Re: [Python-Dev] copy confusion

[Alex Martelli]

On 2005 Jan 11, at 23:58, Guido van Rossum wrote:
   ...
    cls = type(x)
    copier = _copy_dispatch.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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