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python类特殊方法
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<h4>__init__</h4>
<p>__init__方法在定义实例的时候会被调用</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">test</span><span class="p">(</span><span class="nb">object</span><span class="p">);</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span>
<span class="k">class</span> <span class="nc">test1</span><span class="p">(</span><span class="n">test</span><span class="p">):</span>
<span class="k">pass</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">test1</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">t</span><span class="o">.</span><span class="n">x</span><span class="p">)</span>
</pre></div>
<p>执行上面的代码，会打印出10。说明在子类没有显示定义__init__方法时，在定义子类实例时，父类的__init__方法会自动被调用。
不过如果子类定义__init__方法，定义子类实例时不会自动调用父类的__init__方法</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">test</span><span class="p">(</span><span class="nb">object</span><span class="p">);</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">test_x</span> <span class="o">=</span> <span class="n">x</span>
<span class="k">class</span> <span class="nc">test1</span><span class="p">(</span><span class="n">test</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">test1_x</span> <span class="o">=</span> <span class="n">x</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">test1</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">t</span><span class="o">.</span><span class="n">test_x</span><span class="p">)</span>
</pre></div>
<p>执行上面的代码，会产生如下异常，说明父类的__init__方法在定义实例时（准确的说是定义子类实例）未被调用。</p>
<div class="highlight"><pre><span></span>AttributeError: tesst1 instance has no attribute <span class="s1">'test_x'</span>
</pre></div>
<h4>__new__</h4>
<p><code>__new__</code>才是 Python 事实上的构造方法，而 <code>__init__</code> 更确切的说是初始化方法。<code>__new__</code> 会产生一个实例对象，再把这个实例对象传给<code>__init__</code> 绑定一些属性。
因此 <code>__new__</code> 是一个类方法，虽然它没有加 classsmethod 装饰器。不过我们大部分时间都不要实现__new__ 方法，因为 Python 解释器会调用 object 对象的 <code>__new__</code> 给我们返回一个实例对象。</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">test</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">super</span><span class="p">(</span><span class="n">test</span><span class="p">,</span> <span class="bp">cls</span><span class="p">)</span><span class="o">.</span><span class="fm">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
<h4>__call__</h4>
<p>当一个类实现了__call__方法时，它的实例就是可调用的。即像函数一样使用。</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CallTest</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="k">def</span> <span class="fm">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"sum:"</span><span class="p">,</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">)</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="nv">t</span> <span class="o">=</span> CallTest<span class="o">(</span><span class="s2">"test"</span><span class="o">)</span>
t<span class="o">(</span><span class="m">1</span>,2<span class="o">)</span>
</pre></div>
<p>上面将会输出:</p>
<div class="highlight"><pre><span></span><span class="nb">test</span>
sum: <span class="m">3</span>
</pre></div>
<p>从输出结果我们可以看到，__init__在创建实例的时候被调用，用来初始化一些属性。而__call__在实例被像函数一样调用时被调用。</p>
<blockquote>
<p>python3中，类是type的实例，用dir(type)查看type的方法时，可以看见type实现了__call__方法，因此在定义类的时候会调用对应的__call__方法，这也是
元类的工作方式。关于元类，可以查看我的其他文章。同样，函数也实现了__call__方法</p>
</blockquote>
<h4>__getitem__ __getattribute__ __getattr__</h4>
<p>这里把三个属性放在一起讨论，方便区别</p>
<ul>
<li>__getitem__ 在a['a'] 时被调用</li>
<li>__getattr__ 在某个实例属性不存在的时候被调用</li>
<li>__getattribute__ 是无条件被调用.对任何对象的属性访问时,都会隐式的调用__getattribute__方法</li>
</ul>
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