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<title>Extending Wrapped Objects in Python</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Extending Wrapped Objects in Python</b></font>
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<p>
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Thanks to Python's flexibility, you can easily add new methods to a class,
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even after it was already created:</p>
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<code><pre>
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<span class=special>>>> </span><span class=keyword>class </span><span class=identifier>C</span><span class=special>(</span><span class=identifier>object</span><span class=special>): </span><span class=identifier>pass
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</span><span class=special>>>>
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>>> </span>##<span class=identifier>a </span><span class=identifier>regular </span><span class=identifier>function
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</span><span class=special>>>> </span><span class=identifier>def </span><span class=identifier>C_str</span><span class=special>(</span><span class=identifier>self</span><span class=special>): </span><span class=keyword>return </span><span class=literal>'A C instance!'
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</span><span class=special>>>>
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>>> </span>##<span class=identifier>now </span><span class=identifier>we </span><span class=identifier>turn </span><span class=identifier>it </span><span class=identifier>in </span><span class=identifier>a </span><span class=identifier>member </span><span class=identifier>function
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</span><span class=special>>>> </span><span class=identifier>C</span><span class=special>.</span><span class=identifier>__str__ </span><span class=special>= </span><span class=identifier>C_str
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</span><span class=special>>>>
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>>> </span><span class=identifier>c </span><span class=special>= </span><span class=identifier>C</span><span class=special>()
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>>> </span><span class=identifier>print </span><span class=identifier>c
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</span><span class=identifier>A </span><span class=identifier>C </span><span class=identifier>instance</span><span class=special>!
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>>> </span><span class=identifier>C_str</span><span class=special>(</span><span class=identifier>c</span><span class=special>)
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</span><span class=identifier>A </span><span class=identifier>C </span><span class=identifier>instance</span><span class=special>!
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</span></pre></code>
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<p>
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Yes, Python rox. <img src="theme/smiley.gif"></img></p>
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<p>
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We can do the same with classes that were wrapped with Boost.Python. Suppose
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we have a class <tt>point</tt> in C++:</p>
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<code><pre>
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<span class=keyword>class </span><span class=identifier>point </span><span class=special>{...};
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</span><span class=identifier>BOOST_PYTHON_MODULE</span><span class=special>(</span><span class=identifier>_geom</span><span class=special>)
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{
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</span><span class=identifier>class_</span><span class=special><</span><span class=identifier>point</span><span class=special>>(</span><span class=string>"point"</span><span class=special>)...;
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}
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</span></pre></code>
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<p>
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If we are using the technique from the previous session, <a href="creating_packages.html">
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Creating Packages</a>, we can code directly into <tt>geom/__init__.py</tt>:</p>
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<code><pre>
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<span class=identifier>from </span><span class=identifier>_geom </span><span class=identifier>import </span><span class=special>*
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</span>##<span class=identifier>a </span><span class=identifier>regular </span><span class=identifier>function
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</span><span class=identifier>def </span><span class=identifier>point_str</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
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</span><span class=keyword>return </span><span class=identifier>str</span><span class=special>((</span><span class=identifier>self</span><span class=special>.</span><span class=identifier>x</span><span class=special>, </span><span class=identifier>self</span><span class=special>.</span><span class=identifier>y</span><span class=special>))
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</span>##<span class=identifier>now </span><span class=identifier>we </span><span class=identifier>turn </span><span class=identifier>it </span><span class=identifier>into </span><span class=identifier>a </span><span class=identifier>member </span><span class=identifier>function
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</span><span class=identifier>point</span><span class=special>.</span><span class=identifier>__str__ </span><span class=special>= </span><span class=identifier>point_str
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</span></pre></code>
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<p>
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<b>All</b> point instances created from C++ will also have this member function!
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This technique has several advantages:</p>
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<ul><li>Cut down compile times to zero for these additional functions</li><li>Reduce the memory footprint to virtually zero</li><li>Minimize the need to recompile</li><li>Rapid prototyping (you can move the code to C++ if required without changing the interface)</li></ul><p>
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You can even add a little syntactic sugar with the use of metaclasses. Let's
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create a special metaclass that "injects" methods in other classes.</p>
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<code><pre>
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##<span class=identifier>The </span><span class=identifier>one </span><span class=identifier>Boost</span><span class=special>.</span><span class=identifier>Python </span><span class=identifier>uses </span><span class=keyword>for </span><span class=identifier>all </span><span class=identifier>wrapped </span><span class=identifier>classes</span><span class=special>.
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</span>##<span class=identifier>You </span><span class=identifier>can </span><span class=identifier>use </span><span class=identifier>here </span><span class=identifier>any </span><span class=keyword>class </span><span class=identifier>exported </span><span class=identifier>by </span><span class=identifier>Boost </span><span class=identifier>instead </span><span class=identifier>of </span><span class=string>"point"
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</span><span class=identifier>BoostPythonMetaclass </span><span class=special>= </span><span class=identifier>point</span><span class=special>.</span><span class=identifier>__class__
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</span><span class=keyword>class </span><span class=identifier>injector</span><span class=special>(</span><span class=identifier>object</span><span class=special>):
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</span><span class=keyword>class </span><span class=identifier>__metaclass__</span><span class=special>(</span><span class=identifier>BoostPythonMetaclass</span><span class=special>):
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</span><span class=identifier>def </span><span class=identifier>__init__</span><span class=special>(</span><span class=identifier>self</span><span class=special>, </span><span class=identifier>name</span><span class=special>, </span><span class=identifier>bases</span><span class=special>, </span><span class=identifier>dict</span><span class=special>):
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</span><span class=keyword>for </span><span class=identifier>b </span><span class=identifier>in </span><span class=identifier>bases</span><span class=special>:
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</span><span class=keyword>if </span><span class=identifier>type</span><span class=special>(</span><span class=identifier>b</span><span class=special>) </span><span class=keyword>not </span><span class=identifier>in </span><span class=special>(</span><span class=identifier>self</span><span class=special>, </span><span class=identifier>type</span><span class=special>):
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</span><span class=keyword>for </span><span class=identifier>k</span><span class=special>,</span><span class=identifier>v </span><span class=identifier>in </span><span class=identifier>dict</span><span class=special>.</span><span class=identifier>items</span><span class=special>():
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</span><span class=identifier>setattr</span><span class=special>(</span><span class=identifier>b</span><span class=special>,</span><span class=identifier>k</span><span class=special>,</span><span class=identifier>v</span><span class=special>)
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</span><span class=keyword>return </span><span class=identifier>type</span><span class=special>.</span><span class=identifier>__init__</span><span class=special>(</span><span class=identifier>self</span><span class=special>, </span><span class=identifier>name</span><span class=special>, </span><span class=identifier>bases</span><span class=special>, </span><span class=identifier>dict</span><span class=special>)
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</span>##<span class=identifier>inject </span><span class=identifier>some </span><span class=identifier>methods </span><span class=identifier>in </span><span class=identifier>the </span><span class=identifier>point </span><span class=identifier>foo
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</span><span class=keyword>class </span><span class=identifier>more_point</span><span class=special>(</span><span class=identifier>injector</span><span class=special>, </span><span class=identifier>point</span><span class=special>):
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</span><span class=identifier>def </span><span class=identifier>__repr__</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
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</span><span class=keyword>return </span><span class=literal>'Point(x=%s, y=%s)' </span><span class=special>% (</span><span class=identifier>self</span><span class=special>.</span><span class=identifier>x</span><span class=special>, </span><span class=identifier>self</span><span class=special>.</span><span class=identifier>y</span><span class=special>)
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</span><span class=identifier>def </span><span class=identifier>foo</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
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</span><span class=identifier>print </span><span class=literal>'foo!'
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</span></pre></code>
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<p>
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Now let's see how it got:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>print </span><span class=identifier>point</span><span class=special>()
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</span><span class=identifier>Point</span><span class=special>(</span><span class=identifier>x</span><span class=special>=</span><span class=number>10</span><span class=special>, </span><span class=identifier>y</span><span class=special>=</span><span class=number>10</span><span class=special>)
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>>> </span><span class=identifier>point</span><span class=special>().</span><span class=identifier>foo</span><span class=special>()
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</span><span class=identifier>foo</span><span class=special>!
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</span></pre></code>
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<p>
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Another useful idea is to replace constructors with factory functions:</p>
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<code><pre>
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<span class=identifier>_point </span><span class=special>= </span><span class=identifier>point
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</span><span class=identifier>def </span><span class=identifier>point</span><span class=special>(</span><span class=identifier>x</span><span class=special>=</span><span class=number>0</span><span class=special>, </span><span class=identifier>y</span><span class=special>=</span><span class=number>0</span><span class=special>):
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</span><span class=keyword>return </span><span class=identifier>_point</span><span class=special>(</span><span class=identifier>x</span><span class=special>, </span><span class=identifier>y</span><span class=special>)
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</span></pre></code>
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<p>
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In this simple case there is not much gained, but for constructurs with
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many overloads and/or arguments this is often a great simplification, again
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with virtually zero memory footprint and zero compile-time overhead for
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the keyword support.</p>
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<br>
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<hr size="1"><p class="copyright">Copyright © 2002-2003 David Abrahams<br>Copyright © 2002-2003 Joel de Guzman<br><br>
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<font size="2">Permission to copy, use, modify, sell and distribute this document
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is granted provided this copyright notice appears in all copies. This document
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is provided "as is" without express or implied warranty, and with
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