While static types (PyTypeObject values) don't themselves ever change (oncePyType_Ready() has run), they do hold mutable data. This means they cannot be safely shared between multiple interpreters without a common GIL (and without state leaking between).

Mutable data:

• the object header (e.g. refcount), as with all objects
• otherwise immutable objects:
  • ob_type (__class__)
  • tp_base (__base__) - set byPyType_Ready() if not set
  • tp_bases (__bases__) - always set byPyType_Ready()
  • tp_mro (__mro__) - always set byPyType_Ready()
  • tp_dict (__dict__) - set byPyType_Ready() if not set
• mutable containers:
  • tp_subclasses (__subclasses__)
  • tp_weaklist

(Seehttps://docs.python.org/3/c-api/typeobj.html#tp-slots.)
(Note thattp_cache is no longer used.)

For the object header, if PEP 683 (immortal objects) is accepted then we can make static types immortal.

For the otherwise immutable objects, we can make sure each is immortal and then we're good. Eventp_dict is fine since it gets hidden behindtypes.MappingProxyType. We'd also need to either make sure each contained item is immortal.

Fortp_subclasses we will need a per-interpreter copy, and do the proper lookup in the__subclasses__ getter. The cache could be stored onPyInterpreterState or even as a dict intp_subclasses.

Fortp_weaklist it's a similar story as fortp_subclasses. Note thattp_weaklist isn't very important for static types since they are never deallocated.

(The above is also discussed inPEP 684.)

CC@kumaraditya303

Linked PRs

• gh-94673: Isolate the _io module to Each Interpreter #102663
• gh-94673: More Per-Interpreter Fields for Builtin Static Types #103912
• gh-94673: Ensure Builtin Static Types are Readied Properly #103940
• gh-94673: Fix _PyTypes_InitTypes() and get_type_attr_as_size() #103961
• gh-94673: Properly Initialize and Finalize Static Builtin Types for Each Interpreter #104072
• gh-94673: Hide Objects in PyTypeObject Behind Accessors #104074
• gh-94673: Ensure subtypes are readied only once in math.trunc() #105465
• [3.12] gh-94673: Ensure subtypes are readied only once in math.trunc() (gh-105465) #105471
• gh-94673: Clarify About Runtime State Related to Static Builtin Types #117761
• gh-94673: Fix compiler warning in typeobject.c #117980