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

Alex Martelli aleax at aleax.it
Wed Jan 12 00:09:17 CET 2005

On 2005 Jan 11, at 23:58, Guido van Rossum wrote: ...

cls = type(x) copier = copydispatch.get(cls) if copier: return copier(x) ... is this a bug, or a feature of the revised copy/pickle design? [Phillip] Looks like a bug to me; it breaks the behavior of classic classes, since type(classicInstance) returns InstanceType. I'm not so sure. I can't seem to break this for classic classes.

You can't, _copy_dispatch deals with those.

The only thing this intends to break, and then only for new-style classes, is the ability to have copy be an instance variable (whose value should be a callable without arguments) -- it must be a method on the class. This is the same thing that I've done for all built-in operations (add, getitem etc.).

And a wonderful idea it is.

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.) Sorry, my head just exploded. :-( I think I did this change (for all slots) to make the operations more efficient by avoiding dict lookups. It does have the desirable property of not confusing a class's attributes with its metaclass's attributes, but only as long as you use the operation's native syntax (e.g. x[y]) rather than the nominally "equivalent" method call (e.g. x.getitem(y)).

Unfortunately, we do have a problem with the code in copy.py:

class MetaCopyableClass(type): def copy(cls): """ code to copy CLASSES of this metaclass """ # etc, etc, snipped

class CopyableClass: metaclass = MetaCopyableClass # rest of class snipped

x = CopyableClass()

import copy y = copy.copy(x)

kallisti:/tmp alex$ python x.py Traceback (most recent call last): File "x.py", line 14, in ? y = copy.copy(x) File "/usr/local/lib/python2.4/copy.py", line 79, in copy return copier(x) TypeError: copy() takes exactly 1 argument (2 given) kallisti:/tmp alex$

See? copy.copy(x) ends up using MetaCopyableClass.copy -- because of a getattr on CopyableClass for 'copy', which gets the BOUND-METHOD defined in the metaclass, with im_self being CopyableClass.

I had exactly the same metabug in the pep 246 reference implementation, Armin Rigo showed how to fix it in his only recent post.

Alex

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