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    <p>Python looks for special methods like <code>__getattr__</code> in an instance's bases <code>__dict__</code>s, not in the instance's <code>__dict__</code>.</p> <p><code>self.tool</code> is a class. <code>self.tool.test</code>, therefore, will call the <code>__getattr__</code> of <code>self.tool</code>'s class (which is <code>object</code>) -- this is not what we want to happen.</p> <p>Instead, make <code>self.tool</code> an instance, whose class has a <code>__getattr__</code>:</p> <pre><code>class Foo(object): def __init__(self): self.NameList=[] # add new class to ourself toolcls = type('tool', (object,), { '__getattr__' : self.__getattr__, } ) self.tool = toolcls() self.NameList.append( "test" ) # should be called by our newly added "tool" object but is only called sometimes... def __getattr__(self, attr): # print("__getattr__: (%s, %s)" % (self.__class__.__name__, attr) ) if( attr in self.NameList ): return( 99 ) raise AttributeError("--%r object has no attribute %r" % ( type(self).__name__, attr)) foo = Foo() print( "foo.tool.__getattr__=%d" % foo.tool.__getattr__("test") ) print( "foo.tool.test=%d" % foo.tool.test ) </code></pre> <p>yields</p> <pre><code>foo.tool.__getattr__=99 foo.tool.test=99 </code></pre> <p>Also, beware that the above code can lead to infinite recursion if an instance of <code>Foo</code> is made without <code>self.NameList</code> being defined. See <a href="http://nedbatchelder.com/blog/201010/surprising_getattr_recursion.html">Ned Batchelder's post on this suprising pitfall</a>.</p> <p>To protect against the possibility of infinite recursion here, use</p> <pre><code>def __getattr__(self, attr): # print("__getattr__: (%s, %s)" % (self.__class__.__name__, attr) ) if attr == 'NameList': raise AttributeError() if( attr in self.NameList ): return( 99 ) raise AttributeError("--%r object has no attribute %r" % ( type(self).__name__, attr)) </code></pre>
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    1. POpython - trying to override __getattr__ for dynamically added class but only sort of works
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    1. COThanks and thanks for the explanation. I need to read it a few more times to grok it. I also got it to work by defining a class outside Foo but the end result is the same as your - yours is much more succinct and compact and self-contained.
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    2. COOpps, hit return too early. Why does py use the base's __dict__[__getattr__] and not the instance's? And, related, why is your instantiation of toolcls that adds the __getattr__ functionally different than mine where I did the "setattr(self.tool...)
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    3. CO@staggart: I wish I could find a link to documentation which explains this, but unfortunately I haven't been able to find it. That special methods are looked for in the class rather than the instance is true for `new-style` classes, not classic classes. With classic classes, you could put an `__add__` method in an instance's `__dict__` to get special behavior for `x + y`, but not so with new-style classes. Now you must put `__add__` in the classes's `__dict__`.
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