cbarton at metavr
Jun 20, 2007, 3:17 AM
Post #3 of 10
(424 views)
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Re: Calling Methods from Pythons C API with Keywords
[In reply to]
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Hrvoje Nikšić wrote:
> On Wed, 2007-06-20 at 00:21 +1000, Campbell Barton wrote:
>> I want to add my own call's before and after PyLists standard functions
>> but have a proplem with functons that use keywords and have no API
>> equivalent.
>> For example, I cant use the API's PyList_Sort because that dosnt support
>> keywords like...
>>
>> ls.sort(key=lambda a: a.foo))
>>
>> And the Problem with PyObject_CallMethod is that it dosnt accept keywords.
>
> Note that you can always simply call PyObject_Call on the bound method
> object retrieved using PyObject_GetAttrString. The hardest part is
> usually constructing the keywords dictionary, a job best left to
> Py_BuildValue and friends. When I need that kind of thing in more than
> one place, I end up with a utility function like this one:
>
> /* Equivalent to PyObject_CallMethod but accepts keyword args. The
> format... arguments should produce a dictionary that will be passed
> as keyword arguments to obj.method.
>
> Usage example:
> PyObject *res = call_method(lst, "sort", "{s:O}", "key", keyfun));
> */
>
> PyObject *
> call_method(PyObject *obj, const char *methname, char *format, ...)
> {
> va_list va;
> PyObject *meth = NULL, *args = NULL, *kwds = NULL, *ret = NULL;
>
> args = PyTuple_New(0);
> if (!args)
> goto out;
> meth = PyObject_GetAttrString(obj, methname);
> if (!meth)
> goto out;
>
> va_start(va, format);
> kwds = Py_VaBuildValue(format, va);
> va_end(va);
> if (!kwds)
> goto out;
>
> ret = PyObject_Call(meth, args, kwds);
> out:
> Py_XDECREF(meth);
> Py_XDECREF(args);
> Py_XDECREF(kwds);
> return ret;
> }
>
> It would be nice for the Python C API to support a more convenient way
> of calling objects and methods with keyword arguments.
Thanks for the hint, I ended up using PyObject_Call.
This seems to work, EXPP_PyTuple_New_Prepend - is a utility function
that returns a new tuple with self at the start (needed so args starts
with self)
I dont think I can use PyObject_GetAttrString because the subtype would
return a reference to this function - rather then the lists original
function, Id need an instance of a list and dont have one at that point.
______________________
static PyObject * MaterialList_sort(BPy_MaterialList *self, PyObject
*args, PyObject *keywds )
{
PyObject *ret;
PyObject *newargs = EXPP_PyTuple_New_Prepend(args, (PyObject *)self);
sync_list_from_materials__internal(self); # makes sure the list matches
blenders materials
ret = PyObject_Call(PyDict_GetItemString(PyList_Type.tp_dict, "sort"),
newargs, keywds);
Py_DECREF(newargs);
if (ret)
sync_materials_from_list__internal(self); # makes blenders materials
match the lists
return ret;
}
_____________________
Later on Ill probably avoid using PyDict_GetItemString on
PyList_Type.tp_dict all the time since the methods for lists does not
change during python running. - Can probably be assigned to a constant.
--
Campbell J Barton (ideasman42)
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