Issue 11988: special method lookup docs don't address some important details (original) (raw)

Created on 2011-05-03 20:47 by r.david.murray, last changed 2022-04-11 14:57 by admin. This issue is now closed.

Messages (5)
msg135068 - (view)	Author: R. David Murray (r.david.murray) *	Date: 2011-05-03 20:47
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?
msg135074 - (view)	Author: Eric Snow (eric.snow) *	Date: 2011-05-03 21:18
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)
msg135075 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2011-05-03 21:19
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.
msg135080 - (view)	Author: R. David Murray (r.david.murray) *	Date: 2011-05-03 21:43
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.
msg135082 - (view)	Author: R. David Murray (r.david.murray) *	Date: 2011-05-03 22:01
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 :(