On 2005 Jan 16, at 11:17, Raymond Hettinger wrote:
[Alex]So, as per discussion here, I have prepared a patch (to themaintenancebranch of 2.3, to start with) which adds unit tests to highlight these issues, and fixes them in copy.py. This patch should go in the maintenance of 2.3 and 2.4, but in 2.5 a different approach based on new special descriptors for special methods is envisaged (though keeping compatibility with classic extension types may also require some patching to copy.py along the lines of my patch).
For Py2.5, do you have in mind changing something other than copy.py? If so, please outline your plan. I hope your not planning on wrapping all special method access as descriptor look-ups -- that would be a somewhat radical change.
The overall plan does appear to be exactly the "somewhat radical change" which you hope is not being proposed, except it's not my plan -- it's Guido's. Quoting his first relevant post on the subject:
'''
From: [EMAIL PROTECTED]
Subject: Re: getting special from type, not instance (was Re: [Python-Dev] copy confusion)
Date: 2005 January 12 18:59:13 CET
...
I wonder if the following solution wouldn't be more useful (since less
code will have to be changed).
The descriptor for __getattr__ and other special attributes could claim to be a "data descriptor" which means that it gets first pick *even if there's also a matching entry in the instance __dict__*. Quick illustrative example:
class C(object):
foo = property(lambda self: 42) # a property is always a "data descriptor"
42a = C() a.foo
42a.__dict__["foo"] = "hello" a.foo
Normal methods are not data descriptors, so they can be overridden by something in __dict__; but it makes some sense that for methods implementing special operations like __getitem__ or __copy__, where the instance __dict__ is already skipped when the operation is invoked using its special syntax, it should also be skipped by explicit attribute access (whether getattr(x, "__getitem__") or x.__getitem__ -- these are entirely equivalent).
We would need to introduce a new decorator so that classes overriding these methods can also make those methods "data descriptors", and so that users can define their own methods with this special behavior (this would be needed for __copy__, probably).
I don't think this will cause any backwards compatibility problems -- since putting a __getitem__ in an instance __dict__ doesn't override the x[y] syntax, it's unlikely that anybody would be using this. "Ordinary" methods will still be overridable.
PS. The term "data descriptor" now feels odd, perhaps we can say "hard descriptors" instead. Hard descriptors have a __set__ method in addition to a __get__ method (though the __set__ method may always raise an exception, to implement a read-only attribute). '''
All following discussion was, I believe, in the same thread, mostly among Guido, Phillip and Armin. I'm focusing on getting copy.py fixed in 2.3 and 2.4, w/o any plan yet to implement Guido's idea. If you dislike Guido's idea (which Phillip, Armin and I all liked, in different degrees), it might be best for you to read that other thread and explain the issues there, I think.
Alex
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