What is the appropriate way to return a tuple containing a None value from a C extension to Python? I know that a single None value can be created using
result = Py_BuildValue("");
and that a tuple can be created using something like
result = Py_BuildValue("(ii)", 123, 456);
but I do not see a way to combine this to get, for example, a tuple containing a proper integer and a None value.
There's only ever one None object in Python, and you can always refer to it in C as Py_None.
The code...
result = Py_BuildValue("");
...just increments the refcount of Py_None and returns its pointer, so it's the same as...
Py_INCREF(Py_None);
result = Py_None;
If you just want to return None, then use...
Py_INCREF(Py_None);
return Py_None;
...or, since Python 2.4, you can do the same thing with a macro...
Py_RETURN_NONE;
If you return it in a tuple, like this...
return Py_BuildValue("(O)", Py_None);
...there's no need to increment the refcount - Py_BuildValue() does that for you.
But if you return it in a tuple like this...
PyObject* tuple = PyTuple_New(1);
Py_INCREF(Py_None);
PyTuple_SetItem(tuple, 0, Py_None);
return tuple;
...then you have to increment the refcount, because PyTuple_SetItem() doesn't do it for you.