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    <p>In Python 2.6, a class decorator that's also a custom descriptor matches the specs you give:</p> <pre><code>class InnerClassDescriptor(object): def __init__(self, cls): self.cls = cls def __get__(self, instance, outerclass): class Wrapper(self.cls): outer = instance Wrapper.__name__ = self.cls.__name__ return Wrapper class Outer(object): @InnerClassDescriptor class Inner(object): def __init__(self): print self.outer o = Outer() i = o.Inner() print 'Outer is a', type(Outer) print 'Inner is a', type(o.Inner) </code></pre> <p>This emits:</p> <pre><code>&lt;__main__.Outer object at 0x82f90&gt; Outer is a &lt;type 'type'&gt; Inner is a &lt;type 'type'&gt; </code></pre> <p>just to confirm that </p> <blockquote> <p>o.Inner [[is]] a class object, not something weird like a closure</p> </blockquote> <p>as per your peculiar specs. Of course it needs to be a <em>different</em> class each time for reentrancy -- even in a single-threaded world, the following:</p> <pre><code>o1 = Outer() o2 = Outer() i1 = o1.Inner i2 = o2.Inner print i1(), i2(), i1(), i2() </code></pre> <p>should work cleanly, and stashing o1 vs o2 anywhere else than in the classes returned by <code>o1.Inner</code> vs <code>o2.Inner</code> (e.g., in TLS) would mean horrible results for this use.</p> <p>But then you <em>didn't</em> specify "<code>o.Inner</code> has to be <strong>exactly</strong> the same class object for <em>every</em> possible <code>o</code> that's an instance of <code>Outer</code>", so this code fully meets the specs you <em>did</em> give;-).</p>
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    1. POInner Classes: How can I get the outer-class object at construction time?
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    1. USAlex Martelli
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    1. COMarvelous! Now I must meditate whether I can live with o1.Inner != o2.Inner != Outer.Inner. Of course, bound methods != unbound methods, so it only makes sense that bound child classes != unbound child classes.
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      1. USLarry Hastings
    2. COAs written your approach returns a new class every time o.Inner is referenced, so "assert o.Inner == o.Inner" would fail. Caching the Wrapper class in the instance, like setattr(instance, "__cache__" + self.cls.__name__), then preferring the cached version where available, solves that at least.
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    3. COAnother tweak: if "instance" is None, then the __get__ should return self.cls. Without this isinstance(i, Outer.Inner) returns False.
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