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POHow do I dynamically add mixins as base classes without getting MRO errors?
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copied!<p>Say I have a class <code>A</code>, <code>B</code> and <code>C</code>.</p> <p>Class <code>A</code> and <code>B</code> are both mixin classes for Class <code>C</code>.</p> <pre><code>class A( object ): pass class B( object ): pass class C( object, A, B ): pass </code></pre> <p>This will not work when instantiating class C. I would have to remove <code>object</code> from class C to make it work. (Else you'll get MRO problems).</p> <blockquote> <p>TypeError: Error when calling the metaclass bases<br> Cannot create a consistent method resolution<br> order (MRO) for bases B, object, A</p> </blockquote> <p>However, my case is a bit more complicated. In my case class <code>C</code> is a <em>server</em> where <code>A</code> and <code>B</code> will be plugins that are loaded on startup. These are residing in their own folder.</p> <p>I also have a Class named <code>Cfactory</code>. In Cfactory I have a <code>__new__</code> method that will create a fully functional object C. In the <code>__new__</code> method I <em>search</em> for plugins, load them using <code>__import__</code>, and then assign them to <code>C.__bases__ += (loadedClassTypeGoesHere, )</code></p> <p>So the following is a possibility: (made it quite abstract)</p> <pre><code>class A( object ): def __init__( self ): pass def printA( self ): print "A" class B( object ): def __init__( self ): pass def printB( self ): print "B" class C( object ): def __init__( self ): pass class Cfactory( object ): def __new__( cls ): C.__bases__ += ( A, ) C.__bases__ += ( B, ) return C() </code></pre> <p>This again will not work, and will give the MRO errors again: </p> <blockquote> <p>TypeError: Cannot create a consistent method resolution<br> order (MRO) for bases object, A</p> </blockquote> <p>An easy fix for this is removing the <code>object</code> baseclass from <code>A</code> and <code>B</code>. However this will make them old-style objects which should be avoided when these plugins are being run stand-alone (which should be possible, UnitTest wise)</p> <p>Another easy fix is removing <code>object</code> from <code>C</code> but this will also make it an old-style class and <code>C.__bases__</code> will be unavailable thus I can't add extra objects to the base of <code>C</code></p> <p>What would be a good architectural solution for this and how would you do something like this? For now I can live with old-style classes for the plugins themselves. But I rather not use them.</p>

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