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POBase class deleted before subclass during python __del__ processing
 

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    <p><strong>Context</strong></p> <p>I am aware that if I ask a question about Python destructors, the standard argument will be to use contexts instead. Let me start by explaining why I am not doing that.</p> <p>I am writing a subclass to <code>logging.Handler</code>. When an instance is closed, it posts a sentinel value to a Queue.Queue. If it doesn't, a second thread will be left running forever, waiting for Queue.Queue.get() to complete.</p> <p>I am writing this with other developers in mind, so I don't want a failure to call close() on a handler object to cause the program to hang.</p> <p>Therefore, I am adding a check in __del__() to ensure the object was closed properly.</p> <p>I understand circular references may cause it to fail in some circumstances. There's not a lot I can do about that.</p> <p><strong>Problem</strong></p> <p>Here is some simple example code:</p> <pre><code>explicit_delete = True class Base: def __del__(self): print "Base class cleaning up." class Sub(Base): def __del__(self): print "Sub-class cleaning up." Base.__del__(self) x = Sub() if explicit_delete: del x print "End of thread" </code></pre> <p>When I run this I get, as expected:</p> <pre><code>Sub-class cleaning up. Base class cleaning up. End of thread </code></pre> <p>If I set <code>explicit_delete</code> to <code>False</code> in the first line, I get:</p> <pre><code>End of thread Sub-class cleaning up. Exception AttributeError: "'NoneType' object has no attribute '__del__'" in &lt;bound method Sub.__del__ of &lt;__main__.Sub instance at 0x00F0B698&gt;&gt; ignored </code></pre> <p>It seems the definition of Base is removed before the <code>x.__del__()</code> is called.</p> <p>The Python Documentation on __del__() warns that the subclass needs to call the base-class to get a clean deletion, but here that appears to be impossible.</p> <p>Can you see where I made a bad step?</p>
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