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<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Anniywell</title>
<link href="/atom.xml" rel="self"/>
<link href="https://anniywell.github.io/"/>
<updated>2017-06-23T06:50:47.148Z</updated>
<id>https://anniywell.github.io/</id>
<author>
<name>Anniywell</name>
</author>
<generator uri="http://hexo.io/">Hexo</generator>
<entry>
<title>Windows软件清单</title>
<link href="https://anniywell.github.io/2017/06/23/Windows%E8%BD%AF%E4%BB%B6%E6%B8%85%E5%8D%95/"/>
<id>https://anniywell.github.io/2017/06/23/Windows软件清单/</id>
<published>2017-06-23T06:17:09.000Z</published>
<updated>2017-06-23T06:50:47.148Z</updated>
<content type="html"><![CDATA[<h2 id="日常"><a href="#日常" class="headerlink" title="日常"></a>日常</h2><h3 id="浏览器"><a href="#浏览器" class="headerlink" title="浏览器"></a>浏览器</h3><p><a href="https://www.mozilla.org/en-US/firefox/all/" target="_blank" rel="external">Firefox国际版</a><br><a href="https://www.google.cn/chrome/browser/desktop/index.html" target="_blank" rel="external">Chrome</a><br><a href="https://vivaldi.com/download/" target="_blank" rel="external">Vivaldi</a></p>
<h3 id="下载"><a href="#下载" class="headerlink" title="下载"></a>下载</h3><p><a href="">迅雷极速版</a></p>
<h3 id="压缩归档"><a href="#压缩归档" class="headerlink" title="压缩归档"></a>压缩归档</h3><p><a href="http://www.7-zip.org/" target="_blank" rel="external">7-Zip</a></p>
<h3 id="编辑阅读"><a href="#编辑阅读" class="headerlink" title="编辑阅读"></a>编辑阅读</h3><p><a href="http://ep.wps.cn/product/wps-office-download.html" target="_blank" rel="external">WPS专业增强版</a><br><a href="https://www.sumatrapdfreader.org/download-free-pdf-viewer.html" target="_blank" rel="external">SumatraPDF</a></p>
<a id="more"></a>
<h3 id="输入法"><a href="#输入法" class="headerlink" title="输入法"></a>输入法</h3><p><a href="">搜狗拼音</a><br><a href="">Bing拼音</a><br><a href="">Google拼音</a></p>
<h3 id="音视频"><a href="#音视频" class="headerlink" title="音视频"></a>音视频</h3><p><a href="http://www.foobar2000.org/download" target="_blank" rel="external">foobar2000</a><br><a href="http://potplayer.daum.net/?lang=zh_CN" target="_blank" rel="external">PotPlayer</a></p>
<h3 id="截图"><a href="#截图" class="headerlink" title="截图"></a>截图</h3><p><a href="https://getsharex.com/" target="_blank" rel="external">ShareX</a></p>
<h3 id="邮件"><a href="#邮件" class="headerlink" title="邮件"></a>邮件</h3><p><a href="http://www.foxmail.com/" target="_blank" rel="external">Foxmail</a><br><a href="https://www.mozilla.org/zh-CN/thunderbird/" target="_blank" rel="external">Thunderbird</a><br><a href="https://www.microsoft.com/zh-cn/outlook-com/" target="_blank" rel="external">Outlook</a></p>
<h2 id="开发"><a href="#开发" class="headerlink" title="开发"></a>开发</h2><h3 id="编辑器"><a href="#编辑器" class="headerlink" title="编辑器"></a>编辑器</h3><p><a href="https://code.visualstudio.com/" target="_blank" rel="external">Visual Studio Code</a><br><a href="https://atom.io/" target="_blank" rel="external">Atom</a><br><a href="http://www.sublimetext.com/3" target="_blank" rel="external">Sublime Text 3</a></p>
<h3 id="IDE"><a href="#IDE" class="headerlink" title="IDE"></a>IDE</h3><p><a href="http://www.itellyou.cn/?esbwbc=caz8f2" target="_blank" rel="external">Visual Studio</a><br><a href="http://www.jetbrains.com/" target="_blank" rel="external">JetBrains全家桶</a><br><a href="http://mirrors.ustc.edu.cn/qtproject/official_releases/qtcreator/" target="_blank" rel="external">Qt Creator</a></p>
<h3 id="Markdown"><a href="#Markdown" class="headerlink" title="Markdown"></a>Markdown</h3><p><a href="http://pad.haroopress.com/user.html#download" target="_blank" rel="external">Haroopad</a></p>
<h3 id="VCS"><a href="#VCS" class="headerlink" title="VCS"></a>VCS</h3><p><a href="https://git-scm.com/" target="_blank" rel="external">git</a><br><a href="http://subversion.apache.org/" target="_blank" rel="external">svn</a><br><a href="https://www.sourcetreeapp.com/" target="_blank" rel="external">SourceTree</a><br><a href="http://www.syntevo.com/smartgit/download" target="_blank" rel="external">SmartGit</a><br><a href="https://tortoisesvn.net/downloads.html" target="_blank" rel="external">TortoiseSVN</a><br><a href="https://tortoisegit.org/download/" target="_blank" rel="external">TortoiseGit</a></p>
<h3 id="文件对比"><a href="#文件对比" class="headerlink" title="文件对比"></a>文件对比</h3><p><a href="http://www.scootersoftware.com/download.php" target="_blank" rel="external">Beyond Compare</a></p>
<h3 id="终端增强"><a href="#终端增强" class="headerlink" title="终端增强"></a>终端增强</h3><p><a href="http://cmder.net/" target="_blank" rel="external">cmder</a></p>
<h3 id="文件搜索"><a href="#文件搜索" class="headerlink" title="文件搜索"></a>文件搜索</h3><p><a href="http://www.listary.com/download" target="_blank" rel="external">Listary</a><br><a href="http://www.getwox.com/" target="_blank" rel="external">Wox</a></p>
<h3 id="文件管理"><a href="#文件管理" class="headerlink" title="文件管理"></a>文件管理</h3><p><a href="http://www.ghisler.com/" target="_blank" rel="external">Total Commander</a><br><a href="http://cn.ejie.me/" target="_blank" rel="external">Clover</a></p>
<h3 id="剪贴板"><a href="#剪贴板" class="headerlink" title="剪贴板"></a>剪贴板</h3><p><a href="http://ditto-cp.sourceforge.net/" target="_blank" rel="external">Ditto</a></p>
<h3 id="强制解锁"><a href="#强制解锁" class="headerlink" title="强制解锁"></a>强制解锁</h3><p><a href="">Unlocker</a></p>
<h3 id="网络抓包"><a href="#网络抓包" class="headerlink" title="网络抓包"></a>网络抓包</h3><p><a href="https://www.wireshark.org/download.html" target="_blank" rel="external">Wireshark</a><br><a href="http://www.telerik.com/fiddler" target="_blank" rel="external">Fiddler</a></p>
<h3 id="编译工具"><a href="#编译工具" class="headerlink" title="编译工具"></a>编译工具</h3><p><a href="http://www.msys2.org/" target="_blank" rel="external">MSYS2</a><br><a href="http://www.mingw.org/" target="_blank" rel="external">Mingw</a><br><a href="http://www.mingw-w64.org/doku.php/" target="_blank" rel="external">Mingw-w64</a><br><a href="http://www.cygwin.com/" target="_blank" rel="external">Cygwin</a><br><a href="https://cmake.org/" target="_blank" rel="external">CMake</a><br><a href="">GYP</a><br><a href="">Ninja</a><br><a href="https://www.microsoft.com/en-us/download/details.aspx?id=48159" target="_blank" rel="external">Microsoft Build Tools</a></p>
<h3 id="开发工具"><a href="#开发工具" class="headerlink" title="开发工具"></a>开发工具</h3><p><a href="http://www.oracle.com/technetwork/java/javase/downloads/index.html" target="_blank" rel="external">JDK</a><br><a href="https://www.python.org/downloads/" target="_blank" rel="external">Python</a><br><a href="https://nodejs.org/en/download/" target="_blank" rel="external">Node.js</a></p>
<h3 id="虚拟机"><a href="#虚拟机" class="headerlink" title="虚拟机"></a>虚拟机</h3><p><a href="https://www.virtualbox.org/" target="_blank" rel="external">VirtualBox</a><br><a href="https://www.genymotion.com/" target="_blank" rel="external">Genymotion</a></p>
<h3 id="代码阅读"><a href="#代码阅读" class="headerlink" title="代码阅读"></a>代码阅读</h3><p><a href="https://www.sourceinsight.com/" target="_blank" rel="external">Source Insight</a><br><a href="https://www.slickedit.com/" target="_blank" rel="external">SlickEdit</a><br><a href="https://scitools.com/" target="_blank" rel="external">Understand</a></p>
<h3 id="数据库访问"><a href="#数据库访问" class="headerlink" title="数据库访问"></a>数据库访问</h3><p><a href="https://www.navicat.com/en/products" target="_blank" rel="external">Navicat</a></p>
<h3 id="UML建模"><a href="#UML建模" class="headerlink" title="UML建模"></a>UML建模</h3><p><a href="">Enterprise Architect</a><br><a href="http://staruml.io/" target="_blank" rel="external">StarUML</a></p>
<h3 id="科学上网"><a href="#科学上网" class="headerlink" title="科学上网"></a>科学上网</h3><p><a href="https://laod.cn/hosts/" target="_blank" rel="external">Hosts</a><br><a href="https://github.com/getlantern/forum/issues/833" target="_blank" rel="external">lantern</a></p>
]]></content>
<summary type="html">
<h2 id="日常"><a href="#日常" class="headerlink" title="日常"></a>日常</h2><h3 id="浏览器"><a href="#浏览器" class="headerlink" title="浏览器"></a>浏览器</h3><p><a href="https://www.mozilla.org/en-US/firefox/all/" target="_blank" rel="external">Firefox国际版</a><br><a href="https://www.google.cn/chrome/browser/desktop/index.html" target="_blank" rel="external">Chrome</a><br><a href="https://vivaldi.com/download/" target="_blank" rel="external">Vivaldi</a></p>
<h3 id="下载"><a href="#下载" class="headerlink" title="下载"></a>下载</h3><p><a href="">迅雷极速版</a></p>
<h3 id="压缩归档"><a href="#压缩归档" class="headerlink" title="压缩归档"></a>压缩归档</h3><p><a href="http://www.7-zip.org/" target="_blank" rel="external">7-Zip</a></p>
<h3 id="编辑阅读"><a href="#编辑阅读" class="headerlink" title="编辑阅读"></a>编辑阅读</h3><p><a href="http://ep.wps.cn/product/wps-office-download.html" target="_blank" rel="external">WPS专业增强版</a><br><a href="https://www.sumatrapdfreader.org/download-free-pdf-viewer.html" target="_blank" rel="external">SumatraPDF</a></p>
</summary>
<category term="Software" scheme="https://anniywell.github.io/categories/Software/"/>
<category term="Software" scheme="https://anniywell.github.io/tags/Software/"/>
</entry>
<entry>
<title>Python协程</title>
<link href="https://anniywell.github.io/2017/06/23/Python%E5%8D%8F%E7%A8%8B/"/>
<id>https://anniywell.github.io/2017/06/23/Python协程/</id>
<published>2017-06-23T06:08:15.000Z</published>
<updated>2017-06-23T06:50:32.549Z</updated>
<content type="html"><![CDATA[<h2 id="协程简介"><a href="#协程简介" class="headerlink" title="协程简介"></a>协程简介</h2><p>协程，即协作式程序，又称微线程、纤程，英文名Coroutine。</p>
<p>其思想是，一系列互相依赖的协程间依次使用CPU，每次只有一个协程工作，而其他协程处于休眠状态。协程可以在运行期间的某个点上暂停执行，并在恢复运行时从暂停的点上继续执行。<br>协程已经被证明是一种非常有用的程序组件，不仅被python、lua、ruby等脚本语言广泛采用，而且被新一代面向多核的编程语言如golang rust-lang等采用作为并发的基本单位。<br>协程可以被认为是一种用户空间线程，与传统的线程相比，有2个主要的优点：</p>
<ul>
<li>与线程不同，协程是自己主动让出CPU，并交付他期望的下一个协程运行，而不是在任何时候都有可能被系统调度打断。因此协程的使用更加清晰易懂，并且多数情况下不需要锁机制。</li>
<li>与线程相比，协程的切换由程序控制，发生在用户空间而非内核空间，因此切换的代价非常小。</li>
</ul>
<p>总结起来是一句话：协程可以认为是一种用户态线程，与系统提供的线程不同点是，它需要主动让出CPU时间，而不是由系统进行调度，即控制权在程序员手上。</p>
<h2 id="Python协程史"><a href="#Python协程史" class="headerlink" title="Python协程史"></a>Python协程史</h2><ul>
<li>Python 2.2 中的生成器让代码执行过程可以暂停 (yield)</li>
<li>Python 2.5 中可以将值返回给暂停的生成器，这使得 Python 中协程的概念成为可能 (send)</li>
<li>Python 3.3 中的 yield from，使得重构生成器与将它们串联起来都很简单 (yield from)</li>
<li>Python 3.4 以后通过标准库 asyncio 获得了事件循环的特性 (asyncio)</li>
<li>Python 3.5 使用async/await语法引入对协程的显式支持 (async/await)</li>
<li>Python 3.6 增强asyncio，支持异步生成器、异步解析式</li>
</ul>
<a id="more"></a>
<h2 id="yield"><a href="#yield" class="headerlink" title="yield"></a>yield</h2><p>为了理解什么是 yield, 你必须理解什么是生成器(generator)。</p>
<p>关于生成器我的理解是是：生成器保存的是算法，需要时再计算(惰性计算)</p>
<p>创建生成器有两种方式：</p>
<p>第一种方法：把一个列表生成式的[]改成()，就创建了一个generator：</p>
<figure class="highlight python"><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">l = [x * x <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">10</span>)] <span class="comment"># l [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]</span></div><div class="line">g = (x * x <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">10</span>)) <span class="comment"># g <generator object <genexpr> at 0x1022ef630></span></div></pre></td></tr></table></figure>
<p>第二种方式：在函数中使用yield关键字，函数就变成了一个generator。</p>
<p>函数里有了yield后，执行到yield就会停住，当需要再往下算时才会再往下算。所以生成器函数即使是有无限循环也没关系，它需要算到多少就会算多少，不需要就不往下算。</p>
<p>例如你想要自己实现一个 range() 函数，你可以用立即计算的方式创建一个整数列表：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">eager_range</span><span class="params">(up_to)</span>:</span></div><div class="line"> <span class="string">"""Create a list of integers, from 0 to up_to, exclusive."""</span></div><div class="line"> sequence = []</div><div class="line"> index = <span class="number">0</span></div><div class="line"> <span class="keyword">while</span> index < up_to:</div><div class="line"> sequence.append(index)</div><div class="line"> index += <span class="number">1</span></div><div class="line"> <span class="keyword">return</span> sequence</div><div class="line"></div><div class="line">l = eager_range(<span class="number">1000000</span>)</div></pre></td></tr></table></figure>
<p>然而这里存在的问题是，如果你想创建从0到1,000,000这样一个很大的序列，你不得不创建能容纳1,000,000个整数的列表。<br>但是当加入了生成器之后，你可以不用创建完整的序列，你只需要能够每次保存一个整数的内存即可。</p>
<figure class="highlight python"><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><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">lazy_range</span><span class="params">(up_to)</span>:</span></div><div class="line"> <span class="string">"""Generator to return the sequence of integers from 0 to up_to, exclusive."""</span></div><div class="line"> index = <span class="number">0</span></div><div class="line"> <span class="keyword">while</span> index < up_to:</div><div class="line"> <span class="keyword">yield</span> index</div><div class="line"> index += <span class="number">1</span></div><div class="line"></div><div class="line">g = lazy_range(<span class="number">1000000</span>) <span class="comment"># <generator object lazy_range at 0x040A25D0></span></div><div class="line">next(g)</div><div class="line">...</div><div class="line">...</div><div class="line">...</div><div class="line">next(g)</div><div class="line">Traceback (most recent call last):</div><div class="line"> File <span class="string">"<stdin>"</span>, line <span class="number">1</span>, <span class="keyword">in</span> <module></div><div class="line">StopIteration</div></pre></td></tr></table></figure>
<p>让函数遇到 yield 表达式时暂停执行 – 虽然在 Python 2.5 以前它只是一条语句 – 并且能够在后面重新执行，这对于减少内存使用、生成无限序列非常有用。</p>
<p>你有可能已经发现，生成器完全就是关于迭代器的。有一种更好的方式生成迭代器当然很好（尤其是当你可以给一个生成器对象添加 <strong>iter</strong>() 方法时），<br>但是人们知道，如果可以利用生成器“暂停”的部分，添加“将东西发送回生成器”的功能，那么 Python 突然就有了协程的概念（当然这里的协程仅限于 Python 中的概念；Python 中真实的协程在后面才会讨论）。<br>将东西发送回暂停了的生成器这一特性通过 PEP 342添加到了 Python 2.5。<br>与其它特性一起，PEP 342 为生成器引入了 send() 方法。这让我们不仅可以暂停生成器，而且能够传递值到生成器暂停的地方。<br>还是以我们的 range() 为例，你可以让序列向前或向后跳过几个值：</p>
<figure class="highlight python"><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><div class="line">16</div><div class="line">17</div><div class="line">18</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">jumping_range</span><span class="params">(up_to)</span>:</span></div><div class="line"> <span class="string">"""Generator for the sequence of integers from 0 to up_to, exclusive.</span></div><div class="line"> Sending a value into the generator will shift the sequence by that amount.</div><div class="line"> """</div><div class="line"> index = <span class="number">0</span></div><div class="line"> <span class="keyword">while</span> index < up_to:</div><div class="line"> jump = <span class="keyword">yield</span> index</div><div class="line"> <span class="keyword">if</span> jump <span class="keyword">is</span> <span class="keyword">None</span>:</div><div class="line"> jump = <span class="number">1</span></div><div class="line"> index += jump</div><div class="line"></div><div class="line">iterator = jumping_range(<span class="number">5</span>)</div><div class="line">print(next(iterator)) <span class="comment"># 0</span></div><div class="line">print(iterator.send(<span class="number">2</span>)) <span class="comment"># 2</span></div><div class="line">print(next(iterator)) <span class="comment"># 3</span></div><div class="line">print(iterator.send(<span class="number">-1</span>)) <span class="comment"># 2</span></div><div class="line"><span class="keyword">for</span> x <span class="keyword">in</span> iterator:</div><div class="line"> print(x) <span class="comment"># 3, 4</span></div></pre></td></tr></table></figure>
<p>其实next()和send()在一定意义上作用是相似的，区别是send()可以传递yield表达式的值进去，而next()不能传递特定的值，只能传递None进去。</p>
<p>因此，我们可以看做next(g) == g.send(None)</p>
<p>需要注意的是，第一次调用时，请使用next()语句或是send(None)，不能使用send发送一个非None的值，否则会出错，因为没有yield语句来接收这个值。</p>
<h2 id="yield-from"><a href="#yield-from" class="headerlink" title="yield from"></a>yield from</h2><p>在PEP 380 为 Python 3.3 添加了 yield from之前，生成器都没有变动。<br>严格来说，这一特性让你能够从迭代器（生成器刚好也是迭代器）中返回任何值，从而可以干净利索的方式重构生成器。</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><div class="line"><span class="keyword">yield</span> <span class="keyword">from</span> iterator</div><div class="line"><span class="comment"># (本质上)相当于：</span></div><div class="line"><span class="keyword">for</span> x <span class="keyword">in</span> iterator:</div><div class="line"> <span class="keyword">yield</span> x</div></pre></td></tr></table></figure>
<figure class="highlight python"><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"><span class="function"><span class="keyword">def</span> <span class="title">lazy_range</span><span class="params">(up_to)</span>:</span></div><div class="line"> <span class="string">"""Generator to return the sequence of integers from 0 to up_to, exclusive."""</span></div><div class="line"> index = <span class="number">0</span></div><div class="line"> <span class="function"><span class="keyword">def</span> <span class="title">gratuitous_refactor</span><span class="params">()</span>:</span></div><div class="line"> <span class="keyword">while</span> index < up_to:</div><div class="line"> <span class="keyword">yield</span> index</div><div class="line"> index += <span class="number">1</span></div><div class="line"> <span class="keyword">yield</span> <span class="keyword">from</span> gratuitous_refactor()</div></pre></td></tr></table></figure>
<p>yield from 通过让重构变得简单，也让你能够将生成器串联起来，使返回值可以在调用栈中上下浮动，而不需对编码进行过多改动。</p>
<figure class="highlight python"><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><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">bottom</span><span class="params">()</span>:</span></div><div class="line"> <span class="string">"""Returning the yield lets the value that goes up the call stack to come right back down"""</span></div><div class="line"> <span class="keyword">return</span> (<span class="keyword">yield</span> <span class="number">42</span>)</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">middle</span><span class="params">()</span>:</span></div><div class="line"> <span class="keyword">return</span> (<span class="keyword">yield</span> <span class="keyword">from</span> bottom())</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">top</span><span class="params">()</span>:</span></div><div class="line"> <span class="keyword">return</span> (<span class="keyword">yield</span> <span class="keyword">from</span> middle())</div><div class="line"></div><div class="line"><span class="comment"># Get the generator.</span></div><div class="line">gen = top()</div><div class="line">value = next(gen)</div><div class="line">print(value) <span class="comment"># Prints '42'.</span></div><div class="line"><span class="keyword">try</span>:</div><div class="line"> value = gen.send(value * <span class="number">2</span>)</div><div class="line"><span class="keyword">except</span> StopIteration <span class="keyword">as</span> exc:</div><div class="line"> value = exc.value</div><div class="line">print(value) <span class="comment"># Prints '84'.</span></div></pre></td></tr></table></figure>
<h2 id="asyncio"><a href="#asyncio" class="headerlink" title="asyncio"></a>asyncio</h2><p>asyncio是一个基于事件循环的异步I/O库，Python3.4将其引入标准库，Python3.3可通过pip安装</p>
<p>asyncio包括的内容很多很复杂，这里只会做基本的两点：协同程序和事件循环。</p>
<p>协程的基本概念前面已经讲过，这里先来说一下事件循环</p>
<p>通俗来说，事件循环 “是一种等待程序分配事件或消息的编程架构”，其提供一种循环机制，让你可以“在A发生时，执行B”。基本上来说事件循环就是监听当有什么发生时，同时事件循环也关心这件事并执行相应的代码，本质上是以队列的方式来重新分配时间片。</p>
<p>在asyncio中事件循环扮演的是个调度器的角色，被用来安排协同程序的执行。</p>
<p>PEP 342中通过asyncio.coroutine装饰的函数为协程，这里的协程是和asyncio及其事件循环一起使用的。</p>
<p>这赋予了 Python 第一个对于协程的明确定义，也就是基于生成器的协程</p>
<p>这意味着突然之间所有实现了协程接口的生成器，即便它们并不是要以协程方式应用，都符合这一定义。为了修正这一点，asyncio 要求所有要用作协程的生成器必须由asyncio.coroutine修饰。</p>
<p>使用以下语法声明生成器协程：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><div class="line"><span class="meta">@asyncio.coroutine</span></div><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">generator_coroutine</span><span class="params">()</span>:</span></div><div class="line"> <span class="keyword">pass</span></div><div class="line"></div><div class="line">a = generator_coroutine()</div><div class="line">print(a) <span class="comment"># <generator object coro at 0x040F0B48></span></div></pre></td></tr></table></figure>
<p>yield from在asyncio模块中得以发扬光大。通过yield from，我们可以用asyncio.sleep将协程控制权交给事件循环，然后挂起当前协程；之后，由事件循环决定何时唤醒asyncio.sleep,接着向后执行代码。<br>先看示例代码：</p>
<figure class="highlight python"><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"><span class="keyword">import</span> asyncio</div><div class="line"></div><div class="line"><span class="meta">@asyncio.coroutine</span></div><div class="line"><span class="function"><span class="keyword">def</span> <span class="title">countdown</span><span class="params">(number, n)</span>:</span></div><div class="line"> <span class="keyword">while</span> n > <span class="number">0</span>:</div><div class="line"> print(<span class="string">'T-minus'</span>, n, <span class="string">'({})'</span>.format(number))</div><div class="line"> <span class="keyword">yield</span> <span class="keyword">from</span> asyncio.sleep(<span class="number">1</span>)</div><div class="line"> n -= <span class="number">1</span></div><div class="line"></div><div class="line">loop = asyncio.get_event_loop()</div><div class="line">tasks = [</div><div class="line"> asyncio.ensure_future(countdown(<span class="string">"A"</span>, <span class="number">2</span>)),</div><div class="line"> asyncio.ensure_future(countdown(<span class="string">"B"</span>, <span class="number">3</span>))]</div><div class="line">loop.run_until_complete(asyncio.wait(tasks))</div><div class="line">loop.close()</div></pre></td></tr></table></figure>
<p>在解释上面例子之前，需要先简单了解一下asyncio.Future</p>
<p>Future可以理解为延迟结果的抽象，在其他语言中也称作Promise.<br>你可以对任何asyncio.Future对象使用 yield from，从而将其传递给事件循环，暂停协程的执行来等待某些事情的发生（ future 对象并不重要，只是asyncio细节的实现）。<br>一旦 future 对象获取了事件循环，它会一直在那里监听，直到完成它需要做的一切。<br>当 future 完成自己的任务之后，事件循环会察觉到，暂停并等待在那里的协程会通过send()方法获取future对象的返回值并开始继续执行。</p>
<p>以上面的代码为例, 事件循环启动每一个 countdown() 协程，一直执行到遇见其中一个协程的 yield from 和 asyncio.sleep() 。这样会返回一个 asyncio.Future对象并将其传递给事件循环，同时暂停这一协程的执行。事件循环会监控这一future对象，直到倒计时1秒钟之后（同时也会检查其它正在监控的对象，比如像其它协程）。1秒钟的时间一到，事件循环会选择刚刚传递了future对象并暂停了的 countdown() 协程，将future对象的结果返回给协程，然后协程可以继续执行。这一过程会一直持续到所有的 countdown() 协程执行完毕，事件循环也被清空。稍后我会给你展示一个完整的例子，用来说明协程/事件循环之类的这些东西究竟是如何运作的，但是首先我想要解释一下async和await。</p>
<p>关于asyncio这里只做了简单的介绍，它其实包括以下内容，大家可以去查看<a href="https://docs.python.org/3/library/asyncio.html?highlight=asyncio#module-asyncio" target="_blank" rel="external">官方文档</a>：</p>
<ul>
<li>事件循环</li>
<li>任务和协程</li>
<li>传输和协议</li>
<li>基于协程的流</li>
<li>子进程</li>
<li>同步原语</li>
<li>队列</li>
</ul>
<h2 id="async与await"><a href="#async与await" class="headerlink" title="async与await"></a>async与await</h2><p>PEP 492引入async/await语法，中明确了协程类型(原生协程)，用于区别于基于生成器的协程</p>
<p>在以前，我们可以用生成器实现协程（PEP 342），后来又对其进行了改进，引入了yield from语法（PEP 380）。但仍有一些缺点：</p>
<ol>
<li>协程和普通生成器使用相同的语法，所以很容易把它们搞混，初学者更是如此。</li>
<li>一个函数是否是一个协程，取决于它里面是否出现了yield或yield from语句。这并不明显，容易在重构函数的时候搞乱，导致出错。</li>
<li>异步调用被yield语法限制了，我们不能获得、使用更多的语法特性，比如with和for。</li>
</ol>
<p>这个PEP把协程从生成器独立出来，成为Python的一个原生事物。这会消除协程和生成器之间的混淆，方便编写不依赖特定库的协程代码。</p>
<p>使用以下语法声明原生协程：</p>
<figure class="highlight python"><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"><span class="keyword">async</span> <span class="function"><span class="keyword">def</span> <span class="title">native_coroutine</span><span class="params">()</span>:</span></div><div class="line"> <span class="keyword">pass</span></div><div class="line"></div><div class="line">a = native_coroutine()</div><div class="line">print(a) <span class="comment"># <coroutine object a at 0x000000000567EFC0></span></div></pre></td></tr></table></figure>
<p>原生协程语法的关键点：</p>
<ul>
<li>async def函数必定是协程，即使里面不含有await语句。</li>
<li>如果在async函数里面使用yield或yield from语句，会引发SyntaxError异常。</li>
<li>协程在调用时会返回一个coroutine对象</li>
<li>协程不再抛出StopIteration异常，而是替代为RuntimeError</li>
<li>当协程进行垃圾回收时，一个从未被await的协程会抛出RuntimeWarning异常</li>
</ul>
<p><strong>await</strong>表达式</p>
<p>下面新的await表达式用于获取协程执行结果：</p>
<figure class="highlight python"><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"><span class="keyword">async</span> <span class="function"><span class="keyword">def</span> <span class="title">read_data</span><span class="params">(db)</span>:</span></div><div class="line"> data = <span class="keyword">await</span> db.fetch(<span class="string">'SELECT ...'</span>)</div><div class="line"> <span class="keyword">pass</span></div></pre></td></tr></table></figure>
<p>await与yield from相似，挂起read_data协程的执行直到db.fetch这个awaitable对象完成并返回结果数据。</p>
<p>原生协程与生成器协程的区别与联系</p>
<ul>
<li>原生协程对象不实现<strong>iter</strong>和<strong>next</strong>方法。因此，他们不能够通过iter()，list()，tuple()和其他一些内置函数进行迭代。他们也不能用于for…in循环。在原生协程中尝试使用<strong>iter</strong>或者__next会触发TypeError异常。</li>
<li>未被装饰的生成器不能够yield from一个原生协程：这样会引发TypeError。</li>
<li>基于生成器的协程(asyncio代码必须使用@asyncio.coroutine)可以yield from一个原生协程。</li>
<li>对原生协程对象和原生协程函数调用inspect.isgenerator()和inspect.isgeneratorfunction()会返回False。</li>
<li>协程内部基于生成器，原生协程与生成器协程共享实现过程。类似于生成器对象，原生协程包含throw()，send()和close()方法。</li>
</ul>
<h2 id="异步生成器与异步解析式"><a href="#异步生成器与异步解析式" class="headerlink" title="异步生成器与异步解析式"></a>异步生成器与异步解析式</h2><p>PEP 492 引入支持原生协程和async /await的语法到Python 3.5。 在Python 3.5实现里的一个值得注意的局限性就在于它不可能使用await和yield在同一个函数体中。<br>而在Python 3.6中，这个限制已解除，这使得定义异步生成器成为可能：</p>
<figure class="highlight python"><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"><span class="keyword">async</span> <span class="function"><span class="keyword">def</span> <span class="title">ticker</span><span class="params">(delay, to)</span>:</span></div><div class="line"> <span class="string">"""Yield numbers from 0 to *to* every *delay* seconds."""</span></div><div class="line"> <span class="keyword">for</span> i <span class="keyword">in</span> range(to):</div><div class="line"> <span class="keyword">yield</span> i</div><div class="line"> <span class="keyword">await</span> asyncio.sleep(delay)</div></pre></td></tr></table></figure>
<p>PEP 530 添加了对async for在list、set、dict解析式以及generator表达式中的使用支持：</p>
<pre><code>result = [i async for i in aiter() if i % 2]
</code></pre><p>此外，所有解析式都支持“await”表达式：</p>
<pre><code>result = [await fun() for fun in funcs if await condition()]
</code></pre><h2 id="gevent"><a href="#gevent" class="headerlink" title="gevent"></a>gevent</h2><p>gevent是一个基于协同的Python网络库，它使用greenlet在libev事件循环之上提供高级同步API。</p>
<p>主要特性：</p>
<ul>
<li>基于libev的快速事件循环</li>
<li>基于greenlet的轻量级执行单元</li>
<li>重用python标准api(event,queue)</li>
<li>协同的socket和ssl模块</li>
<li>使用标准库和第三方模块写标准阻塞socket(gevent.monkey)</li>
<li>通过线程池或c-ares执行的DNS查询。</li>
<li>内置TCP/UDP/HTTP服务器</li>
<li>支持子进程(gevent.subprocess)</li>
<li>支持线程池</li>
</ul>
<p>下面简单介绍gevent的使用</p>
<p><img src="http://xlambda.com/gevent-tutorial/flow.gif" alt="gevent-flow"></p>
<p>gevent.spawn(function, <em>args, **kwargs)<br>创建一个新的Greenlet对象并安排它运行function(</em>args，**kwargs)</p>
<p>注意：这时function还没有启动，它的运行依赖于gevent的事件循环，只有启动事件循环，它才会被调度</p>
<p>gevent.sleep(seconds=0)</p>
<p>将当前的greenlet睡眠seconds秒</p>
<p>使用gevent.sleep相当于切换上下文，让出执行权</p>
<p>gevent.joinall</p>
<p>等待多个greenlet执行结束</p>
<p>有时需要知道greenlet运行的状态，在greenlet中有一些标志， 让你可以监视它的线程内部状态:</p>
<ul>
<li>started – Boolean, 指示此Greenlet是否已经启动</li>
<li>ready() – Boolean, 指示此Greenlet是否已经停止</li>
<li>successful() – Boolean, 指示此Greenlet是否已经停止而且没抛异常</li>
<li>value – 任意值, 此Greenlet代码返回的值</li>
<li>exception – 异常, 此Greenlet内抛出的未捕获异常</li>
</ul>
<p>更多<a href="http://www.gevent.org/gevent.html#module-gevent" target="_blank" rel="external">gevent api</a>介绍</p>
<h2 id="参考文档"><a href="#参考文档" class="headerlink" title="参考文档"></a>参考文档</h2><ul>
<li><a href="https://docs.python.org/3/library/asyncio.html" target="_blank" rel="external">Python官方文档</a></li>
<li><a href="https://www.python.org/dev/peps/pep-0255/" target="_blank" rel="external">PEP 255 – Simple Generators</a></li>
<li><a href="https://www.python.org/dev/peps/pep-0342/" target="_blank" rel="external">PEP 342 – Coroutines via Enhanced Generators</a></li>
<li><a href="https://www.python.org/dev/peps/pep-0380/" target="_blank" rel="external">PEP 380 – Syntax for Delegating to a Subgenerator</a></li>
<li><a href="https://www.python.org/dev/peps/pep-3156/" target="_blank" rel="external">PEP 3156 – Asynchronous IO Support Rebooted: the “asyncio” Module</a></li>
<li><a href="https://www.python.org/dev/peps/pep-0492/" target="_blank" rel="external">PEP 492 – Coroutines with async and await syntax</a></li>
<li><a href="http://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000" target="_blank" rel="external">廖雪峰Python教程</a></li>
<li><a href="https://www.python.org/dev/peps/pep-0525/" target="_blank" rel="external">PEP 525</a></li>
<li><a href="https://www.python.org/dev/peps/pep-0530/" target="_blank" rel="external">PEP 530</a></li>
<li><a href="https://github.com/SimonXming/my-blog/issues/23" target="_blank" rel="external">Python 3.5协程原理</a></li>
<li><a href="http://www.gevent.org/contents.html" target="_blank" rel="external">gevent官方文档</a></li>
<li><a href="http://sdiehl.github.io/gevent-tutorial/" target="_blank" rel="external">gevent教程</a></li>
</ul>
]]></content>
<summary type="html">
<h2 id="协程简介"><a href="#协程简介" class="headerlink" title="协程简介"></a>协程简介</h2><p>协程，即协作式程序，又称微线程、纤程，英文名Coroutine。</p>
<p>其思想是，一系列互相依赖的协程间依次使用CPU，每次只有一个协程工作，而其他协程处于休眠状态。协程可以在运行期间的某个点上暂停执行，并在恢复运行时从暂停的点上继续执行。<br>协程已经被证明是一种非常有用的程序组件，不仅被python、lua、ruby等脚本语言广泛采用，而且被新一代面向多核的编程语言如golang rust-lang等采用作为并发的基本单位。<br>协程可以被认为是一种用户空间线程，与传统的线程相比，有2个主要的优点：</p>
<ul>
<li>与线程不同，协程是自己主动让出CPU，并交付他期望的下一个协程运行，而不是在任何时候都有可能被系统调度打断。因此协程的使用更加清晰易懂，并且多数情况下不需要锁机制。</li>
<li>与线程相比，协程的切换由程序控制，发生在用户空间而非内核空间，因此切换的代价非常小。</li>
</ul>
<p>总结起来是一句话：协程可以认为是一种用户态线程，与系统提供的线程不同点是，它需要主动让出CPU时间，而不是由系统进行调度，即控制权在程序员手上。</p>
<h2 id="Python协程史"><a href="#Python协程史" class="headerlink" title="Python协程史"></a>Python协程史</h2><ul>
<li>Python 2.2 中的生成器让代码执行过程可以暂停 (yield)</li>
<li>Python 2.5 中可以将值返回给暂停的生成器，这使得 Python 中协程的概念成为可能 (send)</li>
<li>Python 3.3 中的 yield from，使得重构生成器与将它们串联起来都很简单 (yield from)</li>
<li>Python 3.4 以后通过标准库 asyncio 获得了事件循环的特性 (asyncio)</li>
<li>Python 3.5 使用async/await语法引入对协程的显式支持 (async/await)</li>
<li>Python 3.6 增强asyncio，支持异步生成器、异步解析式</li>
</ul>
</summary>
<category term="Python" scheme="https://anniywell.github.io/categories/Python/"/>
<category term="Python" scheme="https://anniywell.github.io/tags/Python/"/>
<category term="协程" scheme="https://anniywell.github.io/tags/%E5%8D%8F%E7%A8%8B/"/>
</entry>
<entry>
<title>浅析Python元类及应用</title>
<link href="https://anniywell.github.io/2017/06/16/%E6%B5%85%E6%9E%90Python%E5%85%83%E7%B1%BB%E5%8F%8A%E5%BA%94%E7%94%A8/"/>
<id>https://anniywell.github.io/2017/06/16/浅析Python元类及应用/</id>
<published>2017-06-16T07:28:36.000Z</published>
<updated>2017-06-16T10:19:59.113Z</updated>
<content type="html"><![CDATA[<h2 id="写这篇文章的缘由"><a href="#写这篇文章的缘由" class="headerlink" title="写这篇文章的缘由"></a>写这篇文章的缘由</h2><p>前几天去一家公司面试，面试的岗位是Python后台开发，面试中被问到了一道题，考虑很久想不到答案，顾回来查阅资料在此总结一下。<br>题目的描述是这样的：<br><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">SQLAlchemy中Model中定义类成员时，如何做到声明顺序与数据库表的列顺序一致</div><div class="line">提示：普通类成员是存储在__dict__，而dict本身是无序的</div></pre></td></tr></table></figure></p>
<p>通过阅读SQLAlchemy源码得知，其内部实现是使用元类编程来完成的，元类编程是Python的一种高级黑暗魔法，经常听其大名，并没有进入深入了解，借此机会稍微总结一下。</p>
<h2 id="元编程"><a href="#元编程" class="headerlink" title="元编程"></a>元编程</h2><p>元编程我个人理解是：<strong>用代码生成（操纵）代码</strong>的编程手法。<br>Python中元编程的几种手段：</p>
<ul>
<li>预定义方法</li>
<li>函数赋值</li>
<li>descriptor</li>
<li>eval</li>
<li>元类</li>
</ul>
<p>上面的几种方式相对于元类来说简单一些，这里暂时先不讲了，以后有时间再去补充<br><a id="more"></a></p>
<h2 id="元类"><a href="#元类" class="headerlink" title="元类"></a>元类</h2><h3 id="类也是对象"><a href="#类也是对象" class="headerlink" title="类也是对象"></a>类也是对象</h3><p>在Python中一切皆对象，同样“类”也是对象<br><figure class="highlight python"><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"><span class="meta">>>> </span><span class="class"><span class="keyword">class</span> <span class="title">OneClass</span>:</span></div><div class="line"><span class="meta">... </span> <span class="keyword">pass</span></div><div class="line">...</div><div class="line"><span class="meta">>>> </span>OneClass</div><div class="line"><<span class="class"><span class="keyword">class</span> '<span class="title">__main__</span>.<span class="title">OneClass</span>'></span></div></pre></td></tr></table></figure></p>
<p>当你在定义一个class时，会在内存中一个对象，对象的名字就是类的名字。<br><strong>这个对象（类）自身拥有创建对象（类实例）的能力，而这就是为什么它是一个类的原因</strong></p>
<h3 id="动态的创建类"><a href="#动态的创建类" class="headerlink" title="动态的创建类"></a>动态的创建类</h3><p>最简单的动态创建方式是将类的定义写在逻辑判断中，不同条件生成不同的类，但这还不够动态，你仍然需要自己去编写整个类的代码。如果想要更动态的创建方式，那我们先来了解一个Python的内建函数<strong>type</strong><br><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">class type(object): # 当传入一个参数时，返回该对象的类型</div><div class="line">class type(name, bases, dict) # 创建新类型，name-类名，bases-父类元组，dict-属性字典</div></pre></td></tr></table></figure></p>
<p>了解到这里，我们就可以用type动态创建class了，比如：<br><figure class="highlight"><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">>>> A = type('A', (), {})</div><div class="line">>>> A</div><div class="line"><class '__main__.A'></div><div class="line">>>> B = type('B', (), {'b': True})</div><div class="line">>>> B</div><div class="line"><class '__main__.B'></div><div class="line">>>> C = type('C', (B,), {'c': False})</div><div class="line">>>> C</div><div class="line"><class '__main__.C'></div><div class="line">>>> c = C()</div><div class="line">>>> c.b</div><div class="line">True</div><div class="line">>>> c.c</div><div class="line">False</div><div class="line">>>></div></pre></td></tr></table></figure></p>
<p>现在我们终于接触到元类了，上面使用的type就是元类，type就是Python在背后用来创建所有类的元类。为了证明这一点我们可以使用<strong>class</strong>来验证一下：<br><figure class="highlight"><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><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div></pre></td><td class="code"><pre><div class="line">>>> class AA:</div><div class="line">... pass</div><div class="line">...</div><div class="line">>>> def func():</div><div class="line">... pass</div><div class="line">...</div><div class="line">>>> var = 3</div><div class="line">>>> AA.__class__</div><div class="line"><class 'type'></div><div class="line">>>> AA.__class__.__class__</div><div class="line"><class 'type'></div><div class="line">>>> func.__class__</div><div class="line"><class 'function'></div><div class="line">>>> func.__class__.__class__</div><div class="line"><class 'type'></div><div class="line">>>> var.__class__</div><div class="line"><class 'int'></div><div class="line">>>> var.__class__.__class__</div><div class="line"><class 'type'></div></pre></td></tr></table></figure></p>
<h3 id="自定义元类"><a href="#自定义元类" class="headerlink" title="自定义元类"></a>自定义元类</h3><p>除了使用Python内建的type元类外，我们还可以使用metaclass来控制类的创建行为，如：<br><figure class="highlight python"><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></pre></td><td class="code"><pre><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">ListMetaclass</span><span class="params">(type)</span>:</span></div><div class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__new__</span><span class="params">(cls, name, bases, attrs)</span>:</span></div><div class="line"> attrs[<span class="string">'add'</span>] = <span class="keyword">lambda</span> self, value: self.append(value)</div><div class="line"> <span class="keyword">return</span> type.__new__(cls, name, bases, attrs)</div><div class="line"></div><div class="line"></div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyList</span><span class="params">(list, metaclass=ListMetaclass)</span>:</span></div><div class="line"> <span class="keyword">pass</span></div><div class="line"></div><div class="line"></div><div class="line"><span class="meta">>>> </span>L = MyList()</div><div class="line"><span class="meta">>>> </span>L.add(<span class="number">1</span>)</div><div class="line">>> L</div><div class="line">[<span class="number">1</span>]</div></pre></td></tr></table></figure></p>
<p>回归开篇的那道题目，实现大致是这样的：<br><figure class="highlight python"><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><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">from</span> collections <span class="keyword">import</span> OrderedDict</div><div class="line"></div><div class="line"></div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">OrderedClass</span><span class="params">(type)</span>:</span></div><div class="line"><span class="meta"> @classmethod</span></div><div class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__prepare__</span><span class="params">(mcs, name, bases)</span>:</span></div><div class="line"> <span class="keyword">return</span> OrderedDict()</div><div class="line"></div><div class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__new__</span><span class="params">(cls, name, bases, classdict)</span>:</span></div><div class="line"> result = type.__new__(cls, name, bases, dict(classdict))</div><div class="line"> result.__fields__ = list(classdict.keys())</div><div class="line"> <span class="keyword">return</span> result</div><div class="line"></div><div class="line"></div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">Column</span>:</span></div><div class="line"> <span class="keyword">pass</span></div><div class="line"></div><div class="line"></div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyClass</span><span class="params">(metaclass=OrderedClass)</span>:</span></div><div class="line"> mycol2 = Column()</div><div class="line"> mycol3 = Column()</div><div class="line"> zut = Column()</div><div class="line"> cool = Column()</div><div class="line"> menfin = Column()</div><div class="line"> a = Column()</div><div class="line"></div><div class="line"></div><div class="line">print(MyClass.__fields__)</div></pre></td></tr></table></figure></p>
<p>在元类中使用<strong>prepare</strong>返回有序字典来存储类成员定义，在元类中创建类，并将有序的属性名赋值给类的<strong>fields</strong>，通过访问<strong>fields</strong>得到与定义顺序相同的属性名称。<br>想了解更多可以看一下<a href="https://www.python.org/dev/peps/pep-3115/" target="_blank" rel="external">PEP 3115 – Metaclasses in Python 3000</a><br>另外，关于类属性定义顺序问题，在Python3.6已经改成默认有序了，详情参考<a href="https://www.python.org/dev/peps/pep-0520/" target="_blank" rel="external">PEP 520 – Preserving Class Attribute Definition Order</a></p>
<h2 id="究竟为什么要使用元类？"><a href="#究竟为什么要使用元类？" class="headerlink" title="究竟为什么要使用元类？"></a>究竟为什么要使用元类？</h2><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">“元类就是深度的魔法，99%的用户应该根本不必为此操心。如果你想搞清楚究竟是否需要用到元类，那么你就不需要它。那些实际用到元类的人都非常清楚地知道他们需要做什么，而且根本不需要解释为什么要用元类。” —— Python界的领袖 Tim Peters</div></pre></td></tr></table></figure>
<p>元类的主要用途是创建API，一个典型的应用就是数据库的ORM，通过一个class来映射成一张table，通过一个对象来映射成一行数据，将原本复杂的代码变的很简介，这些很多都是元类的功劳。</p>
]]></content>
<summary type="html">
<h2 id="写这篇文章的缘由"><a href="#写这篇文章的缘由" class="headerlink" title="写这篇文章的缘由"></a>写这篇文章的缘由</h2><p>前几天去一家公司面试，面试的岗位是Python后台开发，面试中被问到了一道题，考虑很久想不到答案，顾回来查阅资料在此总结一下。<br>题目的描述是这样的：<br><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">SQLAlchemy中Model中定义类成员时，如何做到声明顺序与数据库表的列顺序一致</div><div class="line">提示：普通类成员是存储在__dict__，而dict本身是无序的</div></pre></td></tr></table></figure></p>
<p>通过阅读SQLAlchemy源码得知，其内部实现是使用元类编程来完成的，元类编程是Python的一种高级黑暗魔法，经常听其大名，并没有进入深入了解，借此机会稍微总结一下。</p>
<h2 id="元编程"><a href="#元编程" class="headerlink" title="元编程"></a>元编程</h2><p>元编程我个人理解是：<strong>用代码生成（操纵）代码</strong>的编程手法。<br>Python中元编程的几种手段：</p>
<ul>
<li>预定义方法</li>
<li>函数赋值</li>
<li>descriptor</li>
<li>eval</li>
<li>元类</li>
</ul>
<p>上面的几种方式相对于元类来说简单一些，这里暂时先不讲了，以后有时间再去补充<br>
</summary>
<category term="Python" scheme="https://anniywell.github.io/categories/Python/"/>
<category term="Python" scheme="https://anniywell.github.io/tags/Python/"/>
<category term="元编程" scheme="https://anniywell.github.io/tags/%E5%85%83%E7%BC%96%E7%A8%8B/"/>
</entry>
<entry>
<title>Hello World</title>
<link href="https://anniywell.github.io/2017/06/16/hello-world/"/>
<id>https://anniywell.github.io/2017/06/16/hello-world/</id>
<published>2017-06-16T00:38:45.178Z</published>
<updated>2017-06-16T03:22:09.875Z</updated>
<content type="html"><![CDATA[<p>Welcome to <a href="https://hexo.io/" target="_blank" rel="external">Hexo</a>! This is your very first post. Check <a href="https://hexo.io/docs/" target="_blank" rel="external">documentation</a> for more info. If you get any problems when using Hexo, you can find the answer in <a href="https://hexo.io/docs/troubleshooting.html" target="_blank" rel="external">troubleshooting</a> or you can ask me on <a href="https://github.com/hexojs/hexo/issues" target="_blank" rel="external">GitHub</a>.</p>
<h2 id="Quick-Start"><a href="#Quick-Start" class="headerlink" title="Quick Start"></a>Quick Start</h2><h3 id="Create-a-new-post"><a href="#Create-a-new-post" class="headerlink" title="Create a new post"></a>Create a new post</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">$ hexo new <span class="string">"My New Post"</span></div></pre></td></tr></table></figure>
<p>More info: <a href="https://hexo.io/docs/writing.html" target="_blank" rel="external">Writing</a><br><a id="more"></a></p>
<h3 id="Run-server"><a href="#Run-server" class="headerlink" title="Run server"></a>Run server</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">$ hexo server</div></pre></td></tr></table></figure>
<p>More info: <a href="https://hexo.io/docs/server.html" target="_blank" rel="external">Server</a></p>
<h3 id="Generate-static-files"><a href="#Generate-static-files" class="headerlink" title="Generate static files"></a>Generate static files</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">$ hexo generate</div></pre></td></tr></table></figure>
<p>More info: <a href="https://hexo.io/docs/generating.html" target="_blank" rel="external">Generating</a></p>
<h3 id="Deploy-to-remote-sites"><a href="#Deploy-to-remote-sites" class="headerlink" title="Deploy to remote sites"></a>Deploy to remote sites</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">$ hexo deploy</div></pre></td></tr></table></figure>
<p>More info: <a href="https://hexo.io/docs/deployment.html" target="_blank" rel="external">Deployment</a></p>
]]></content>
<summary type="html">
<p>Welcome to <a href="https://hexo.io/" target="_blank" rel="external">Hexo</a>! This is your very first post. Check <a href="https://hexo.io/docs/" target="_blank" rel="external">documentation</a> for more info. If you get any problems when using Hexo, you can find the answer in <a href="https://hexo.io/docs/troubleshooting.html" target="_blank" rel="external">troubleshooting</a> or you can ask me on <a href="https://github.com/hexojs/hexo/issues" target="_blank" rel="external">GitHub</a>.</p>
<h2 id="Quick-Start"><a href="#Quick-Start" class="headerlink" title="Quick Start"></a>Quick Start</h2><h3 id="Create-a-new-post"><a href="#Create-a-new-post" class="headerlink" title="Create a new post"></a>Create a new post</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">$ hexo new <span class="string">"My New Post"</span></div></pre></td></tr></table></figure>
<p>More info: <a href="https://hexo.io/docs/writing.html" target="_blank" rel="external">Writing</a><br>
</summary>
</entry>
<entry>
<title>My blog</title>
<link href="https://anniywell.github.io/2017/06/16/My-blog/"/>
<id>https://anniywell.github.io/2017/06/16/My-blog/</id>
<published>2017-06-15T16:59:11.000Z</published>
<updated>2017-06-16T03:21:10.923Z</updated>
<content type="html"><![CDATA[<h2 id="关于博客"><a href="#关于博客" class="headerlink" title="关于博客"></a>关于博客</h2><p>此blog是基于hexo-theme-even修改而来，目前已经托管到我个人的github仓库中，后面会持续更新。</p>
<h2 id="博客内容"><a href="#博客内容" class="headerlink" title="博客内容"></a>博客内容</h2><p>以技术学习与分享为主，主要关注：</p>
<ol>
<li>Web全栈</li>
<li>JavaScript</li>
<li>Node.js</li>
<li>Python3</li>
<li>Swift</li>
<li><hr>
</li>
</ol>
]]></content>
<summary type="html">
<h2 id="关于博客"><a href="#关于博客" class="headerlink" title="关于博客"></a>关于博客</h2><p>此blog是基于hexo-theme-even修改而来，目前已经托管到我个人的github仓库中，后面会持续更新。</p>
</summary>
<category term="Hexo" scheme="https://anniywell.github.io/categories/Hexo/"/>
<category term="hexo" scheme="https://anniywell.github.io/tags/hexo/"/>
<category term="blog" scheme="https://anniywell.github.io/tags/blog/"/>
</entry>
</feed>