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<h1>
<a href="./python-xin-shi-lei-he-jiu-shi-lei-de-yi-xie-li-jie.html">
python 新式类和旧式类的一些理解
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<blockquote>
<p>下面是两个类,一个新式类，一个旧式类</p>
</blockquote>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">OldClass</span><span class="p">():</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">NewClass</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">pass</span>
<span class="n">old_instance</span> <span class="o">=</span> <span class="n">OldClass</span><span class="p">()</span>
<span class="n">new_instance</span> <span class="o">=</span> <span class="n">NewClass</span><span class="p">()</span>
</pre></div>
<blockquote>
<p>下面是我在ipython运行测试的结果</p>
</blockquote>
<div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">15</span><span class="p">]:</span> <span class="k">class</span> <span class="nc">OldClass</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">16</span><span class="p">]:</span> <span class="k">class</span> <span class="nc">NewClass</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">17</span><span class="p">]:</span> <span class="n">old_instance</span> <span class="o">=</span> <span class="n">OldClass</span><span class="p">()</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">18</span><span class="p">]:</span> <span class="n">new_instance</span> <span class="o">=</span> <span class="n">NewClass</span><span class="p">()</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">19</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="n">old_instance</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">19</span><span class="p">]:</span> <span class="n">instance</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">23</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="n">new_instance</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">23</span><span class="p">]:</span> <span class="n">__main__</span><span class="o">.</span><span class="n">NewClass</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">24</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">old_instance</span><span class="p">))</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'instance'</span><span class="o">></span>
<span class="n">In</span> <span class="p">[</span><span class="mi">25</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">new_instance</span><span class="p">))</span>
<span class="o"><</span><span class="k">class</span> <span class="err">'</span><span class="nc">__main__</span><span class="o">.</span><span class="n">NewClass</span><span class="s1">'></span>
<span class="n">In</span> <span class="p">[</span><span class="mi">26</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">26</span><span class="p">]:</span> <span class="nb">int</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">27</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'int'</span><span class="o">></span>
<span class="n">In</span> <span class="p">[</span><span class="mi">31</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="nb">int</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">31</span><span class="p">]:</span> <span class="kc">True</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">32</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="ow">is</span> <span class="nb">int</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">32</span><span class="p">]:</span> <span class="kc">True</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">33</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="n">old_instance</span><span class="p">)</span> <span class="ow">is</span> <span class="n">OldClass</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">33</span><span class="p">]:</span> <span class="kc">False</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">34</span><span class="p">]:</span> <span class="nb">type</span><span class="p">(</span><span class="n">new_instance</span><span class="p">)</span> <span class="ow">is</span> <span class="n">NewClass</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">34</span><span class="p">]:</span> <span class="kc">True</span>
</pre></div>
<ul>
<li>通过上面，我们看到，旧式类，一切实例的类型均是<code>instance</code>。它和内置类型表现形式不一致，这样不能很好的体现实例和类之间的关系。
而新式类则使类和内置类型表现形式一致。为了判断一个实例是否属于一个类，旧式类最可靠的方式时<code>instance()</code>内置方法，新式类可以
通过<code>type</code>判断，和内置类型一致。</li>
</ul>
<blockquote>
<p>下面是类本身的特性</p>
</blockquote>
<div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">37</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'int'</span><span class="o">></span>
<span class="n">In</span> <span class="p">[</span><span class="mi">38</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">OldClass</span><span class="p">))</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'classobj'</span><span class="o">></span>
<span class="n">In</span> <span class="p">[</span><span class="mi">39</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">NewClass</span><span class="p">))</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'type'</span><span class="o">></span>
<span class="n">In</span> <span class="p">[</span><span class="mi">40</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="nb">int</span><span class="p">))</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'type'</span><span class="o">></span>
</pre></div>
<p>新式类的的类型是<code>type</code>，这和内置类型一致。旧式类则不是。普通类是元类的实例，对于新式类，默认的元类是<code>type</code>。</p>
<hr>
<p><em>注:最新的python版本将内置类型也统一成类的形式，<code>print(type(int)</code>将打印<code><class 'type'></code>。以后我觉得更精准的称呼应该是内置类和自定义类</em></p>
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