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Initial commit of Python queries and QL libraries.

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Mark Shannon
2018-11-19 13:13:39 +00:00
committed by Mark Shannon
parent 90c75cd362
commit 5f58824d1b
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python/ql/src/Classes/SuperclassDelCalledMultipleTimes.qhelp Normal file
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<!DOCTYPE qhelp PUBLIC
"-//Semmle//qhelp//EN"
"qhelp.dtd">
<qhelp>
<overview>
<p>Python, unlike statically typed languages such as Java, allows complete freedom when calling methods during object destruction.
However, standard object-oriented principles apply to Python classes using deep inheritance hierarchies.
Therefore the developer has responsibility for ensuring that objects are properly cleaned up when
there are multiple <code>__del__</code> methods that need to be called.
</p>
<p>
Calling a <code>__del__</code> method more than once during object destruction risks resources being released multiple
times. The relevant <code>__del__</code> method may not be designed to be called more than once.
</p>
<p>There are a number of ways that a <code>__del__</code> method may be be called more than once.</p>
<ul>
<li>There may be more than one explicit call to the method in the hierarchy of <code>__del__</code> methods.</li>
<li>A class using multiple inheritance directly calls the <code>__del__</code> methods of its base types.
One or more of those base types uses <code>super()</code> to pass down the inheritance chain.</li>
</ul>
</overview>
<recommendation>
<p>Either be careful not to explicitly call a <code>__del__</code> method more than once, or
use <code>super()</code> throughout the inheritance hierarchy.</p>
<p>Alternatively refactor one or more of the classes to use composition rather than inheritance.</p>
</recommendation>
<example>
<p>In the first example, explicit calls to <code>__del__</code> are used, but <code>SportsCar</code> erroneously calls
both <code>Vehicle.__del__</code> and <code>Car.__del__</code>.
This can be fixed by removing the call to <code>Vehicle.__del__</code>, as shown in <code>FixedSportsCar</code>.
</p>
<sample src="SuperclassDelCalledMultipleTimes.py" />
<p>In the second example, there is a mixture of explicit calls to <code>__del__</code> and calls using <code>super()</code>.
To fix this example, <code>super()</code> should be used throughout.
</p>
<sample src="SuperclassInitCalledMultipleTimes2.py" />
</example>
<references>
<li>Python Tutorial: <a href="https://docs.python.org/2/tutorial/classes.html">Classes</a>.</li>
<li>Python Standard Library: <a href="https://docs.python.org/2/library/functions.html#super">super</a>.</li>
<li>Artima Developer: <a href="http://www.artima.com/weblogs/viewpost.jsp?thread=236275">Things to Know About Python Super</a>.</li>
<li>Wikipedia: <a href="http://en.wikipedia.org/wiki/Composition_over_inheritance">Composition over inheritance</a>.</li>
</references>
</qhelp>