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

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Wed May 16 22:16:26 CEST 2012

 On Wed, 16 May 2012 20:49:18 +0100, mark florisson 
 <markflorisson88 at gmail.com> wrote:
> On 16 May 2012 20:15, 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.
>> Stefan
>> _______________________________________________
>> cython-devel mailing list
>> cython-devel at python.org
>> http://mail.python.org/mailman/listinfo/cython-devel
> This works really well for local variables, but for globals, def
> methods or callbacks as attributes, this won't work so well, as they
> may be rebound at any time outside of the module scope. I think in

 +1. The python-dev discussion is pretty focused on the world of a 
 manually written C extension. But code generation is an entirely 
 different matter. Python puts in place pretty efficient boundaries 
 against full-program static analysis, so there's really not much we can 

 Here's some of my actual code I have for wrapping a C++ library:

 cdef class CallbackEventReceiver(BasicEventReceiver):
     cdef object callback

     def __init__(self, callback):
         self.callback = callback

     cdef dispatch_event(self, ...):

 The idea is that you can subclass BasicEventReceiver in Cython for 
 speed, but if you want to use a Python callable then this converter is 

 This code is very performance critical. And, the *loop* in question 
 sits deep inside a C++ library.

 Good luck pre-acquiring the function pointer of self.callback in any 
 useful way. Even if it is not exported by the class, that could be 
 overridden by a subclass. I stress the fact that this is real world code 
 by yours truly (unfortunately not open source, it wraps a closed source 

 Yes, you can tell users to be mindful of this and make as much as 
 possible local variables, introduce final modifiers and __nomonkey__ and 
 whatnot, but that's a large price to pay to avoid hacking tp_flags.


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