getting special from type, not instance (was Re: [Python-Dev] copy confusion) (original) (raw)

Alex Martelli aleax at aleax.it
Thu Jan 13 11:47:59 CET 2005

On 2005 Jan 12, at 18:59, Guido van Rossum wrote: ...

[Alex]

Armin's fix was to change: ... [And then proceeds to propose a new API to improve the situation] 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. ... 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).

A very nice idea for how to proceed in the future, and I think definitely the right solution for Python 2.5. But maybe we need to think about a bugfix for 2.3/2.4, too.

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.

...in new-style classes, yes. And classic types and old-style classes would keep behaving the old-way (with per-instance override) so the bug that bit the effbot would disappear... in Python 2.5. But the bug is there in 2.3 and 2.4, and it seems to me we should still find a fix that is applicable there, even though the fix won't need to get into the 2.5 head, just the 2.3 and 2.4 bugfix branches.

"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).

Good terminology point, and indeed explaining the data'' in "data descriptor" has always been a problem. "Hard" or "Get-Set" descriptors or other terminology yet will make explanation easier; to pick the best terminology we should also think of the antonym, since non-data'' won't apply any more ("soft descriptors", "get-only descriptors", ...). strong'' descriptors having a __set__, and weak'' ones not having it, is another possibility.

But back to the bugfix for copy.py (and I believe at least pprint.py too, though of course that's more marginal than copy.py!) in 2.3 and 2.4: am I correct that this new descriptor idea is too big/invasive for ths bugfix, and thus we should still be considering localized changes (to copy.py and pprint.py) via a function copy._get_special (or whatever) in 2.3.5 and 2.4.1?

This small, local, minimally invasive change to copy.py would go well with the other one we need (as per my latest post with Subject: Re: [Python-Dev] Re: copy confusion Date: 2005 January 12 10:52:10 CET ) -- having the check for issubclass(cls, type) in copy.copy() just as we have it in copy.deepcopy() and for the same reason (which is a bit wider than the comment in copy.deepcopy about old versions of Boost might suggest).

Alex

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