Python C-API Making len(...) work with extension class
When creating a class in Python, I can simply make a def __len__(self):
method to make the len(InstanceOfMyClass)
work, however I can't find out how to do this with an extension class via the C-API.
I tried adding a __len__
method, but that appears to not work
{"__len__",(PyCFunction)&TestClass_GetLen,METH_NOARGS,""},
Python test code:
def test(my_instance):
x = len(my_instance)#exception
return x
TypeError: object of type 'test_module.test_class' has no len()
Code for TestClass
struct TestClass;
static int TestClass_Init(TestClass *self, PyObject *args, PyObject* kwds);
static void TestClass_Dealloc(TestClass *self);
static PyObject* TestClass_GetLen(TestClass *self);
struct TestClass
{
PyObject_HEAD;
};
static PyMethodDef TestClass_methods[] =
{
{"__len__",(PyCFunction)&TestClass_GetLen,METH_O,""},
{NULL}
};
static PyTypeObject TestClass_type = {PyObject_HEAD_INIT(NULL)};
bool InitTestClass(PyObject *module)
{
TestClass_type.tp_basicsize = sizeof(TestClass);
TestClass_type.tp_name = PY_MODULE_NAME".TestClass";
TestClass_type.tp_doc = "";
TestClass_type.tp_flags = Py_TPFLAGS_DEFAULT;
TestClass_type.tp_methods = TestClass_methods;
TestClass_type.tp_new = PyType_GenericNew;
TestClass_type.tp_init = (initproc)TestClass_Init;
TestClass_type.tp_dealloc = (destructor)TestClass_Dealloc;
if(PyType_Ready(TestClass_type) < 0) return false;
Py_INCREF(TestClass_type);
PyModule_AddObject(module, "TestClass", (PyObject*)&TestClass_type);
return true;
};
void TestClass_Dealloc(TestClass *self)
{
Py_TYPE(self)->tp_free((PyObject*)self);
}
int TestClass_Init(TestClass *self, P开发者_StackOverflow社区yObject *args, PyObject* kwds)
{
return 0;
}
PyObject* TestClass_GetLen(TestClass *self)
{
return PyLong_FromLong(55);
}
As Igniacio says, the correct way is to fill the tp_as_sequence
member of your typeobject. Here is a minimal example:
#include <Python.h>
/**
* C structure and methods definitions
*/
typedef struct {
PyObject_HEAD;
} TestObject;
static Py_ssize_t
TestClass_len(TestObject* self)
{
return 55;
}
/**
* Python definitions
*/
static PySequenceMethods TestClass_sequence_methods = {
TestClass_len, /* sq_length */
};
static PyTypeObject TestClass = {
PyObject_HEAD_INIT(NULL)
0, /*ob_size*/
"testmodule.TestClass", /*tp_name*/
sizeof(TestObject), /*tp_basicsize*/
};
/**
* Module entry point
*/
#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
inittestmodule(void)
{
PyObject* m;
TestClass.tp_new = PyType_GenericNew;
TestClass.tp_as_sequence = &TestClass_sequence_methods;
TestClass.tp_flags = Py_TPFLAGS_DEFAULT;
if (PyType_Ready(&TestClass) < 0)
return;
m = Py_InitModule3("testmodule", NULL, "");
Py_INCREF(&TestClass);
PyModule_AddObject(m, "TestClass", (PyObject *)&TestClass);
}
Another (more complicated and slower) way is to add a __len__
method to your class dynamically in the module's init function with PyCFunction_New
, PyMethod_New
and PyObject_SetAttr
. This would look more like the code you pasted, except that you defined __len__
in the C methods table, which doesn't work.
The tp_as_sequence
member of the typeobject structure should be filled in with something that has its sq_length
member populated.
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