[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c

[Guido van Rossum]

On 3/29/06, Armin Rigo <arigo at tunes.org> wrote:
> Hi all,
>
> On Tue, Mar 28, 2006 at 09:50:49AM -0800, Guido van Rossum wrote:
> > C extensions are my main worry -- OTOH if += for a list can already
> > passes arbitrary types as the argument, then any extension types
> > should be ready to expect this, right?
>
> Yes, I don't think C extensions are going to segfault.  My worry is
> about returning a different result than before.  Actually I believe the
> problem is not specific to C extensions.  Here are some typical behavior
> changes that could be observed in pure Python already:
>
> class X(object):
>     def __radd__(self, other):
>         return 42
>     def __iter__(self):
>         return iter("xyz")
>     def __rmul__(self, other):
>         return 42
>     def __index__(self):
>         return 5
>
> t = []
> t += X()
> print t            # current: 42   new: ['x', 'y', 'z']
> print [1] * X()    # current: 42   new: [1, 1, 1, 1, 1]
>
> Another visible difference is that the __add__/__iadd__/__mul__/__imul__
> methods of lists, tuples, strings etc., will return NotImplemented
> instead of raising the TypeError themselves.  This could impact user
> subclasses of these built-in types trying to override and call the super
> methods, not expecting a NotImplemented result (a reason why
> NotImplemented should have been an exception in the first place IMHO).
>
> (A different bug I found is that [1].__mul__(X()) with an __index__able
> class X currently raises TypeError, even though [1]*X() works just
> fine.)
>
> This seems to be it on the incompatibility side.  I'd vote for the
> change anyway because the language specs -- as well as PyPy and probably
> all Python implementations other than CPython -- don't have this
> double-slot inconsistency and already show the "new" behavior.  For what
> it's worth no CPython test breaks on top of PyPy because of this.
>
> If this change is accepted I'll submit a patch for 2.5.

I trust you in these matters. Go for it.

--
--Guido van Rossum (home page: http://www.python.org/~guido/)