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Sat Oct 18 18:44:46 EDT 2003 · https://mail.python.org/pipermail/python-dev/2003-October/038999.html

[Alex Martelli]
 >[Kevin Jacobs]
 >>[Guido van Rossum]
>>>I don't like the trick of avoiding the copy if the refcount is one;
>>>AFAIK it can't be done in Jython.
>>
>>There is also a problem with the strategy if if gets called by a
>>C only extension.  It is perfectly feasible for a C extension to
>>hold the reference to an object, call the copying sort (directly
>>or indirectly), and then be very surprised that the copy did not
>>take place.
> 
> Alas, I fear you're right.  Darn--so much for a possible little but
> cheap optimization (which might have been neat in PySequence_List
> even if copysort never happens and the optimization is only for
> CPython -- I don't see why an optimization being impossible in
> Jython should stop CPython from making it, as long as semantics
> remain compatible).  It's certainly possible for C code to call
> PySequence_List or whatever while holding the only reference,
> and count on the returned and argument objects being distinct:-(.

I'm afraid I'm confused here.  If the C code is like:

    ... at this point PTR refers to an object with refcount 1
    OTHER = <invoke sortcopy>(PTR)
    ... Then it might be that PTR == OTHER ...

What possible harm could come?  The C code should expect a
sortcopy method to recycle the object referred to by PTR
if "the Trick" isn't used.  I am a trifle confused about
what harm occurs.  Seems to me that list(v) (and alist[:])
could quite happily implement "the Trick" without fear of
failure.

-Scott David Daniels
Scott.Danies at Acm.Org

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[Python-Dev] Re: Python-Dev Digest, Vol 3, Issue 59