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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?