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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="python.iterators"></a>Iterators</h2></div></div></div> |
| <p> |
| In C++, and STL in particular, we see iterators everywhere. Python also has |
| iterators, but these are two very different beasts. |
| </p> |
| <p> |
| <span class="bold"><strong>C++ iterators:</strong></span> |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| C++ has 5 type categories (random-access, bidirectional, forward, input, |
| output) |
| </li> |
| <li class="listitem"> |
| There are 2 Operation categories: reposition, access |
| </li> |
| <li class="listitem"> |
| A pair of iterators is needed to represent a (first/last) range. |
| </li> |
| </ul></div> |
| <p> |
| <span class="bold"><strong>Python Iterators:</strong></span> |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| 1 category (forward) |
| </li> |
| <li class="listitem"> |
| 1 operation category (next()) |
| </li> |
| <li class="listitem"> |
| Raises StopIteration exception at end |
| </li> |
| </ul></div> |
| <p> |
| The typical Python iteration protocol: <code class="literal"><span class="bold"><strong>for y |
| in x...</strong></span></code> is as follows: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">__iter__</span><span class="special">()</span> <span class="comment"># get iterator |
| </span><span class="keyword">try</span><span class="special">:</span> |
| <span class="keyword">while</span> <span class="number">1</span><span class="special">:</span> |
| <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">iter</span><span class="special">.</span><span class="identifier">next</span><span class="special">()</span> <span class="comment"># get each item |
| </span> <span class="special">...</span> <span class="comment"># process y |
| </span><span class="keyword">except</span> <span class="identifier">StopIteration</span><span class="special">:</span> <span class="keyword">pass</span> <span class="comment"># iterator exhausted |
| </span></pre> |
| <p> |
| Boost.Python provides some mechanisms to make C++ iterators play along nicely |
| as Python iterators. What we need to do is to produce appropriate <code class="computeroutput"><span class="identifier">__iter__</span></code> function from C++ iterators that |
| is compatible with the Python iteration protocol. For example: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">object</span> <span class="identifier">get_iterator</span> <span class="special">=</span> <span class="identifier">iterator</span><span class="special"><</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>();</span> |
| <span class="identifier">object</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">get_iterator</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> |
| <span class="identifier">object</span> <span class="identifier">first</span> <span class="special">=</span> <span class="identifier">iter</span><span class="special">.</span><span class="identifier">next</span><span class="special">();</span> |
| </pre> |
| <p> |
| Or for use in class_<>: |
| </p> |
| <pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"__iter__"</span><span class="special">,</span> <span class="identifier">iterator</span><span class="special"><</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>())</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>range</strong></span> |
| </p> |
| <p> |
| We can create a Python savvy iterator using the range function: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| range(start, finish) |
| </li> |
| <li class="listitem"> |
| range<Policies,Target>(start, finish) |
| </li> |
| </ul></div> |
| <p> |
| Here, start/finish may be one of: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| member data pointers |
| </li> |
| <li class="listitem"> |
| member function pointers |
| </li> |
| <li class="listitem"> |
| adaptable function object (use Target parameter) |
| </li> |
| </ul></div> |
| <p> |
| <span class="bold"><strong>iterator</strong></span> |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| iterator<T, Policies>() |
| </li></ul></div> |
| <p> |
| Given a container <code class="literal">T</code>, iterator is a shortcut that simply |
| calls <code class="literal">range</code> with &T::begin, &T::end. |
| </p> |
| <p> |
| Let's put this into action... Here's an example from some hypothetical bogon |
| Particle accelerator code: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">f</span> <span class="special">=</span> <span class="identifier">Field</span><span class="special">()</span> |
| <span class="keyword">for</span> <span class="identifier">x</span> <span class="keyword">in</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">pions</span><span class="special">:</span> |
| <span class="identifier">smash</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> |
| <span class="keyword">for</span> <span class="identifier">y</span> <span class="keyword">in</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">bogons</span><span class="special">:</span> |
| <span class="identifier">count</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> |
| </pre> |
| <p> |
| Now, our C++ Wrapper: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">class_</span><span class="special"><</span><span class="identifier">F</span><span class="special">>(</span><span class="string">"Field"</span><span class="special">)</span> |
| <span class="special">.</span><span class="identifier">property</span><span class="special">(</span><span class="string">"pions"</span><span class="special">,</span> <span class="identifier">range</span><span class="special">(&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_begin</span><span class="special">,</span> <span class="special">&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_end</span><span class="special">))</span> |
| <span class="special">.</span><span class="identifier">property</span><span class="special">(</span><span class="string">"bogons"</span><span class="special">,</span> <span class="identifier">range</span><span class="special">(&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_begin</span><span class="special">,</span> <span class="special">&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_end</span><span class="special">));</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>stl_input_iterator</strong></span> |
| </p> |
| <p> |
| So far, we have seen how to expose C++ iterators and ranges to Python. Sometimes |
| we wish to go the other way, though: we'd like to pass a Python sequence to |
| an STL algorithm or use it to initialize an STL container. We need to make |
| a Python iterator look like an STL iterator. For that, we use <code class="computeroutput"><span class="identifier">stl_input_iterator</span><span class="special"><></span></code>. |
| Consider how we might implement a function that exposes <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">assign</span><span class="special">()</span></code> to Python: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">list_assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">l</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">o</span><span class="special">)</span> <span class="special">{</span> |
| <span class="comment">// Turn a Python sequence into an STL input range |
| </span> <span class="identifier">stl_input_iterator</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">o</span><span class="special">),</span> <span class="identifier">end</span><span class="special">;</span> |
| <span class="identifier">l</span><span class="special">.</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">);</span> |
| <span class="special">}</span> |
| |
| <span class="comment">// Part of the wrapper for list<int> |
| </span><span class="identifier">class_</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>(</span><span class="string">"list_int"</span><span class="special">)</span> |
| <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"assign"</span><span class="special">,</span> <span class="special">&</span><span class="identifier">list_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">>)</span> |
| <span class="comment">// ... |
| </span> <span class="special">;</span> |
| </pre> |
| <p> |
| Now in Python, we can assign any integer sequence to <code class="computeroutput"><span class="identifier">list_int</span></code> |
| objects: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">list_int</span><span class="special">();</span> |
| <span class="identifier">x</span><span class="special">.</span><span class="identifier">assign</span><span class="special">([</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">,</span><span class="number">5</span><span class="special">])</span> |
| </pre> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2002-2005 Joel |
| de Guzman, David Abrahams<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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