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

Armin Rigo arigo at tunes.org
Tue May 19 16:00:54 CEST 2015 
Hi Nick,

On 16 May 2015 at 10:31, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 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.

It's sadly not the only place, by far, where a behavior of CPython
could be considered an implementation detail, but people rely on it
and so we need to write a workaround.  We don't report all of them,
particularly not the ones that are clearly of the kind "won't be
changed in CPython 2.7".  Maybe we should?

Another example where this same bug occurs is:

    class T(tuple):
       def __radd__(self, other):
          return 42

    lst = [ ]
    lst += T()

which calls T.__radd__ in contradiction to all the general rules.
(Yes, if you print(lst) afterwards, you get 42.  And oops, trying this
out on PyPy does not give 42; only "lst + T()" does.  Probably another
corner case to fix...)


A bientôt,

Armin.
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