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<div class="section" id="support-for-cooperative-multi-inheritance">
<span id="ref-cooperative-multiinheritance"></span><h1>Support for Cooperative Multi-inheritance<a class="headerlink" href="#support-for-cooperative-multi-inheritance" title="Permalink to this headline">ΒΆ</a></h1>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This section is not about sub-classing from more that one Qt class.</p>
</div>
<p>Cooperative multi-inheritance is a technique for implementing classes that
inherit multiple super-classes - typically a main super-class and one or more
mixin classes that add additional behaviour. It makes it easy to add new
mixins at a later date to further extend the behavior, without needing to
change either the implementation of the class or any existing code that creates
an instance of the class.</p>
<p>The technique requires that all the super-class’s <tt class="docutils literal"><span class="pre">__init__</span></tt> methods follow
the same pattern in the way that they handle unrecognised keyword arguments and
use <tt class="docutils literal"><span class="pre">super()</span></tt> to invoke their own super-class’s <tt class="docutils literal"><span class="pre">__init__</span></tt> methods.</p>
<p>PyQt5’s classes follow this pattern.</p>
<p>See Raymond Hettinger’s <a class="reference external" href="http://rhettinger.wordpress.com/2011/05/26/super-considered-super/">Python’s super() considered super!</a> blog
post for some more background on the subject.</p>
<p>As an example, let’s say we have a class that represents a person, and that a
person has a name. The following might be an initial implementation:</p>
<div class="highlight-python"><pre>class Person(QObject):
def __init__(self, name, parent=None)
QObject.__init__(self, parent)
self.name = name</pre>
</div>
<p>An instance would normally be created in one of the following ways:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">person</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="s">"Joe"</span><span class="p">)</span>
<span class="n">person</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="s">"Joe"</span><span class="p">,</span> <span class="n">some_parent</span><span class="p">)</span>
</pre></div>
</div>
<p>This approach has some limitations:</p>
<ul class="simple">
<li>Only a sub-set of the <a class="reference internal" href="api/qobject.html#PyQt5.QtCore.QObject" title="PyQt5.QtCore.QObject"><tt class="xref py py-class docutils literal"><span class="pre">QObject</span></tt></a> API is exposed. For
example you cannot set the value of a Qt property or connect a signal by
passing appropriate keyword arguments to <tt class="docutils literal"><span class="pre">Person.__init__</span></tt>.</li>
<li>Adding another class to <tt class="docutils literal"><span class="pre">Person</span></tt>‘s list of super-classes means that its
<tt class="docutils literal"><span class="pre">__init__</span></tt> implementation needs to be changed. If the new mixin takes
non-optional arguments then every call to create a <tt class="docutils literal"><span class="pre">Person</span></tt> instance will
need changing.</li>
</ul>
<p>Consider this alternative implementation:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">QObject</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
</pre></div>
</div>
<p>The difference is that we only handle arguments that are used by the <tt class="docutils literal"><span class="pre">Person</span></tt>
class itself and we punt all the other arguments to the super-classes by
calling <tt class="docutils literal"><span class="pre">super()</span></tt>.</p>
<p>With this implementation an instance would normally created in one of the
following ways:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">person</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="s">"Joe"</span><span class="p">)</span>
<span class="n">person</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="s">"Joe"</span><span class="p">,</span> <span class="n">parent</span><span class="o">=</span><span class="n">some_parent</span><span class="p">)</span>
</pre></div>
</div>
<p>Here the difference is that we are using keyword arguments to specify any
arguments that are not handled by the <tt class="docutils literal"><span class="pre">Person</span></tt> class itself. Note that we
could use keyword arguments for all arguments - whether or not you do so is
down to personal choice.</p>
<p>The limitations of the first implementation no longer apply. For example,
without any further changes we can also do this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">person</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="s">"Joe"</span><span class="p">,</span> <span class="n">destroyed</span><span class="o">=</span><span class="n">some_callable</span><span class="p">)</span>
</pre></div>
</div>
<p>Let’s say we now want to extend the behaviour of the <tt class="docutils literal"><span class="pre">Person</span></tt> class by adding
a mixin that handles a person’s age. The implementation of the mixin would be
as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Age</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">age</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o">=</span> <span class="n">age</span>
</pre></div>
</div>
<p>This follows a similar pattern to our <tt class="docutils literal"><span class="pre">Person</span></tt> implementation, but notice
that we have provided the <tt class="docutils literal"><span class="pre">age</span></tt> argument with a default value.</p>
<p>The following is our new <tt class="docutils literal"><span class="pre">Person</span></tt> implementation:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">QObject</span><span class="p">,</span> <span class="n">Age</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
</pre></div>
</div>
<p>The only change we have had to make is to add <tt class="docutils literal"><span class="pre">Age</span></tt> to <tt class="docutils literal"><span class="pre">Person</span></tt>‘s list of
super-classes. More importantly we do not need to change any call to create a
<tt class="docutils literal"><span class="pre">Person</span></tt> instance.</p>
<p>If we do want to create a <tt class="docutils literal"><span class="pre">Person</span></tt> instance with a non-default age then we
simply pass it as a keyword argument as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">person</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="s">"Joe"</span><span class="p">,</span> <span class="n">age</span><span class="o">=</span><span class="mi">38</span><span class="p">)</span>
</pre></div>
</div>
<p>This technique increases the use of keyword arguments - while this means a bit
more typing, it significantly increases the readability of application code.</p>
</div>
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