gh-146636: PEP 803: Reference documentation

Doc/c-api/module.rst

@@ -685,6 +685,11 @@ remove it.
    Usually, there is only one variable of this type for each extension module
    defined this way.
 
+   In the :ref:`Stable ABI <stable-abi>` for free-threaded builds (``abi3t``),
+   this struct is opaque, and unusable in practice.
+   The struct, including all members, is part of Stable ABI for
+   non-free-threaded builds (``abi3``).
+
    .. c:member:: PyModuleDef_Base m_base
 
       Always initialize this member to :c:macro:`PyModuleDef_HEAD_INIT`:
@@ -695,6 +700,11 @@ remove it.
 
          The type of :c:member:`!PyModuleDef.m_base`.
 
+         In the :ref:`Stable ABI <stable-abi>` for Free-Threaded Builds
+         (``abi3t``), this struct is opaque, and unusable in practice.
+         The struct is part of Stable ABI for
+         non-free-threaded builds (``abi3``).
+
       .. c:macro:: PyModuleDef_HEAD_INIT
 
          The required initial value for :c:member:`!PyModuleDef.m_base`.

Doc/c-api/structures.rst

@@ -33,6 +33,13 @@ under :ref:`reference counting <countingrefs>`.
    The members must not be accessed directly; instead use macros such as
    :c:macro:`Py_REFCNT` and :c:macro:`Py_TYPE`.
 
+   In the :ref:`Stable ABI <stable-abi>` for Free-Threaded Builds (``abi3t``),
+   this struct is opaque; its size and layout may change between
+   Python versions.
+   In Stable ABI for non-free-threaded builds (``abi3``), the
+   :c:member:`!ob_refcnt` and :c:member:`!ob_type` fields are available,
+   but using them directly is discouraged.
+
    .. c:member:: Py_ssize_t ob_refcnt
 
       The object's reference count, as returned by :c:macro:`Py_REFCNT`.
@@ -72,6 +79,19 @@ under :ref:`reference counting <countingrefs>`.
    instead use macros such as :c:macro:`Py_SIZE`, :c:macro:`Py_REFCNT` and
    :c:macro:`Py_TYPE`.
 
+   In the :ref:`Stable ABI <stable-abi>` for Free-Threaded Builds (``abi3t``),
+   this struct is opaque; its size and layout may change between
+   Python versions.
+   In Stable ABI for non-free-threaded builds (``abi3``), the
+   :c:member:`!ob_base` and :c:member:`!ob_size` fields are available,
+   but using them directly is discouraged.
+
+   .. c:member:: PyObject ob_base
+
+      Common object header.
+      Typically, this field is not accessed directly; instead
+      :c:type:`!PyVarObject` can be cast to :c:type:`PyObject`.
+
    .. c:member:: Py_ssize_t ob_size
 
       A size field, whose contents should be considered an object's internal

Doc/tools/extensions/c_annotations.py

@@ -249,18 +249,17 @@ def _stable_abi_annotation(
         reftype="ref",
         refexplicit="False",
     )
-    struct_abi_kind = record.struct_abi_kind
-    if struct_abi_kind in {"opaque", "members"}:
-        ref_node += nodes.Text(sphinx_gettext("Limited API"))
-    else:
-        ref_node += nodes.Text(sphinx_gettext("Stable ABI"))
+    ref_node += nodes.Text(sphinx_gettext("Stable ABI"))
     emph_node += ref_node
+    struct_abi_kind = record.struct_abi_kind
     if struct_abi_kind == "opaque":
         emph_node += nodes.Text(" " + sphinx_gettext("(as an opaque struct)"))
     elif struct_abi_kind == "full-abi":
         emph_node += nodes.Text(
             " " + sphinx_gettext("(including all members)")
         )
+    elif struct_abi_kind in {"members", "abi3t-opaque"}:
+        emph_node += nodes.Text(" " + sphinx_gettext("(see below)"))
     if record.ifdef_note:
         emph_node += nodes.Text(f" {record.ifdef_note}")
     if stable_added == "3.2":
@@ -271,11 +270,7 @@ def _stable_abi_annotation(
             " " + sphinx_gettext("since version %s") % stable_added
         )
     emph_node += nodes.Text(".")
-    if struct_abi_kind == "members":
-        msg = " " + sphinx_gettext(
-            "(Only some members are part of the stable ABI.)"
-        )
-        emph_node += nodes.Text(msg)
+
     return emph_node
 
 
@@ -378,6 +373,32 @@ def run(self) -> list[nodes.Node]:
         return [node]
 
 
+class VersionHexCheatsheet(SphinxDirective):
+    """Show results of Py_PACK_VERSION(3, x) for a few relevant Python versions
+
+    This is useful for defining version before Python.h is included.
+    It should auto-update with the version being documented, so it must be an
+    extension.
+    """
+    has_content = False
+    required_arguments = 0
+    optional_arguments = 0
+    final_argument_whitespace = True
+
+    def run(self) -> list[nodes.Node]:
+        content = [
+            ".. code-block:: c",
+            "",
+        ]
+        current_minor = int(self.config.version.removeprefix('3.'))
+        for minor in range(current_minor - 5, current_minor + 1):
+            value = (3 << 24) | (minor << 16)
+            content.append(f'    {value:#x}  /* Py_PACK_VERSION(3.{minor}) */')
+        node = nodes.paragraph()
+        self.state.nested_parse(StringList(content), 0, node)
+        return [node]
+
+
 class CorrespondingTypeSlot(SphinxDirective):
     """Type slot annotations
 
@@ -443,6 +464,7 @@ def setup(app: Sphinx) -> ExtensionMetadata:
     app.add_config_value("stable_abi_file", "", "env", types={str})
     app.add_config_value("threadsafety_file", "", "env", types={str})
     app.add_directive("limited-api-list", LimitedAPIList)
+    app.add_directive("version-hex-cheatsheet", VersionHexCheatsheet)
     app.add_directive("corresponding-type-slot", CorrespondingTypeSlot)
     app.connect("builder-inited", init_annotations)
     app.connect("doctree-read", add_annotations)

Doc/whatsnew/3.15.rst

@@ -82,6 +82,7 @@ Summary -- Release highlights
   <whatsnew315-typeform>`
 * :pep:`782`: :ref:`A new PyBytesWriter C API to create a Python bytes object
   <whatsnew315-pybyteswriter>`
+* :pep:`803`: :ref:`Stable ABI for Free-Threaded Builds <whatsnew315-abi3t>`
 * :ref:`The JIT compiler has been significantly upgraded <whatsnew315-jit>`
 * :ref:`Improved error messages <whatsnew315-improved-error-messages>`
 * :ref:`The official Windows 64-bit binaries now use the tail-calling interpreter
@@ -381,6 +382,41 @@ agen() for x in a)``.
 
 (Contributed by Adam Hartz in :gh:`143055`.)
 
+.. _whatsnew315-abi3t:
+
+:pep:`803`: ``abi3t`` -- Stable ABI for Free-Threaded Builds
+------------------------------------------------------------
+
+C extensions that target the :ref:`Stable ABI <stable-abi>` can now be
+compiled for the new *Stable ABI for Free-Threaded Builds* (also known
+as ``abi3t``), which makes them compatible with
+:term:`free-threaded <free-threaded build>` builds of CPython.
+This usually requires some non-trivial changes to the source code;
+specifically:
+
+- Switching to API introduced in :pep:`697` (Python 3.12), such as
+  negative :c:member:`~PyType_Spec.basicsize` and
+  :c:func:`PyObject_GetTypeData`, rather than making :c:type:`PyObject`
+  part of the instance struct; and
+- Switching from a ``PyInit_`` function to a new export hook,
+  :c:func:`PyModExport_* <PyModExport_modulename>`, introduced for this
+  purpose in :pep:`793`.
+
+Note that Stable ABI does not offer all the functionality that CPython
+has to offer.
+Extensions that cannot switch to ``abi3t`` should continue to build for
+the existing Stable ABI (``abi3``) and the version-specific ABI for
+free-threading (``cp315t``) separately.
+
+Stable ABI for Free-Threaded Builds should typically
+be selected in a build tool (such as Setuptools, meson-python, Cython,
+Scikit-build-core, Maturin, and similar).
+At the time of writing this entry, these tools did **not** support ``abi3t``.
+If this is the case for your tool, compile for ``cp315t`` separately.
+If not using a build tool -- or when writing such a tool -- you can select
+``abi3t`` by setting the macro :c:macro:`!Py_TARGET_ABI3T` as discussed
+in :ref:`abi3-compiling`.
+
 
 .. _whatsnew315-improved-error-messages:
 

Misc/NEWS.d/next/C_API/2026-04-03-11-06-20.gh-issue-146636.zR6Jsn.rst

@@ -0,0 +1 @@
+Implement :pep:`803` -- ``abi3t``: Stable ABI for Free-Threaded Builds.

Misc/stable_abi.toml

@@ -1,4 +1,4 @@
-# This file lists the contents of the Limited API and Stable ABI.
+# This file lists the contents of Limited API and Stable ABI.
 # Please append new items at the end.
 
 # The syntax of this file is not fixed.
@@ -46,15 +46,24 @@
 #   - 'opaque': No members are part of the ABI, nor is the size. The Limited
 #     API only handles these via pointers. The C definition should be
 #     incomplete (opaque).
-#   - 'members': Only specific members are part of the stable ABI.
-#     The struct's size may change, so it can't be used in arrays.
+#   - 'abi3t-opaque': 'full-abi' in abi3; 'opaque' in abi3t.
+#     For docs, the generated annotation refers to details that need to
+#     be added to the ReST file manually.
+#   - 'members':
+#     - 'opaque' in abi3t.
+#     - In abi3, only specific members are part of the stable ABI.
+#       The struct's size may change, so it can't be used in arrays.
 #     Do not add new structs of this kind without an extremely good reason.
+#     For docs, the generated annotation refers to details that need to
+#     be added to the ReST file manually.
 # - members: For `struct` with struct_abi_kind = 'members', a list of the
 #   exposed members.
 # - doc: for `feature_macro`, the blurb added in documentation
 # - windows: for `feature_macro`, this macro is defined on Windows.
 #   (This info is used to generate the DLL manifest and needs to be available
 #   on all platforms.)
+# - abi3t_opaque: In abi3t, this struct is opaque (as if `struct_abi_kind`
+#   was 'opaque' and `members` was missing).
 
 # Removing items from this file is generally not allowed, and additions should
 # be considered with that in mind. See the devguide for exact rules:
@@ -107,10 +116,10 @@
     struct_abi_kind = 'full-abi'
 [struct.PyModuleDef_Base]
     added = '3.2'
-    struct_abi_kind = 'full-abi'
+    struct_abi_kind = 'abi3t-opaque'
 [struct.PyModuleDef]
     added = '3.2'
-    struct_abi_kind = 'full-abi'
+    struct_abi_kind = 'abi3t-opaque'
 [struct.PyStructSequence_Field]
     added = '3.2'
     struct_abi_kind = 'full-abi'

Tools/check-c-api-docs/ignored_c_api.txt

@@ -18,8 +18,6 @@ Py_HasFileSystemDefaultEncoding
 Py_UTF8Mode
 # pyhash.h
 Py_HASH_EXTERNAL
-# modsupport.h
-PyABIInfo_FREETHREADING_AGNOSTIC
 # object.h
 Py_INVALID_SIZE
 # pyexpat.h