Author r.david.murray
Recipients docs@python, r.david.murray
Date 2011-05-03.20:47:09
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Message-id <1304455631.01.0.264040156642.issue11988@psf.upfronthosting.co.za>
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Content
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)
-----------------------------

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?
History
Date User Action Args
2011-05-03 20:47:11r.david.murraysetrecipients: + r.david.murray, docs@python
2011-05-03 20:47:11r.david.murraysetmessageid: <1304455631.01.0.264040156642.issue11988@psf.upfronthosting.co.za>
2011-05-03 20:47:09r.david.murraylinkissue11988 messages
2011-05-03 20:47:09r.david.murraycreate