Allow extensions to set a callback to be invoked when a type is modified

[mpage]

BPO	46895
Nosy	@carljm, @DinoV, @itamaro, @mpage

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-01.22:16:51.046>
labels = ['interpreter-core', 'expert-C-API', 'type-feature', '3.11']
title = 'Allow extensions to set a callback to be invoked when a type is modified'
updated_at = <Date 2022-03-01.22:21:58.988>
user = 'https://github.com/mpage'

bugs.python.org fields:

activity = <Date 2022-03-01.22:21:58.988>
actor = 'mpage'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core', 'C API']
creation = <Date 2022-03-01.22:16:51.046>
creator = 'mpage'
dependencies = []
files = []
hgrepos = []
issue_num = 46895
keywords = []
message_count = 1.0
messages = ['414305']
nosy_count = 4.0
nosy_names = ['carljm', 'dino.viehland', 'itamaro', 'mpage']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46895'
versions = ['Python 3.11']

Linked PRs

• gh-91051: fix segfault when using all 8 type watchers #107853
• [3.12] gh-91051: fix segfault when using all 8 type watchers (GH-107853) #107876
• gh-91051: fix type watcher test to be robust to existing watcher #107989
• [3.12] gh-91051: fix type watcher test to be robust to existing watcher (GH-107989) #108053

[mpage]

CPython extensions providing optimized execution of Python bytecode (e.g. the Cinder JIT), or even CPython itself (e.g. the faster-cpython project) may wish to cache access to lookups in the class hierarchy (e.g. when resolving the target of a method call). Extensions that perform these optimizations need to know when to invalidate the cached values. CPython already has a mechanism to invalidate its internal state (e.g. the global method cache) when a type is modified: _PyType_Modified. We propose adding an API to allow extensions to set a callback that will be invoked by _PyType_Modified whenever a type, or any ancestor of the type in the class hierarchy, changes.

Proposed API:

// A callback to be invoked with the modified type and optionally the name of
// the attribute that was modified.
typedef void(*PyType_ModifiedCallback)(PyTypeObject* type, PyObject* attr);

// Set or get the callback. The callback may be cleared by supplying a NULL callback.
void PyType_SetModifiedCallback(PyType_ModifiedCallback callback);
PyType_ModifiedCallback PyType_GetModifiedCallback();

[gvanrossum]

I don't see any discussion here. Are you still interested in getting this in 3.12? What should happen next? Is it a simple PR, or should we have a design discussion first?

[itamaro]

Are you still interested in getting this in 3.12?

yes, that's the plan!

What should happen next? Is it a simple PR, or should we have a design discussion first?

I think this is a simple PR (similar to #91054), so the next step is to write that PR if the proposed API looks reasonable