[Python-Dev] compatibility for C-accelerated types

[Maciej Fijalkowski]

For what is worth, that level of differences already exists on pypy
and it's really hard to get the *exact* same semantics if things are
implemented in python vs C or the other way around.

Example list of differences (which I think OrderedDict already breaks
if moved to C):

* do methods like items call special methods like __getitem__ (I think
it's undecided anyway)

* what happens if you take a method and rebind it to another subclass,
does it automatically become a method (there are differences between
built in and pure python)

* atomicity of operations. Some operations used to be non-atomic in
Python will be atomic now.

I personally think those (and the __class__ issue) are unavoidable

On Mon, Oct 19, 2015 at 11:47 PM, Serhiy Storchaka <storchaka at gmail.com> wrote:
> On 20.10.15 00:00, Guido van Rossum wrote:
>>
>> Apart from Serhiy's detraction of the 3.5 bug report there wasn't any
>> discussion in this thread. I also don't really see any specific
>> questions, so maybe you don't have any. Are you just asking whether it's
>> okay to merge your code? Or are you asking for more code review?
>
>
> I think Eric ask whether it's okay to have some incompatibility between
> Python and C implementations.
>
> 1. Is it okay to have a difference in effect of __class__ assignment. Pure
> Python and extension classes have different restrictions. For example
> (tested example this time) following code works with Python implementation
> in 3.4, but fails with C implementation in 3.5:
>
> from collections import OrderedDict
> od = OrderedDict()
> class D(dict): pass
>
> od.__class__ = D
>
> 2. Is it okay to use obj.__class__ in Python implementation and type(obj) in
> C implementation for the sake of code simplification? Can we ignore subtle
> differences?
>
> 3. In general, is it okay to have some incompatibility between Python and C
> implementations for the sake of code simplification, and where the border
> line lies?
>
>
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