<!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 initialization.

However, standard object-oriented principles apply to Python classes using deep inheritance hierarchies.

Therefore the developer has responsibility for ensuring that objects are properly initialized when

there are multiple <code>__init__</code> methods that need to be called.

</p>

<p>

If the <code>__init__</code> method of a superclass is not called during object initialization it is likely that

that object will end up in an incorrect state.

</p>

<p>A call to the <code>__init__</code> method of a superclass during object initialization may be omitted:

</p>

<ul>

<li>When a subclass calls the <code>__init__</code> method of the wrong class.</li>

<li>When a call to the <code>__init__</code> method of one its base classes is omitted.</li>

<li>When multiple inheritance is used and a class inherits from several base classes,

and at least one of those does not use <code>super()</code> in its own <code>__init__</code> method.</li>

</ul>

</overview>

<recommendation>

<p>Either be careful to explicitly call the <code>__init__</code> of the correct base class, 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 this example, explicit calls to <code>__init__</code> are used, but <code>SportsCar</code> erroneously calls

<code>Vehicle.__init__</code>. This is fixed in <code>FixedSportsCar</code> by calling <code>Car.__init__</code>.

</p>

<sample src="MissingCallToInit.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>