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Python Enhancement Proposals

PEP 689 – Unstable C API tier

Petr Viktorin <encukou at>
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Standards Track
27-Apr-2022, 25-Aug-2022, 27-Oct-2022
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Table of Contents


This PEP is a historical document. The up-to-date, canonical documentation can now be found at Changing Python’s C API.


User-facing documentation is at Unstable C API.

See PEP 1 for how to propose changes.


Some functions and types of the C-API are designated unstable, meaning that they will not change in patch (bugfix/security) releases, but may change between minor releases (e.g. between 3.11 and 3.12) without deprecation warnings.

Any C API with a leading underscore is designated internal, meaning that it may change or disappear without any notice.

Motivation & Rationale

Unstable C API tier

The Python C-API is currently divided into three stability tiers:

  • Limited API, with high compatibility expectations
  • Public API, which follows the backwards compatibility policy, and requires deprecation warnings before changes
  • Internal (private) API, which can change at any time.

Tools requiring access to CPython internals (e.g. advanced debuggers and JIT compilers) are often built for minor series releases of CPython, and assume that the C-API internals used do not change in patch releases. To support these tools, we need a tier between the Public and Private C-API, with guarantees on stability throughout the minor-series release: the proposed Unstable tier.

Some functions, like PyCode_New(), are documented as unstable (“Calling [it] directly can bind you to a precise Python version”), and also often change in practice. The unstable tier should make their status obvious even to people who don’t read the docs carefully enough, making them hard to use accidentally.

Reserving leading underscores for Private API

Currently, CPython developers don’t agree on the exact meaning of a leading underscore in API names. It is used to mean two different things:

  • API that may change between minor releases, as in the Unstable tier proposed here (e.g. functions introduced in PEP 523).
  • API that is private and should not be used outside of CPython at all (e.g. because it may change without notice, or it relies on undocumented assumptions that non-CPython code cannot guarantee).

The unclear meaning makes the underscore less useful than it could be. If it only marked private API, CPython developers could change underscored functions, or remove unused ones, without researching how they’re documented or used outside CPython.

With the introduction of a dedicated unstable tier, we can clarify the meaning of the leading underscore. It should mark private API only.

Not breaking code unnecessarily

This PEP specifies that API in the unstable tier should have a special name prefix. This means functions (macros, etc.) will need to be renamed. After a rename, the old name should continue to be available until an incompatible change is made (i.e. until call sites need to be updated anyway). In other words, just changing the tier of a function shouldn’t break users’ code.


The C API is divided by stability expectations into three “sections” (internal, public, and limited). We’ll now call these stability tiers, or tiers for short.

An Unstable tier will be added.

APIs (functions, types, etc.) in this tier will named with the PyUnstable_ prefix, with no leading underscore.

They will be declared in headers used for public API (Include/*.h, rather than in a subdirectory like Include/unstable/).

Several rules for dealing with the unstable tier will be introduced:

  • Unstable API should have no backwards-incompatible changes across patch releases, but may change or be removed in minor releases (3.x.0, including Alpha and Beta releases of 3.x.0). Such changes must be documented and mentioned in the What’s New document.
  • Backwards-incompatible changes to these APIs should be made so that code that uses them will need to be updated to compile with the new version (e.g. arguments should be added/removed, or a function should be renamed, but the semantic meaning of an argument should not change).
  • Unstable API should be documented and tested.
  • To move an API from the public tier to the unstable tier, it should be exposed under the new PyUnstable_* name.

    The old name should be deprecated (e.g. with Py_DEPRECATED), but continue to be available until an incompatible change is made to the API. Per Python’s backwards compatibility policy (PEP 387), this deprecation needs to last at least two releases (without an SC exceptions). But it can also last indefinitely – for example, if PEP 590’s “provisional” _PyObject_Vectorcall was added today, it would be initially named PyUnstable_Object_Vectorcall and there would be no plan to remove this name.

    In the following cases, an incompatible change (and thus removing the deprecated name) is allowed without an SC exception, as if the function was already part of the Unstable tier:

    • Any API introduced before Python 3.12 that is documented to be less stable than default.
    • Any API introduced before Python 3.12 that was named with a leading underscore.

    For examples, see the initial unstable API specified in this PEP.

  • To move an internal API to the unstable tier, it should be exposed under the new PyUnstable_* name.

    If the old name is documented, or widely used externally, it should continue to be available until an incompatible change is made (and call sites need to be updated). It should start raising deprecation warnings (e.g. using Py_DEPRECATED).

  • To move an API from the unstable tier to the public tier, it should be exposed without the PyUnstable_* prefix.

    The old name should remain available until the API is deprecated or removed.

  • Adding new unstable API for existing features is allowed even after Beta feature freeze, up until the first Release Candidate. Consensus on Core Development Discourse or is needed in the Beta period.

These rules will be documented in the devguide, and user documentation will be updated accordingly.

Reference docs for C API named PyUnstable_* will automatically show notes with links to the unstable tier documentation.

Leading underscore

C API named with a leading underscore, as well as API only available with Py_BUILD_CORE, will be considered internal. This means:

  • It may change or be removed without notice in minor releases (3.x.0, including Alpha and Beta releases of 3.x.0). API changes in patch releases or Release Candidates should only be done if absolutely necessary.
  • It should be documented in source comments or Devguide only, not in the public documentation.
  • API introduced before Python 3.12 that is documented or widely used externally should be moved to the Unstable tier as explained above.

    This might happen long after this PEP is accepted. Consequently, for a few years core devs should do some research before changing underscored API, especially if it doesn’t need Py_BUILD_CORE.

Users of the C API are encouraged to search their codebase for _Py and _PY identifier prefixes, and treat any hits as issues to be eventually fixed – either by switching to an existing alternative, or by opening a CPython issue to request exposing public API for their use case, and eventually switching to that.

Initial unstable API

The following API will be moved to the Unstable tier in the initial implementation as proof of the concept.

Code object constructors:

  • PyUnstable_Code_New() (renamed from PyCode_New)
  • PyUnstable_Code_NewWithPosOnlyArgs() (renamed from PyCode_NewWithPosOnlyArgs)

Code extra information (PEP 523):

  • PyUnstable_Eval_RequestCodeExtraIndex() (renamed from _PyEval_RequestCodeExtraIndex)
  • PyUnstable_Code_GetExtra() (renamed from _PyCode_GetExtra)
  • PyUnstable_Code_SetExtra() (renamed from _PyCode_SetExtra)

More are expected in Python 3.12, without the need for another PEP.

Backwards Compatibility

The C API backwards compatibility expectations will be made clearer.

All renamed API will be available under old names for as long as feasible.

How to Teach This

The changes affect advanced C programmers, who should consult the updated reference documentation, devguide and/or What’s New document.

Reference Implementation…encukou:unstable-tier

Rejected Ideas

No special prefix

In the initial version of this PEP, unstable API didn’t have the PyUnstable prefix. Instead, defining Py_USING_UNSTABLE_API made the API available in a given source file, signifying acknowledgement that the file as a whole will potentially need to be revisited for each Python release.

However, it was decided that unstable-ness needs to be exposed in the individual names.

Underscore prefix

It would be possible to mark both private and unstable API with leading underscores. However, that would dilute the meaning of _Py prefix. Reserving the prefix for internal API only makes it trivial to search for.

New header directory

Other API tiers have dedicated directories for headers (Include/cpython/, Include/internal/).

Since the unstable tier uses a very obvious naming convention and the names are always available, a directory like Include/unstable/ is unnecessary.

Python API

It might be good to add a similar tier in the Python (not C) API, e.g. for types.CodeType. However, the mechanism for that would need to be different. This is outside the scope of the PEP.


Last modified: 2023-09-09 17:39:29 GMT