[Python-Dev] Calling Methods from Pythons C API with Keywords (original) (raw)

Hrvoje Nikšić hrvoje.niksic at avl.com
Wed Jun 20 09:34:49 CEST 2007

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 PyListSort because that dosnt support keywords like...

ls.sort(key=lambda a: a.foo)) And the Problem with PyObjectCallMethod 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.

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