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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">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;();</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_&lt;&gt;:
</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">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;())</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&lt;Policies,Target&gt;(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&lt;T, Policies&gt;()
</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 &amp;T::begin, &amp;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">&lt;</span><span class="identifier">F</span><span class="special">&gt;(</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">(&amp;</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_begin</span><span class="special">,</span> <span class="special">&amp;</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">(&amp;</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_begin</span><span class="special">,</span> <span class="special">&amp;</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">&lt;&gt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</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">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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&lt;int&gt;
</span><span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;(</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">&amp;</span><span class="identifier">list_assign</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;)</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>
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