Python C API and references

[Gabriel Genellina]

En Thu, 12 Nov 2009 06:23:54 -0300, lallous <lallous at lgwm.org> escribió:

> Everytime I use PyObject_SetAttrString(obj, attr_name, py_val) and I  
> don't need the reference to py_val I should decrement the reference  
> after this call?

If you own a reference to py_val, and you don't need it anymore, you must
decrement it. It doesn't matter if you call PyObject_SetAttrString or
whatever, except when the called function says it "steals" a reference.

> So for example:
>
> PyObject *py_val = PyInt_FromLong(5)
> PyObject_SetAttrString(py_obj, "val", py_val);
> Py_DECREF(py_val)
>
> Right?

Yes, because PyInt_FromLong returns a new reference, and you own it.

> If so, take sysmodule.c:
>
> 	if (PyObject_SetAttrString(builtins, "_", Py_None) != 0)
> 		return NULL;
>
> Shouldn't they also call Py_DECREF(Py_None) ?

No, because the reference count of Py_None was not incremented previously;
the code doesn't own a reference to Py_None at that time. It's not the
same as the example above.

> Same logic applies to PyDict_SetItemString() and the reference should be  
> decrement after setting the item (ofcourse if the value is not needed).

Yes, same as your first example.

-- 
Gabriel Genellina