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copied!<p>Your <code>__del__</code> method assumes that the class is still present by the time it is called.</p> <p>This assumption is incorrect. <code>Groupclass</code> has already been cleared when your Python program exits and is now set to <code>None</code>.</p> <p>Test if the global reference to the class still exists first:</p> <pre><code>def __del__(self): if Groupclass: Groupclass.count -= 1 </code></pre> <p>or use <code>type()</code> to get the local reference:</p> <pre><code>def __del__(self): type(self).count -= 1 </code></pre> <p>but do note that this means that the semantics for <code>count</code> change if <code>Groupclass</code> is subclassed (each subclass gets a <code>.count</code> attribute versus only <code>Groupclass</code> having a <code>.count</code> attribute).</p> <p>Quoting from the <code>__del__</code> hook documentation:</p> <blockquote> <p><strong>Warning</strong>: Due to the precarious circumstances under which <code>__del__()</code> methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to <code>sys.stderr</code> instead. Also, when <code>__del__()</code> is invoked in response to a module being deleted (e.g., when execution of the program is done), other globals referenced by the <code>__del__()</code> method may already have been deleted or in the process of being torn down (e.g. the import machinery shutting down). For this reason, <code>__del__()</code> methods should do the absolute minimum needed to maintain external invariants. Starting with version 1.5, Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the <code>__del__()</code> method is called.</p> </blockquote> <p>If you are using Python 3, two additional notes apply:</p> <ul> <li><p>CPython 3.3 automatically applies a <a href="https://docs.python.org/3/reference/datamodel.html#object.__hash__" rel="noreferrer">randomized hash salt</a> to the <code>str</code> keys used in a <code>globals</code> dictionary; this also affects the order in which globals are cleaned up, and it <em>could</em> be that you see the problem on only <em>some</em> of the runs.</p></li> <li><p>CPython 3.4 no longer sets globals to <code>None</code> (in most cases), as per <a href="https://docs.python.org/3/whatsnew/3.4.html#whatsnew-pep-442" rel="noreferrer">Safe Object Finalization</a>; see <a href="http://www.python.org/dev/peps/pep-0442/" rel="noreferrer">PEP 442</a>.</p></li> </ul>

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