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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.