Idiomatic code is

    if (PyModule_AddObject(module, "name", create_new_object()) < 0)
        goto error;

If you already have a reference and need to use it later:

    obj = create_new_object();
    ... /* use obj */
    Py_INCREF();
    if (PyModule_AddObject(module, "name", create_new_object()) < 0)
        goto error;
    ... /* use obj */
    Py_DECREF(obj);

error:
    Py_XDECREF(obj);

Many current code use above idioms, but it doesn't work as expected.

It is almost impossible to write correct code with current behavior. And _decimal.c is not an exception, it has leaks.