[Cython] [Python-Dev] C-level duck typing

Robert Bradshaw robertwb at gmail.com
Wed May 16 22:25:44 CEST 2012

On Wed, May 16, 2012 at 12:15 PM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> "Martin v. Löwis", 16.05.2012 20:33:
>>> Does this use case make sense to everyone?
>>> The reason why we are discussing this on python-dev is that we are looking
>>> for a general way to expose these C level signatures within the Python
>>> ecosystem. And Dag's idea was to expose them as part of the type object,
>>> basically as an addition to the current Python level tp_call() slot.
>> The use case makes sense, yet there is also a long-standing solution
>> already to expose APIs and function pointers: the capsule objects.
>> If you want to avoid dictionary lookups on the server side, implement
>> tp_getattro, comparing addresses of interned strings.
> I think Martin has a point there. Why not just use a custom attribute on
> callables that hold a PyCapsule? Whenever we see inside of a Cython
> implemented function that an object variable that was retrieved from the
> outside, either as a function argument or as the result of a function call,
> is being called, we try to unpack a C function pointer from it on all
> assignments to the variable. If that works, we can scan for a suitable
> signature (either right away or lazily on first access) and cache that. On
> each subsequent call through that variable, the cached C function will be used.
> That means we'd replace Python variables that are being called by multiple
> local variables, one that holds the object and one for each C function with
> a different signature that it is being called with. We set the C function
> variables to NULL when the Python function variable is being assigned to.
> When the C function variable is NULL on call, we scan for a matching
> signature and assign it to the variable.  When no matching signature can be
> found, we set it to (void*)-1.
> Additionally, we allow explicit user casts of Python objects to C function
> types, which would then try to unpack the C function, raising a TypeError
> on mismatch.
> Assignments to callable variables can be expected to occur much less
> frequently than calls to them, so this will give us a good trade-off in
> most cases. I don't see why this kind of caching would be any slower inside
> of loops than what we were discussing so far.

I like the idea, but that only helps if you're doing multiple calls
(e.g. in a loop). Definitely worth implementing in my mind, but
orthogonal to making the lookup itself as fast as possible.

- Robert

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