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copied!<p>Alright, it's the usual "<code>super()</code> cannot be used with an old-style class".</p> <p>However, the important point is that <strong>the correct test</strong> for "is this a new-style <strong>instance</strong> (i.e. object)?" is</p> <pre><code>>>> class OldStyle: pass >>> instance = OldStyle() >>> issubclass(instance.__class__, object) False </code></pre> <p>and not (as in the question):</p> <pre><code>>>> isinstance(instance, object) True </code></pre> <p>For <strong>classes</strong>, the correct "is this a new-style class" test is:</p> <pre><code>>>> issubclass(OldStyle, object) # OldStyle is not a new-style class False >>> issubclass(int, object) # int is a new-style class True </code></pre> <p>The <strong>crucial point</strong> is that with old-style classes, the <em>class</em> of an instance and its <em>type</em> are distinct. Here, <code>OldStyle().__class__</code> is <code>OldStyle</code>, which does not inherit from <code>object</code>, while <code>type(OldStyle())</code> is the <code>instance</code> type, which <em>does</em> inherit from <code>object</code>. Basically, an old-style class just creates objects of type <code>instance</code> (whereas a new-style class creates objects whose type is the class itself). This is probably why the instance <code>OldStyle()</code> is an <code>object</code>: its <code>type()</code> inherits from <code>object</code> (the fact that its class does <em>not</em> inherit from <code>object</code> does not count: old-style classes merely construct new objects of type <code>instance</code>). Partial reference: <a href="https://stackoverflow.com/a/9699961/42973">https://stackoverflow.com/a/9699961/42973</a>.</p> <p>PS: The difference between a new-style class and an old-style one can also be seen with:</p> <pre><code>>>> type(OldStyle) # OldStyle creates objects but is not itself a type classobj >>> isinstance(OldStyle, type) False >>> type(int) # A new-style class is a type type </code></pre> <p>(old-style classes are <em>not</em> types, so they cannot be the type of their instances).</p>

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