[Python-Dev] PEP 399: Pure Python/C Accelerator Module Compatibiilty Requirements

[Stefan Behnel]

Raymond Hettinger, 18.04.2011 19:26:
> On Apr 18, 2011, at 10:11 AM, Maciej Fijalkowski wrote:
>>
>> * we usually target CPython version that's already frozen, which is
>> pretty inconvinient to post this changes back. Example would be a
>> socket module where it has changed enough in 3.x that 2.7 changes make
>> no sense.
>
> Do you have any thoughts on the problem with the concrete C API
> not working well with subclasses of builtin types?
>
> I'm thinking that the PEP should specifically ban the practice
> of using the concrete api unless it is known for sure that
> an object is an exact type match.

Absolutely.


> It is okay to write PyList_New() followed by PyList_SetItem()
> but not okay to use PyList_SetItem() on a user supplied
> argument that is known to be a subclass of list.  A fast path
> can be provided for an exact path, but there would also need
> to a be a slower path that either converts the object to
> an exact list or that uses PyObject_SetItem().

For what it's worth, Cython generates code that contains optimistic 
optimisations for common cases, such as iteration, x.append() calls, etc. 
When it finds such a pattern, it generates separate code paths for the most 
likely (builtin type) case and a slower fallback for the more unlikely case 
of a user provided type. So you get both speed and compatibility for free, 
just by writing idiomatic code like "for item in some_iterable".

Stefan