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

[Nathaniel Smith]

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

-- 
Nathaniel J. Smith -- http://vorpus.org