added note about leaving the rules file open too long

iterators.html

@@ -371,6 +371,11 @@ rules = LazyRules()</code></pre>
 <li><strong>Separation of code and data.</strong> All the patterns are stored in a separate file. Code is code, and data is data, and never the twain shall meet.
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+<blockquote class=note>
+<p><span class=u>&#x261E;</span>Is this really nirvana? Well, yes and no. Here&#8217;s something to consider with the <code>LazyRules</code> exampl: the pattern file is opened (during <code>__init__()</code>, and it remains open until the final rule is reached. Python will evenutally close the file when it exits, or after the last instantiation of the <code>LazyRules</code> class is destroyed, but still, that could be a <em>long</em> time. If this class is part of a long-running Python process, the Python interpreter may never exit, and the <code>LazyRules</code> object may never get destroyed.
+<p>There are ways around this. Instead of opening the file during <code>__init__()</code> and leaving it open while you read rules one line at a time, you could open the file, read all the rules, and immediately close the file. Or you could open the file, read one rule, save the file position with the <a href=files.html#read><code>tell()</code> method</a>, close the file, and later re-open it and use the <a href=files.html#read><code>seek()</code> method</a> to continue reading where you left off. Or you could not worry about it and just leave the file open, like this example code does. Programming is design, and design is all about trade-offs and constraints. Leaving a file open too long might be a problem; making your code more complicated might be a problem. Which one is the bigger problem depends on your development team, your application, and your runtime environment.
+</blockquote>
+
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 <h2 id=furtherreading>Further Reading</h2>