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The following code:
--------------------------------
class X(list):
 def __contains__(self, key):
 print('X contains:', key)
class Y():
 def __init__(self, x):
 self.x = x
 def __getattr__(self, key):
 return getattr(self.x, key)
 def __iter__(self):
 print('Y iter')
 return iter([1,2])
x = X()
y = Y(x)
print('res:', 1 in y)
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prints True. It has been explained to me that this is because of:
http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes
However, there is no way in the world that I would guess the behavior above from the documentation provided (and I find it surprising...I expected x's __contains__ to get called because Y (a class, not an instance) doesn't have a __contains__ method).
Can anyone explain it more clearly and update the documentation?

In 2.7 I get the following (if Y does not inherit from object):
>>> print('res:', 1 in y)
('X contains:', 1)
('res:', False)
This makes sense with old style classes. With Y(object):
>>> print('res:', 1 in y)
Y iter
('res:', True)

Not sure I understand your issue here. How should "1 in y" get at X.__contains__ given the special method lookup rules? The __getattr__ is not called since y.__contains__ isn't looked up.

Well, then I suppose my question is why isn't __contains__ looked up? Other special methods that don't exist on Y do cause __getattr__ to be called. Why is __contains__ special? The docs for __getattr__ don't hint at this possibility either.
I think the people that understand this must have a mental model of how the interpreter looks up methods that has some pieces that are missing from mine. I guess I'd like those bits to be made explicit in the special method lookup docs, or if they are already documented somewhere, for there to be a pointer to them in the special method lookup docs.

Ah, that's what my problem is. My test example was poorly conceived (I used __del__!) so I *thought* the other special methods were triggering getattr.
I'd have figured it out if I hadn't screwed up my test :(