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<title>How can I automatically convert my custom string type to and from a Python string?</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="faq.how_can_i_automatically_convert_"></a><a class="link" href="how_can_i_automatically_convert_.html" title="How can I automatically convert my custom string type to and from a Python string?">How can I automatically
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convert my custom string type to and from a Python string?</a>
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</h3></div></div></div>
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<p>
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<span class="emphasis"><em>Ralf W. Grosse-Kunstleve provides these notes:</em></span>
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</p>
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<p>
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Below is a small, self-contained demo extension module that shows how to
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do this. Here is the corresponding trivial test:
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</p>
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<pre class="programlisting"><span class="identifier">import</span> <span class="identifier">custom_string</span>
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<span class="identifier">assert</span> <span class="identifier">custom_string</span><span class="special">.</span><span class="identifier">hello</span><span class="special">()</span> <span class="special">==</span> <span class="string">"Hello world."</span>
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<span class="identifier">assert</span> <span class="identifier">custom_string</span><span class="special">.</span><span class="identifier">size</span><span class="special">(</span><span class="string">"california"</span><span class="special">)</span> <span class="special">==</span> <span class="number">10</span>
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</pre>
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<p>
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If you look at the code you will find:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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A custom <code class="computeroutput"><span class="identifier">to_python</span></code> converter
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(easy): <code class="computeroutput"><span class="identifier">custom_string_to_python_str</span></code>
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</li>
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<li class="listitem">
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A custom lvalue converter (needs more code): <code class="computeroutput"><span class="identifier">custom_string_from_python_str</span></code>
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</li>
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</ul></div>
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<p>
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The custom converters are registered in the global Boost.Python registry
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near the top of the module initialization function. Once flow control has
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passed through the registration code the automatic conversions from and to
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Python strings will work in any module imported in the same process.
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">module</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">def</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">to_python_converter</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">namespace</span> <span class="identifier">sandbox</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="special">{</span>
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<span class="keyword">class</span> <span class="identifier">custom_string</span>
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<span class="special">{</span>
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<span class="keyword">public</span><span class="special">:</span>
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<span class="identifier">custom_string</span><span class="special">()</span> <span class="special">{}</span>
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<span class="identifier">custom_string</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">value</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">value_</span><span class="special">(</span><span class="identifier">value</span><span class="special">)</span> <span class="special">{}</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">value_</span><span class="special">;</span> <span class="special">}</span>
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<span class="keyword">private</span><span class="special">:</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">value_</span><span class="special">;</span>
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<span class="special">};</span>
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<span class="keyword">struct</span> <span class="identifier">custom_string_to_python_str</span>
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<span class="special">{</span>
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<span class="keyword">static</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">custom_string</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">s</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">incref</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">s</span><span class="special">.</span><span class="identifier">value</span><span class="special">()).</span><span class="identifier">ptr</span><span class="special">());</span>
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<span class="special">}</span>
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<span class="special">};</span>
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<span class="keyword">struct</span> <span class="identifier">custom_string_from_python_str</span>
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<span class="special">{</span>
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<span class="identifier">custom_string_from_python_str</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">converter</span><span class="special">::</span><span class="identifier">registry</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">(</span>
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<span class="special">&</span><span class="identifier">convertible</span><span class="special">,</span>
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<span class="special">&</span><span class="identifier">construct</span><span class="special">,</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">type_id</span><span class="special"><</span><span class="identifier">custom_string</span><span class="special">>());</span>
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<span class="special">}</span>
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<span class="keyword">static</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">convertible</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">obj_ptr</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">if</span> <span class="special">(!</span><span class="identifier">PyString_Check</span><span class="special">(</span><span class="identifier">obj_ptr</span><span class="special">))</span> <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="identifier">obj_ptr</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">construct</span><span class="special">(</span>
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<span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">obj_ptr</span><span class="special">,</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">converter</span><span class="special">::</span><span class="identifier">rvalue_from_python_stage1_data</span><span class="special">*</span> <span class="identifier">data</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">PyString_AsString</span><span class="special">(</span><span class="identifier">obj_ptr</span><span class="special">);</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">value</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">throw_error_already_set</span><span class="special">();</span>
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<span class="keyword">void</span><span class="special">*</span> <span class="identifier">storage</span> <span class="special">=</span> <span class="special">(</span>
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<span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">converter</span><span class="special">::</span><span class="identifier">rvalue_from_python_storage</span><span class="special"><</span><span class="identifier">custom_string</span><span class="special">>*)</span>
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<span class="identifier">data</span><span class="special">)-></span><span class="identifier">storage</span><span class="special">.</span><span class="identifier">bytes</span><span class="special">;</span>
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<span class="keyword">new</span> <span class="special">(</span><span class="identifier">storage</span><span class="special">)</span> <span class="identifier">custom_string</span><span class="special">(</span><span class="identifier">value</span><span class="special">);</span>
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<span class="identifier">data</span><span class="special">-></span><span class="identifier">convertible</span> <span class="special">=</span> <span class="identifier">storage</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="special">};</span>
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<span class="identifier">custom_string</span> <span class="identifier">hello</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">custom_string</span><span class="special">(</span><span class="string">"Hello world."</span><span class="special">);</span> <span class="special">}</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">custom_string</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">s</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">value</span><span class="special">().</span><span class="identifier">size</span><span class="special">();</span> <span class="special">}</span>
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<span class="keyword">void</span> <span class="identifier">init_module</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">;</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">to_python_converter</span><span class="special"><</span>
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<span class="identifier">custom_string</span><span class="special">,</span>
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<span class="identifier">custom_string_to_python_str</span><span class="special">>();</span>
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<span class="identifier">custom_string_from_python_str</span><span class="special">();</span>
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<span class="identifier">def</span><span class="special">(</span><span class="string">"hello"</span><span class="special">,</span> <span class="identifier">hello</span><span class="special">);</span>
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<span class="identifier">def</span><span class="special">(</span><span class="string">"size"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="special">}}</span> <span class="comment">// namespace sandbox::<anonymous></span>
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<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">custom_string</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">sandbox</span><span class="special">::</span><span class="identifier">init_module</span><span class="special">();</span>
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<span class="special">}</span>
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</pre>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2015 David
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Abrahams, Stefan Seefeld<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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