[Python-Dev] Python-versus-CPython question for __mul__ dispatch

Nick Coghlan ncoghlan at gmail.com
Sat May 16 10:31:34 CEST 2015

On 16 May 2015 at 07:35, Nathaniel Smith <njs at pobox.com> wrote:
> On Thu, May 14, 2015 at 11:53 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> On Thu, May 14, 2015 at 9:29 PM, Guido van Rossum <guido at python.org> wrote:
>>> I expect you can make something that behaves like list by defining __mul__
>>> and __rmul__ and returning NotImplemented.
>> Hmm, it's fairly tricky, and part of the trick is that you can never
>> return NotImplemented (because you have to pretty much take over and
>> entirely replace the normal dispatch rules inside __mul__ and
>> __rmul__), but see attached for something I think should work.
>> So I guess this is just how Python's list, tuple, etc. work, and PyPy
>> and friends need to match...
> For the record, it looks like PyPy does already have a hack to
> implement this -- they do it by having a hidden flag on the built-in
> sequence types which the implementations of '*' and '+' check for, and
> if it's found it triggers a different rule for dispatching to the
> __op__ methods:
>     https://bitbucket.org/pypy/pypy/src/a1a494787f4112e42f50c6583e0fea18db3fb4fa/pypy/objspace/descroperation.py?at=default#cl-692

Oh, that's rather annoying that the PyPy team implemented bug-for-bug
compatibility there, and didn't follow up on the operand precedence
bug report to say that they had done so. We also hadn't previously
been made aware that NumPy is relying on this operand precedence bug
to implement publicly documented API behaviour, so fixing it *would*
break end user code :(

I guess that means someone in the numeric community will need to write
a PEP to make this "try the other operand first" "feature" part of the
language specification, so that other interpreters can implement it up
front, rather than all having to come up with their own independent
custom hacks just to make NumPy work.


P.S. It would also be nice if someone could take on the PEP for a
Python level buffer API for 3.6: http://bugs.python.org/issue13797

Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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