GeezerVison is showing github.com — Unstable C API tier (PEP 689) · Issue #101101 · python/cpython
Unstable C API tier (PEP 689) #101101DescriptionAdd an Unstable C API tier as per PEP 689.
Other candidates for inclusion are below.
These usually need discussion first. This is a checklist for having the discussion.
Please try to not hold the individual discussions in this issue.
- Dict watching API
- Functions from PEP-523 not specified in PEP-689 (
_PyEval_EvalFrameDefault & co.) -- this is a can of worms tho
- FrameStack API (Add "unstable" frame stack api #91371)
-
_Py_HashDouble, see PEP 689 -- Add an unstable C-API tier #91744 (comment)
- PyBytesObject.ob_shash sounds like a good candidate: see https://discuss.python.org/t/15108 (guess fields need a slightly different naming convention though. Sigh. That should have been in the PEP.)
- non-opaque access to frame structs and any other key APIs needed to implement alternate eval loops with comparable performance to the default eval loop (unless & until we can figure out stable public APIs that can deliver equivalent performance) - see Nick's reply
- C APIs that provide access to compiled code whether in AST or opcode form (the API itself may be stable, but the compiled code isn't) - see Nick's reply
-
PyLong_FromByteArray/PyUnstable_LongToBase30Digits: https://discuss.python.org/t/20045
- Anything that starts with an underscore and is documented
-
Fast access to PyLong contents [PyLong_Export]
Got any more?
I offer to move other API to the unstable tier myself, if there's a good usage example to base docs & regression tests on [edit 2026:] and a C API WG decision. Please discuss new ideas on Discourse.
Linked PRs
Activitycorona10 commented on Mar 1, 2023 CAM-Gerlach commented on May 2, 2023 Hey @encukou , with the feature freeze coming up, do you have an update on the status here? encukou commented on May 2, 2023 Changes described the PEP are in. Waiting for SC approval for a change before I mark it Final: python/steering-council#185
I won't be able to start all the discussions I want before beta, so the issue will stay open. vstinner commented on Jan 27, 2025 encukou commented on Jan 28, 2025 Sadly I haven't had as much time for the C API as I hoped. If you want to work on one of the items above, or move this to the C API WG, go ahead. mbechard commented on Feb 24, 2026 I would like to request that the AST parsing be exposed as an Unstable C API as well. I use this to parse simple expressions that run code native to my app, rather than going through the Python interpreter which ultimately ends up back in C module code I've made using the Python C API. This is useful for common used expressions, as it gives a large speedup for the simple/common cases. The code falls back to the full interpreter when an expression includes items my parser doesn't handle.
In particular, I use
PyParser_ASTFromString
And then traverse the AST using the various enums and structs from pycore_ast.h vstinner commented on Mar 2, 2026 Include/internal/pycore_ast.h and Include/internal/pycore_asdl.h cannot easily be exposed in the public C API, even as PyUnstable. Names are not prefixed by Py prefix. Using directly structure members is more like to cause API compatibility issues if a structure changes. For example, PEP 810 (lazy imports) recently added int is_lazy; to Import and ImportFrom structures, and added int is_lazy parameter to _PyAST_Import() and _PyAST_ImportFrom() functions.
mbechard commented on Mar 2, 2026 API incompatibility through struct changes is ok though, as I would expect that in an unstable API. encukou commented on Mar 3, 2026 The original post asks:
Please try to not hold the individual discussions in this issue.
Nowadays the place for new ideas is Discourse.
That said, I like the idea in general, but I think exposing this even in the unstable API would be a huge amount of work. This is all optimized, generated code.
I use this to parse simple expressions that run code native to my app
You're essentially forking CPython; depending on a specific, tested build.
I'm thinking of closing this issue -- the world has moved on; unstable API went in a slightly different direction than I thought.
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