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Calling an unbound method in C using the Public API


Thanks Serhiy. This does come with a performance penalty (in particular for function taking keyword arguments) but this is to be expected. Also the PyList_Insert solution does not sadly work for all the methods that I need to wrap.

Best

Matthieu

> On Aug 30, 2018, at 11:09 AM, Serhiy Storchaka <storchaka at gmail.com> wrote:
> 
> 29.08.18 17:33, Matthieu Dartiailh ????:
>> I tried to look at the public C API for a way to call an unbound method with a minimal cost (in term of speed and memory). It seems to me, but please correct me if I am wrong, that one cannot call a MethodDef using only the public API. To use the public C API, one has to use PyCFunction_Call (or a variant) that expect a PyCFunctionObject which binds a the MethodDef to an instance. In my case, to avoid creating a temporary PyCFunctionObject each time I call list.insert on my custom subclass instance, I have to store that PyCFunctionObject for each instance. But this means storing  7 PyCFunctionObject per instance (one for each method of list I need to wrap). So I can either use the public API and increase the memory footprint or slow down the code by creating PyCFunctionObject for each call
>>  , or use large amount of the private API.
>> Am I missing something ?
> 
> In general, you need to cache the unbound method object, and call it with self as the first argument.
> 
> list_insert = PyObject_GetAttrString((PyObject *)&PyList_Type, "insert");
> ...
> res = PyObject_CallFunctionObjArgs(list_insert, self, index, value, NULL);
> 
> But in the particular case of the insert method it will be easier and more efficient to use PyList_Insert().
> 
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