Once I discussed with Eric Snow during a core developer sprint: _Py_IDENTIFIER() should use an "interpreter local storage" for identifiers values. _Py_IDENTIFIER() would only be a "key" and _PyUnicode_FromId() would store the value somewhere in a hash table stored in PyInterpreterState. Something similar to the TSS API:

* PyThread_create_key()
* PyThread_delete_key_value()
* PyThread_set_key_value()
* PyThread_get_key_value()

But per interpreter, rather than being per thread.

The key can be simply the variable address in memory. It only has to be unique in the process.