条语句，可读性上升。</p>
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<p>再来一个例子</p>
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<p>最初版本</p>
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<figure class="highlight plain"><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">a = [1,2,3,4]</span><br><span class="line">if len(a) > 3: #计算 len(a) 一次 </span><br><span class="line"> print(f"a is too long ({len(a)} elements,expected < 3)") # 计算 len(a) 第二次</span><br></pre></td></tr></table></figure>
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<p>我们改写为:</p>
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<p>改进版本</p>
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<figure class="highlight plain"><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">a = [1,2,3,4]</span><br><span class="line">n = len(a) # 计算一次len(a)</span><br><span class="line">if n > 3: # 多了变量n</span><br><span class="line"> print(f"a is too long ({n} elements,expected < 3)") #</span><br></pre></td></tr></table></figure>
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<p>新特性重写:<br>重写版本</p>
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<figure class="highlight plain"><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">a = [1,2,3,4]</span><br><span class="line">if (n:=len(a)) > 3: # 计算一次len(a)，多了变量n，把两行改为一行</span><br><span class="line"> print(f"a is too long ({n} elements,expected < 3)") #</span><br></pre></td></tr></table></figure>
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<p>从上面可以看到，重写版本和改进版本的不同在于：</p>
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<figure class="highlight plain"><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">n = len(a) </span><br><span class="line">if n > 3:</span><br><span class="line"> pass</span><br></pre></td></tr></table></figure>
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<p>与</p>
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<figure class="highlight plain"><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">if (n:=len(a)) > 3:</span><br><span class="line"> pass</span><br></pre></td></tr></table></figure>
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<p>这两个版本的区别在于，<code>:=</code>和<code>=</code>是补充关系并不是替换关系，下面的例子可以看到官方的意图。</p>
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<figure class="highlight plain"><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">x = 5</span><br><span class="line">print(f"x = {x}")</span><br><span class="line"></span><br><span class="line">#能用=解决的就用=解决</span><br><span class="line">y := 5 # SyntaxError: invalid syntax</span><br><span class="line">print(f"y = {y}")</span><br><span class="line"></span><br><span class="line">(z := 5)</span><br><span class="line">print(f"z = {z}")</span><br></pre></td></tr></table></figure>
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<p>由上面可以看出<code>:=</code>和<code>=</code>是互补关系，在应该使用<code>:=</code>的时候才可以使用<code>:=</code>。</p>
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<p>Python语言的一致性，不管是专家还是新手，在同一个问题上都应该有一致的写法，然后这就形成了最pythonic的写法。</p>
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<p>最后带来，新特性带来的最佳实践</p>
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<figure class="highlight plain"><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"># 简化 os.fork </span><br><span class="line">if pid := os.fork():</span><br><span class="line"> # Parent code</span><br><span class="line">else:</span><br><span class="line"> # Child code</span><br><span class="line"></span><br><span class="line"># 直接到把 socket 对象的 read buffer 读完为止</span><br><span class="line">while data := sock.recv(8192):</span><br><span class="line"> print("Received data:", data)</span><br></pre></td></tr></table></figure>
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<h4 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h4><ul>
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<li><a href="https://docs.python.org/zh-cn/3.8/whatsnew/3.8.html" target="_blank" rel="noopener">https://docs.python.org/zh-cn/3.8/whatsnew/3.8.html</a></li>
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<li><a href="https://www.python.org/dev/peps/pep-0572" target="_blank" rel="noopener">https://www.python.org/dev/peps/pep-0572</a></li>
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