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.

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.

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.