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Python super 函数的理解
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<h3>super 是一个类</h3>
<p>当我们调用 super() 的时候，实际上是实例化了一个 super 类。你没看错， super 是个类，既不是关键字也不是函数等其他数据结构:</p>
<div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">1</span><span class="p">]:</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="o">...</span><span class="p">:</span> <span class="k">pass</span>
<span class="o">...</span><span class="p">:</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">2</span><span class="p">]:</span> <span class="n">s</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">Test</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">3</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">3</span><span class="p">]:</span> <span class="nb">super</span>
</pre></div>
<p>在大多数情况下， super 包含了两个非常重要的信息: 一个 MRO 以及 MRO 中的一个类。当以如下方式调用 super 时:</p>
<div class="highlight"><pre><span></span><span class="nb">super</span><span class="p">(</span><span class="n">a_type</span><span class="p">,</span> <span class="n">obj</span><span class="p">)</span>
</pre></div>
<p>MRO 指的是 type(obj) 的 MRO, MRO 中的那个类就是 a_type , 同时 isinstance(obj, a_type) == True 。</p>
<p>当这样调用时:</p>
<div class="highlight"><pre><span></span><span class="nb">super</span><span class="p">(</span><span class="n">type1</span><span class="p">,</span> <span class="n">type2</span><span class="p">)</span>
</pre></div>
<p>MRO 指的是 type2 的 MRO, MRO 中的那个类就是 type1 ，同时 issubclass(type2, type1) == True 。</p>
<h3>super 具体怎么工作</h3>
<p>如上面所说，super 包含一个 MRO 和 MRO 中的一个类，假如 MRO 如下：</p>
<div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">,</span> <span class="n">B</span><span class="p">,</span> <span class="n">C</span><span class="p">,</span> <span class="n">D</span><span class="p">,</span> <span class="n">E</span><span class="p">,</span> <span class="nb">object</span><span class="p">]</span>
</pre></div>
<p>那么 <code>super(C, A).method()</code> 将从 D，E，object 中查找 method 方法。并将 method 方法调用时的 self 设置为 A。</p>
<p>如下的多继承例子：</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">A</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="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">=</span> <span class="mi">2</span>
<span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">m</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'self is </span><span class="si">{0}</span><span class="s1"> @A.add'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">+=</span> <span class="n">m</span>
<span class="k">class</span> <span class="nc">B</span><span class="p">(</span><span class="n">A</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="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">=</span> <span class="mi">3</span>
<span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">m</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'self is </span><span class="si">{0}</span><span class="s1"> @B.add'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="p">))</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">m</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">+=</span> <span class="mi">3</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">A</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="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">=</span> <span class="mi">4</span>
<span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">m</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'self is </span><span class="si">{0}</span><span class="s1"> @C.add'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="p">))</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">m</span><span class="p">)</span> <span class="c1"># 相当于 super(D, self).add(m)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">+=</span> <span class="mi">4</span>
<span class="k">class</span> <span class="nc">D</span><span class="p">(</span><span class="n">B</span><span class="p">,</span> <span class="n">C</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="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">=</span> <span class="mi">5</span>
<span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">m</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'self is </span><span class="si">{0}</span><span class="s1"> @D.add'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="p">))</span>
<span class="nb">super</span><span class="p">(</span><span class="n">C</span><span class="p">,</span> <span class="n">D</span><span class="p">)</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">m</span><span class="p">)</span> <span class="c1"># super 第二个参数为类对象时，需要显示传递self</span>
<span class="bp">self</span><span class="o">.</span><span class="n">n</span> <span class="o">+=</span> <span class="mi">5</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">d1</span> <span class="o">=</span> <span class="n">D</span><span class="p">()</span>
<span class="n">d2</span> <span class="o">=</span> <span class="n">D</span><span class="p">()</span>
<span class="n">d1</span><span class="o">.</span><span class="n">add1</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">d1</span><span class="o">.</span><span class="n">n</span><span class="p">,</span> <span class="s1">'================='</span><span class="p">)</span>
<span class="n">d2</span><span class="o">.</span><span class="n">add2</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">d2</span><span class="o">.</span><span class="n">n</span><span class="p">,</span> <span class="s1">'================='</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">D</span><span class="o">.</span><span class="vm">__mro__</span><span class="p">)</span> <span class="c1"># D 的 MRO</span>
</pre></div>
<p>输出结果如下：</p>
<div class="highlight"><pre><span></span>self is <__main__.D object at 0x10e15e320> @D.add1
self is <__main__.D object at 0x10e15e320> @B.add
self is <__main__.D object at 0x10e15e320> @C.add
self is <__main__.D object at 0x10e15e320> @A.add
<span class="nv">19</span> <span class="o">=================</span>
self is <__main__.D object at 0x10e15e6a0> @D.add2
self is <__main__.D object at 0x10e15e6a0> @A.add
<span class="nv">12</span> <span class="o">=================</span>
<span class="o">(</span><class <span class="s1">'__main__.D'</span>>, <class <span class="s1">'__main__.B'</span>>, <class <span class="s1">'__main__.C'</span>>,
<class <span class="s1">'__main__.A'</span>>, <class <span class="s1">'object'</span>><span class="o">)</span>
</pre></div>
<p>说明：</p>
<blockquote>
<ul>
<li>从输出结果可以看到，super 可以避免重复调用一个方法的问题。</li>
<li>super().add(m) 等于 super(D, self).add(m)，前者是后者的简写形式。</li>
<li>当 super() 的第二个参数是一个类对象时，需要显示的将实例对象传递给父类的方法。这也是 add1 和 add2 的主要区别。
这其实很好理解，因为第二个参数为类对象时，super 返回的是一个非绑定方法，因此需要显示的指定self。</li>
<li>d2.add2(2) 输出结果为12，为什么不是19？因为我们显示的将 super 第一个参数传递为 C，那么将只从 A, object 查找 add 方法，从输出
结果可以看出来这点。</li>
</ul>
</blockquote>
<h3>一些思考</h3>
<ol>
<li>如果把 B 中的 <code>super().add(m)</code> 删除，<code>d1.add1(2)</code> 调用后的 <code>d1.n</code> 值是多少呢？<br>
答案：</li>
</ol>
<div class="highlight"><pre><span></span>self is <__main__.D object at 0x10714b8d0> @D.add1
self is <__main__.D object at 0x10714b8d0> @B.add
<span class="nv">13</span> <span class="o">=================</span>
</pre></div>
<p>从这个问题看出，要让 super 按正常的 MRO 顺序炒作某个方法，父类的方法都必须通过 super 正确关联。</p>
<ol>
<li>如果把 B 中的 <code>super().add(m)</code> 删除，<code>d1.add2(2)</code> 调用后的 <code>d2.n</code> 值是多少呢？<br>
答案：</li>
</ol>
<div class="highlight"><pre><span></span>self is <__main__.D object at 0x10714b668> @D.add2
self is <__main__.D object at 0x10714b668> @A.add
<span class="nv">12</span> <span class="o">=================</span>
</pre></div>
<p>从这个问题看出，某个父类的 super 查找中断，并不影响从其后开始正常查找。所谓正常查找，就是其后的父类方法都通过 super 正确
关联了。</p>
<ol>
<li>在 <code>__new__</code> 方法中调用无参数 <code>super()</code> 会有什么结果<br>
答案: <br>
会抛出异常 <code>TypeError: object.__new__(): not enough arguments</code>。这是因为__new__是一个类方法，还不存在对应的实例对象。如果
在 <code>classmethod</code>装饰器修饰的类方法上调用 <code>super</code>, 则抛出异常 <code>TypeError: add() missing 1 required positional argument: 'm'</code>。
原因和这个差不多。</li>
</ol>
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<a href="https://www.linkedin.com/feed/" title="My LinkedIn" target="_blank" rel="nofollow noopener noreferrer">
<svg xmlns="http://www.w3.org/2000/svg" aria-label="LinkedIn" role="img" viewBox="0 0 512 512" fill="#fff"><rect width="512" height="512" rx="15%" fill="#0077b5"/><circle cx="142" cy="138" r="37"/><path stroke="#fff" stroke-width="66" d="M244 194v198M142 194v198"/><path d="M276 282c0-20 13-40 36-40 24 0 33 18 33 45v105h66V279c0-61-32-89-76-89-34 0-51 19-59 32"/></svg>
</a>
</div>
<h4>Friendship Links</h4>
<ul class="list-of-tags tags-in-article">
<li>
<a href="https://leetcode-cn.com/" title="力扣中国站点" target="_blank" rel="nofollow noopener noreferrer">
LeetCode
</a>
</li>
<li>
<a href="https://coolshell.cn/about" title="陈皓酷壳" target="_blank" rel="nofollow noopener noreferrer">
CoolShell
</a>
</li>
<li>
<a href="https://yikun.github.io/" title="姜毅坤博客" target="_blank" rel="nofollow noopener noreferrer">
Yikun
</a>
</li>
<li>
<a href="https://www.nowcoder.com/" title="牛客网" target="_blank" rel="nofollow noopener noreferrer">
牛客网
</a>
</li>
</ul>
</section>
</div>
</article>
</div>
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