Create an object using Python's C API

Question

Say I have my object layout defined as:

typedef struct {
    PyObject_HEAD
    // Other stuff...
} pyfoo;

...and my type definition:

static PyTypeObject pyfoo_T = {
    PyObject_HEAD_INIT(NULL)
    // ...

    pyfoo_new,
};

How do I create a new instance of pyfoo somewhere within my C extension?

Solution

Call PyObject_New(), followed by PyObject_Init().

EDIT: The best way is to call the class object, just like in Python itself:

/* Pass two arguments, a string and an int. */
PyObject *argList = Py_BuildValue("si", "hello", 42);

/* Call the class object. */
PyObject *obj = PyObject_CallObject((PyObject *) &pyfoo_T, argList);

/* Release the argument list. */
Py_DECREF(argList);

Comments

[Frédéric Hamidi]

I agree the docs are a little terse in that case. I updated my answer with the required call to PyObject_Init().

[detly]

Wait, PyObject_Init() doesn't take any arguments, so how do you pass in the requisite initialisation arguments?

[Frédéric Hamidi]

@detly, you need to call the class object. See my updated answer.

[detly]

Aah, that makes more sense. Is it normal to get a compiler error ("assignment from incompatible pointer type")?

[Frédéric Hamidi]

@detly, since PyTypeObject "derives" from PyObject internally, and an explicit cast is used, there should be no warning. What's your compiler?

[detly]

GCC 4.4.5... and now that I've had a chance to check, it's only a warning, not an error.

[jkp]

If returning this instance to a Python function, I'm assuming I don't need to increment the retain count to be safe?

[Frédéric Hamidi]

@jkp, if I'm not mistaken, the class object should already have INCREFed the object it returned, because it just created it and intends to pass ownership to you in the first place. If you also intend to pass ownership of the new object to your caller, you should not DECREF it either. See edcjones.tripod.com/refcount.html for the gory details.

[Wiktor Tomczak]

one-liner: PyObject_CallFunction((PyObject *)&pyfoo_T, "si", "hello", 42); combines PyObject_CallObject + Py_BuildValue

[dbn]

If there is only one argument, the Py_BuildValue call needs parenthesis in the format string.

[cheshirekow]

Can you expand on what you mean by "best"? Is there any reason to prefer CallObject over PyObject_New and PyObject_Init which are crossed out in the answer?

[theonlygusti]

Is this answer out of date? I can't find it in documentation