[Python-Dev] copy confusion (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Jan 11 23:39:34 CET 2005

At 11:20 PM 1/11/05 +0100, Fredrik Lundh wrote:

I recently discovered that this feature has disappeared in 2.3 and 2.4. in- stead of looking for an instance method, the code now looks at the object's type:

... cls = type(x) copier = copydispatch.get(cls) if copier: return copier(x) copier = getattr(cls, "copy", None) if copier: return copier(x) ... (copy.deepcopy still seems to be able to use deepcopy hooks, though) is this a bug, or a feature of the revised copy/pickle design?

Looks like a bug to me; it breaks the behavior of classic classes, since type(classicInstance) returns InstanceType.

However, it also looks like it might have been introduced to fix the possibility that calling 'copy' on a new-style class with a custom metaclass would result in ending up with an unbound method. (Similar to the "metaconfusion" issue being recently discussed for PEP 246.)

ISTM the way to fix both issues is to switch to using x.class in preference to type(x) to retrieve the copy method from, although this still allows for metaconfusion at higher metaclass levels.

Maybe we need a getclassattr to deal with this issue, since I gather from Armin's post that this problem has popped up other places besides here and PEP 246.

(Follow-up: Guido's checkin comment for the change suggests it was actually done as a performance enhancement while adding a related feature (copy_reg integration), rather than as a fix for possible metaconfusion, even though it also has that effect.)

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