If you want a singleton in a C extension module for CPython you basically have to do the same as when doing this in plain Python: the .__new__() method needs to return the same object each time it is called. Implementing this has a few catches though.

static PyObject *
MyType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    static MyObject *self = NULL;

    if (self == NULL)
        self = (MyObject *)type->tp_alloc(type, 0);
    Py_XINCREF(self);
    return (PyObject *)self;
}

Then assign this function to the tp_new slot in the type. There's two things of interest in this function:

• self is declared as static. Normally this would not be the case, but declaring it as static makes it stay alive after the function has returned and it will still be pointing to the first instance of our object, so the next time a new object is asked for this is simply returned.
• Before returning the pointer to the static self, the reference count is increased. This may seem odd but is the right thing to do, otherwise the reference count will not go up for subsequent calls to the new function since the reference count is increased by PyType_GenericAlloc() (via the call from the pointer in the tp_alloc slot). So if you don't do this you end up with negative reference counts, which doesn't make python very happy. This does mean you never end up deallocating the object since the lowest reference count you have is 1, but you wanted a singleton right?. If you really wanted to get the reference count to drop to 0 then you can always put the Py_INCREF() in an else clause.