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<!DOCTYPE HTML>
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<title>修改 Python 源码 · Python 3 源码分析</title>
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<li class="header">第 1 部分：序章</li>
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<b>1.1.</b>
前言
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<b>1.2.</b>
Python 源代码的组织
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<b>1.5.</b>
修改 Python 源码
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<b>2.1.</b>
Python 对象初探
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<b>2.2.</b>
Python 整数对象
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<b>2.3.</b>
Python 字符串 对象
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<b>2.4.</b>
Python List 对象
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Python Dict 对象
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<b>2.6.</b>
Python Set 对象
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<b>2.7.</b>
实现简版 Python
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<a href=".." >修改 Python 源码</a>
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<h1 id="修改-python-源码">修改 Python 源码</h1>
<h2 id="在源代码中-print">在源代码中 Print</h2>
<p>在接下来研究源码的过程中，我们可能会对某些语句的逻辑感到好奇，需要输出中间结果。
这就需要借助 Python C API 中打印对象的接口：</p>
<p><code>源文件：</code><a href="https://github.com/python/cpython/blob/v3.7.0/Objects/object.c#L339" target="_blank">Objects/object.c</a></p>
<pre class="language-"><code class="lang-c"><span class="token keyword">int</span>
<span class="token function">PyObject_Print</span><span class="token punctuation">(</span>PyObject <span class="token operator">*</span>op<span class="token punctuation">,</span> FILE <span class="token operator">*</span>fp<span class="token punctuation">,</span> <span class="token keyword">int</span> flags<span class="token punctuation">)</span>
</code></pre>
<p>比如，我们希望在解释器交互界面中打印整数值的时候输出一段字符串，则我们可以修改如下函数：</p>
<p><code>源文件：</code><a href="https://github.com/python/cpython/blob/v3.7.0/Objects/longobject.c#L1762" target="_blank">Objects/longobject.c</a></p>
<pre class="language-"><code class="lang-c"><span class="token keyword">static</span> PyObject <span class="token operator">*</span>
<span class="token function">long_to_decimal_string</span><span class="token punctuation">(</span>PyObject <span class="token operator">*</span>aa<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
PyObject <span class="token operator">*</span>str <span class="token operator">=</span> <span class="token function">PyUnicode_FromString</span><span class="token punctuation">(</span><span class="token string">"I am always before int"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">PyObject_Print</span><span class="token punctuation">(</span>str<span class="token punctuation">,</span> <span class="token constant">stdout</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
PyObject <span class="token operator">*</span>v<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">long_to_decimal_string_internal</span><span class="token punctuation">(</span>aa<span class="token punctuation">,</span> <span class="token operator">&</span>v<span class="token punctuation">,</span> <span class="token constant">NULL</span><span class="token punctuation">,</span> <span class="token constant">NULL</span><span class="token punctuation">,</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token constant">NULL</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> v<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<p>函数实现中的前 3 行为我们加入的代码，其中：</p>
<ul>
<li><code>PyUnicode_FromString</code> 用于把 C 中的原生字符数组转换为出 Python 中的字符串（Unicode）对象</li>
<li><code>PyObject_Print</code> 则将转换好的字符串对象打印至我们指定的标准输出（<code>stdout</code>）</li>
</ul>
<p>对 Python 重新进行编译，在 Unix 上可执行：</p>
<pre class="language-"><code class="lang-console"><span class="token function">make</span> <span class="token operator">&&</span> <span class="token function">make</span> bininstall
</code></pre>
<p>运行编译后的 Python，输入 print 语句即可看到我们希望的结果：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> <span class="token keyword">print</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token string">'I am always before int'</span>
<span class="token number">1</span>
</code></pre>
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