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PEP 697: Changes based on initial implementation & docs (GH-2906) 

* PEP 697: Changes based on initial implementation & docs

- Use a flag, `Py_TPFLAGS_ITEMS_AT_END`, rather than a slot. This way the subclass doesn't need to worry about items (if the superclass is set up right).
- The result of `PyObject_GetTypeDataSize` may be higher than requested by -basicsize (e.g. due to alignment). It is safe to use all of it (e.g. with memset).
- Mention that `basicsize == 0` and `itemsize == 0` already work. I’ll add explicit docs & tests though.
- Add link to initial implementation
- Add endorsements
- Add a “big picture” decision tree
- Rewording
- Mention possible flags for alternative item layouts

* Apply suggestions from code review

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>
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@ -24,16 +24,18 @@ Make the mechanism usable with ``PyHeapTypeObject``.
Motivation Motivation
========== ==========
The motivating problem this PEP solves is creating metaclasses (subclasses of The motivating problem this PEP solves is attaching C-level state
:py:class:`python:type`) in “wrappers” – projects that expose another type to custom types --- i.e. metaclasses (subclasses of
system (e.g. C++, Java, Rust) as Python classes. :py:class:`python:type`).
These systems typically need to attach information about the “wrapped”
non-Python class to the Python type object -- that is, extend
``PyHeapTypeObject``.
This should be possible to do in the Limited API, so that these generators This is often needed in “wrappers” that expose another type
can be used to create Stable ABI extensions. (See :pep:`652` for the benefits system (e.g. C++, Java, Rust) as Python classes.
of providing a stable ABI.) These typically need to attach information about the “wrapped” non-Python
class to the Python type object.
This should be possible to do in the Limited API, so that the language wrappers
or code generators can be used to create Stable ABI extensions.
(See :pep:`652` for the benefits of providing a stable ABI.)
Extending ``type`` is an instance of a more general problem: Extending ``type`` is an instance of a more general problem:
extending a class while maintaining loose coupling – that is, extending a class while maintaining loose coupling – that is,
@ -100,7 +102,7 @@ However, this has disadvantages:
use ``PyObject_GetAttrString(obj, "__basicsize__")``. use ``PyObject_GetAttrString(obj, "__basicsize__")``.
This is cumbersome, and unsafe in edge cases (the Python attribute can This is cumbersome, and unsafe in edge cases (the Python attribute can
be overridden). be overridden).
* Variable-size types are not handled (see `var-sized`_ below). * Variable-size objects are not handled (see :ref:`697-var-sized` below).
To make this easy (and even *best practice* for projects that choose loose To make this easy (and even *best practice* for projects that choose loose
coupling over maximum performance), this PEP proposes an API to: coupling over maximum performance), this PEP proposes an API to:
@ -115,35 +117,45 @@ coupling over maximum performance), this PEP proposes an API to:
2. Given an instance, and the subclass ``PyTypeObject*``, 2. Given an instance, and the subclass ``PyTypeObject*``,
get a pointer to the ``SubListState``. get a pointer to the ``SubListState``.
A new function will be added for this. A new function, ``PyObject_GetTypeData``, will be added for this.
The base class is not limited to ``PyListObject``, of course: it can be used to The base class is not limited to ``PyListObject``, of course: it can be used to
extend any base class whose instance ``struct`` is opaque, unstable across extend any base class whose instance ``struct`` is opaque, unstable across
releases, or not exposed at all -- including :py:class:`python:type` releases, or not exposed at all -- including :py:class:`python:type`
(``PyHeapTypeObject``) mentioned earlier, but also other extensions (``PyHeapTypeObject``) or third-party extensions
(for example, NumPy arrays [#f1]_). (for example, NumPy arrays [#f1]_).
For cases where no additional state is needed, a zero ``basicsize`` will be For cases where no additional state is needed, a zero ``basicsize`` will be
allowed: in that case, the base's ``tp_basicsize`` will be inherited. allowed: in that case, the base's ``tp_basicsize`` will be inherited.
(With the current API, the base's ``basicsize`` needs to be passed in.) (This currently works, but lacks explicit documentation and tests.)
The ``tp_basicsize`` of the new class will be set to the computed total size, The ``tp_basicsize`` of the new class will be set to the computed total size,
so code that inspects classes will continue working as before. so code that inspects classes will continue working as before.
.. _var-sized: .. _697-var-sized:
Extending variable-size objects Extending variable-size objects
------------------------------- -------------------------------
Additional considerations are needed to subclass Additional considerations are needed to subclass
:external+python:c:type:`variable-sized objects <PyVarObject>` :external+python:c:type:`variable-sized objects <PyVarObject>`
while maintaining loose coupling as much as possible. while maintaining loose coupling:
the variable-sized data can collide with subclass data (``SubListState`` in
the example above).
Unfortunately, in this case we cannot decouple the subclass from its superclass Currently, CPython doesn't provide a way to prevent such collisions.
entirely. So, the proposed mechanism of extending opaque classes (negative
There are two main memory layouts for variable-sized objects, and the ``base->tp_itemsize``) will *fail* by default.
subclass's author needs to know which one the superclass uses.
We could stop there, but since the motivating type --- ``PyHeapTypeObject`` ---
is variable sized, we need a safe way to allow subclassing it.
A bit of background first:
Variable-size layouts
.....................
There are two main memory layouts for variable-sized objects.
In types such as ``int`` or ``tuple``, the variable data is stored at a fixed In types such as ``int`` or ``tuple``, the variable data is stored at a fixed
offset. offset.
@ -177,56 +189,90 @@ The first layout enables fast access to the items array.
The second allows subclasses to ignore the variable-sized array (assuming The second allows subclasses to ignore the variable-sized array (assuming
they use offsets from the start of the object to access their data). they use offsets from the start of the object to access their data).
Which layout is used is, unfortunately, an implementation detail that the Since this PEP focuses on ``PyHeapTypeObject``, it proposes an API to allow
subclass code must take into account. subclassing for the second variant.
Correspondingly, if a variable-sized type is designed to be extended in C, Support for the first can be added later *as an API-compatible change*
its documentation should note the mechanism used. (though your PEP author doubts it'd be worth the effort).
Since this PEP focuses on ``PyHeapTypeObject``, it proposes API for the second
variant.
Like with fixed-size types, extending a variable-sized type is already
possible: when creating the class, ``base->tp_itemsize`` needs to be passed
as ``PyType_Spec.itemsize``.
This is cumbersome in the Limited API, where one needs to resort to
``PyObject_GetAttrString(obj, "__itemsize__")``, with the same caveats as for
``__basicsize__`` above.
This PEP proposes a mechanism to instruct the interpreter to do this on its
own, without the extension needing to read ``base->tp_itemsize``.
Several alternatives for this mechanism were rejected:
* The easiest way to do this would be to allow leaving ``itemsize`` as 0 to
mean “inherit”.
However, unlike ``basicsize`` zero is a valid value for ``itemsize`` --
it marks fixed-sized types.
Also, in C, zero is the default value used when ``itemsize`` is not specified.
Since extending a variable-sized type requires *some* knowledge of the
superclass, it would be a good idea to require a more explicit way
to request it.
* It would be possible to reserve a special negative value like ``itemsize=-1``
to mean “inherit”.
But this would rule out a possible future where negative ``itemsize``
more closely matches negative ``basicsize`` -- a request for
additional space.
* A new flag would also work, but ``tp_flags`` is running out of free bits.
Reserving one for a flag only used in type creation seems wasteful.
So, this PEP proposes a new :external+python:c:type:`PyType_Slot` to mark
that ``tp_itemsize`` hould be inherited.
When this flag is used, ``itemsize`` must be set to zero.
Like with ``tp_basicsize``, ``tp_itemsize`` will be set to the computed value
as the class is created.
Normalizing the ``PyHeapTypeObject``-like layout Extending classes with the ``PyHeapTypeObject``-like layout
'''''''''''''''''''''''''''''''''''''''''''''''' ...........................................................
This PEP proposes a type flag, ``Py_TPFLAGS_ITEMS_AT_END``, which will indicate
the ``PyHeapTypeObject``-like layout.
This can be set in two ways:
* the superclass can set the flag, allowing subclass authors to not care about
the fact that ``itemsize`` is involved, or
* the new subclass sets the flag, asserting that the author knows the
superclass is suitable (but perhaps hasn't been updated to use the flag yet).
This flag will be necessary to extend a variable-sized type using negative
``basicsize``.
An alternative to a flag would be to require subclass authors to know that the
base uses a compatible layout (e.g. from documentation).
A past version of this PEP proposed a new
``PyType_Slot`` for it.
This turned out to be hard to explain, and goes against the idea of decoupling
the subclass from the base layout.
The new flag will be used to allow safely extending variable-sized types:
creating a type with ``spec->basesize < 0`` and ``base->tp_itemsize > 0``
will require the flag.
Additionally, this PEP proposes a helper function to get the variable-sized Additionally, this PEP proposes a helper function to get the variable-sized
data of a given instance, assuming it uses the ``PyHeapTypeObject``-like layout. data of a given instance, if it uses the new ``Py_TPFLAGS_ITEMS_AT_END`` flag.
This is mainly to make it easier to define and document such types. This hides the necessary pointer arithmetic behind an API
that can potentially be adapted to other layouts in the future (including,
potentially, a VM-managed layout).
This function will not be exposed in the Limited API. Big picture
...........
To make it easier to verify that all cases are covered, here's a scary-looking
big-picture decision tree.
.. note::
The individual cases are easier to explain in isolation (see the
:ref:`reference implementation <697-ref-impl>` for draft docs).
* ``spec->basesize > 0``: No change to the status quo. (The base
class layout is known.)
* ``spec->basesize == 0``: (Inheriting the basicsize)
* ``base->tp_itemsize == 0``: The item size is set to ``spec->tp_itemsize``.
(No change to status quo.)
* ``base->tp_itemsize > 0``: (Extending a variable-size class)
* ``spec->itemsize == 0``: The item size is inherited.
(No change to status quo.)
* ``spec->itemsize > 0``: The item size is set. (This is hard to use safely,
but it's CPython's current behavior.)
* ``spec->basesize < 0``: (Extending the basicsize)
* ``base->tp_itemsize == 0``: (Extending a fixed-size class)
* ``spec->itemsize == 0``: The item size is set to 0.
* ``spec->itemsize > 0``: Fail. (We'd need to add an ``ob_size``, which is
only possible for trivial types -- and the trivial layout must be known.)
* ``base->tp_itemsize > 0``: (Extending a variable-size class)
* ``spec->itemsize == 0``: (Inheriting the itemsize)
* ``Py_TPFLAGS_ITEMS_AT_END`` used: itemsize is inherited.
* ``Py_TPFLAGS_ITEMS_AT_END`` not used: Fail. (Possible conflict.)
* ``spec->itemsize > 0``: Fail. (Changing/extending the item size can't be
done safely.)
Setting ``spec->itemsize < 0`` is always an error.
This PEP does not propose any mechanism to *extend* ``tp->itemsize``
rather than just inherit it.
Relative member offsets Relative member offsets
@ -234,15 +280,15 @@ Relative member offsets
One more piece of the puzzle is ``PyMemberDef.offset``. One more piece of the puzzle is ``PyMemberDef.offset``.
Extensions that use a subclass-specific ``struct`` (``SubListState`` above) Extensions that use a subclass-specific ``struct`` (``SubListState`` above)
will get a way to specify “relative” offsets -- offsets based on this ``struct`` will get a way to specify “relative” offsets (offsets based from this
-- rather than to “absolute” ones (based on ``PyObject*``). ``struct``) rather than “absolute” ones (based off the ``PyObject`` struct).
One way to do it would be to automatically assume “relative” offsets One way to do it would be to automatically assume “relative” offsets
if this PEP's API is used to create a class. when creating a class using the new API.
However, this implicit assumption may be too surprising. However, this implicit assumption would be too surprising.
To be more explicit, this PEP proposes a new flag for “relative” offsets. To be more explicit, this PEP proposes a new flag for “relative” offsets.
At least initially, this flag will serve only a check against misuse At least initially, this flag will serve only as a check against misuse
(and a hint for reviewers). (and a hint for reviewers).
It must be present if used with the new API, and must not be used otherwise. It must be present if used with the new API, and must not be used otherwise.
@ -259,8 +305,9 @@ Relative ``basicsize``
The ``basicsize`` member of ``PyType_Spec`` will be allowed to be zero or The ``basicsize`` member of ``PyType_Spec`` will be allowed to be zero or
negative. negative.
In that case, it will specify the inverse of *extra* storage space instances of In that case, its absolute value will specify how much *extra* storage space
the new class require, in addition to the basicsize of the base class. instances of the new class require, in addition to the basicsize of the
base class.
That is, the basicsize of the resulting class will be: That is, the basicsize of the resulting class will be:
.. code-block:: c .. code-block:: c
@ -268,8 +315,10 @@ That is, the basicsize of the resulting class will be:
type->tp_basicsize = _align(base->tp_basicsize) + _align(-spec->basicsize); type->tp_basicsize = _align(base->tp_basicsize) + _align(-spec->basicsize);
where ``_align`` rounds up to a multiple of ``alignof(max_align_t)``. where ``_align`` rounds up to a multiple of ``alignof(max_align_t)``.
When ``spec->basicsize`` is zero, ``base->tp_basicsize`` will be inherited
directly instead (i.e. set to ``base->tp_basicsize`` without aligning). When ``spec->basicsize`` is zero, basicsize will be inherited
directly instead, i.e. set to ``base->tp_basicsize`` without aligning.
(This already works; explicit tests and documentation will be added.)
On an instance, the memory area specific to a subclass -- that is, the On an instance, the memory area specific to a subclass -- that is, the
“extra space” that subclass reserves in addition its base -- will be available “extra space” that subclass reserves in addition its base -- will be available
@ -288,10 +337,13 @@ Another function will be added to retreive the size of this memory area:
.. code-block:: c .. code-block:: c
Py_ssize_t Py_ssize_t
PyObject_GetTypeDataSize(PyTypeObject *cls) { PyType_GetTypeDataSize(PyTypeObject *cls) {
return cls->tp_basicsize - _align(cls->tp_base->tp_basicsize); return cls->tp_basicsize - _align(cls->tp_base->tp_basicsize);
} }
The result may be higher than requested by ``-basicsize``. It is safe to
use all of it (e.g. with ``memset``).
The new ``*Get*`` functions come with an important caveat, which will be The new ``*Get*`` functions come with an important caveat, which will be
pointed out in documentation: They may only be used for classes created using pointed out in documentation: They may only be used for classes created using
negative ``PyType_Spec.basicsize``. For other classes, their behavior is negative ``PyType_Spec.basicsize``. For other classes, their behavior is
@ -303,40 +355,40 @@ undefined.
Inheriting ``itemsize`` Inheriting ``itemsize``
----------------------- -----------------------
If a new slot, ``Py_tp_inherit_itemsize``, is present in When ``spec->itemsize`` is zero, ``tp_itemsize`` will be inherited
``PyType_Spec.slots``, the new class will inherit from the base.
the base's ``tp_itemsize``. (This already works; explicit tests and documentation will be added.)
If this is the case, CPython will assert that: A new type flag, ``Py_TPFLAGS_ITEMS_AT_END``, will be added.
This flag can only be set on types with non-zero ``tp_itemsize``.
It indicates that the variable-sized portion of an instance
is stored at the end of the instance's memory.
* ``PyType_Spec.itemsize`` must be set to zero. The default metatype (``PyType_Type``) will set this flag.
* The ``Py_tp_inherit_itemsize`` slot's
``~PyType_Slot.pfunc`` must be set to NULL.
A new function, ``PyObject_GetItemData``, will be added to safely access the A new function, ``PyObject_GetItemData``, will be added to access the
memory reserved for items, taking subclasses that extend ``tp_basicsize`` memory reserved for variable-sized content of types with the new flag.
into account.
In CPython it will be defined as: In CPython it will be defined as:
.. code-block:: c .. code-block:: c
void * void *
PyObject_GetItemData(PyObject *obj) { PyObject_GetItemData(PyObject *obj) {
if (!PyType_HasFeature(Py_TYPE(obj), Py_TPFLAGS_ITEMS_AT_END) {
<fail with TypeError>
}
return (char *)obj + Py_TYPE(obj)->tp_basicsize; return (char *)obj + Py_TYPE(obj)->tp_basicsize;
} }
This function will *not* be added to the Limited API. This function will initially *not* be added to the Limited API.
Note that it **is not safe** to use **any** of the functions added in this PEP Extending a class with positive ``base->itemsize`` using
unless **all classes in the inheritance hierarchy** only use negative ``spec->basicsize`` will fail unless ``Py_TPFLAGS_ITEMS_AT_END``
``PyObject_GetItemData`` (or an equivalent) for per-item memory, or don't is set, either on the base or in ``spec->flags``.
use per-item memory at all. (See :ref:`697-var-sized` for a full explanation.)
(This issue already exists for most current classes that use variable-length
arrays in the instance struct, but it's much less obvious if the base struct
layout is unknown.)
The documentation for all API added in this PEP will mention Extending a class with positive ``spec->itemsize`` using negative
the caveat. ``spec->basesize`` will fail.
Relative member offsets Relative member offsets
@ -345,32 +397,23 @@ Relative member offsets
In types defined using negative ``PyType_Spec.basicsize``, the offsets of In types defined using negative ``PyType_Spec.basicsize``, the offsets of
members defined via ``Py_tp_members`` must be relative to the members defined via ``Py_tp_members`` must be relative to the
extra subclass data, rather than the full ``PyObject`` struct. extra subclass data, rather than the full ``PyObject`` struct.
This will be indicated by a new flag, ``PY_RELATIVE_OFFSET``. This will be indicated by a new flag in ``PyMemberDef.flags``:
``Py_RELATIVE_OFFSET``.
In the initial implementation, the new flag will be redundant. It only serves In the initial implementation, the new flag will be redundant. It only serves
to make the offset's changed meaning clear, and to help avoid mistakes. to make the offset's changed meaning clear, and to help avoid mistakes.
It will be an error to *not* use ``PY_RELATIVE_OFFSET`` with negative It will be an error to *not* use ``Py_RELATIVE_OFFSET`` with negative
``basicsize``, and it will be an error to use it in any other context ``basicsize``, and it will be an error to use it in any other context
(i.e. direct or indirect calls to ``PyDescr_NewMember``, ``PyMember_GetOne``, (i.e. direct or indirect calls to ``PyDescr_NewMember``, ``PyMember_GetOne``,
``PyMember_SetOne``). ``PyMember_SetOne``).
CPython will adjust the offset and clear the ``PY_RELATIVE_OFFSET`` flag when CPython will adjust the offset and clear the ``Py_RELATIVE_OFFSET`` flag when
intitializing a type. intitializing a type.
This means that the created type's ``tp_members`` will not match the input This means that:
definition's ``Py_tp_members`` slot, and that any code that reads
``tp_members`` will not need to handle the flag.
* the created type's ``tp_members`` will not match the input
Changes to ``PyTypeObject`` definition's ``Py_tp_members`` slot, and
--------------------------- * any code that reads ``tp_members`` will not need to handle the flag.
Internally in CPython, access to ``PyTypeObject`` “items”
(``_PyHeapType_GET_MEMBERS``) will be changed to use ``PyObject_GetItemData``.
Note that the current implementation is equivalent: it only lacks the
alignment adjustment.
The macro is used a few times in type creation, so no measurable
performance impact is expected.
Public API for this data, ``tp_members``, will not be affected.
List of new API List of new API
@ -381,8 +424,9 @@ The following new functions/values are proposed.
These will be added to the Limited API/Stable ABI: These will be added to the Limited API/Stable ABI:
* ``void * PyObject_GetTypeData(PyObject *obj, PyTypeObject *cls)`` * ``void * PyObject_GetTypeData(PyObject *obj, PyTypeObject *cls)``
* ``Py_ssize_t PyObject_GetTypeDataSize(PyTypeObject *cls)`` * ``Py_ssize_t PyType_GetTypeDataSize(PyTypeObject *cls)``
* ``Py_tp_inherit_itemsize`` slot for ``PyType_Spec.slots`` * ``Py_TPFLAGS_ITEMS_AT_END`` flag for ``PyTypeObject.tp_flags``
* ``Py_RELATIVE_OFFSET`` flag for ``PyMemberDef.flags``
These will be added to the public C API only: These will be added to the public C API only:
@ -414,10 +458,14 @@ None known.
Endorsements Endorsements
============ ============
XXX: The PEP mentions wrapper libraries, so it should get review/endorsement The author of ``pybind11`` originally requested solving the issue
from nanobind, PyO3, JPype, PySide &c. (see point 2 in `this list <https://discuss.python.org/t/15993>`__),
and `has been verifying the implementation <https://discuss.python.org/t/19743/14>`__.
XXX: HPy devs might also want to chime in. Florian from the HPy project `said <https://discuss.python.org/t/19743/3>`__
that the API looks good in general.
(See :ref:`below <697-alignment-performance>` for a possible solution to
performance concerns.)
How to Teach This How to Teach This
@ -428,17 +476,22 @@ and a What's New entry, which should be enough for the target audience
-- authors of C extension libraries. -- authors of C extension libraries.
.. _697-ref-impl:
Reference Implementation Reference Implementation
======================== ========================
XXX: Not quite ready yet A reference implementation is in the `extend-opaque branch <https://github.com/python/cpython/compare/main...encukou:cpython:extend-opaque>`__
in the ``encukou/cpython`` GitHub repo.
Possible Future Enhancements Possible Future Enhancements
============================ ============================
Alignment .. _697-alignment-performance:
---------
Alignment & Performance
-----------------------
The proposed implementation may waste some space if instance structs The proposed implementation may waste some space if instance structs
need smaller alignment than ``alignof(max_align_t)``. need smaller alignment than ``alignof(max_align_t)``.
@ -457,8 +510,16 @@ without breaking the API:
the cost of an extra pointer in the class. the cost of an extra pointer in the class.
- Then, a new ``PyType_Slot`` can specify the desired alignment, to - Then, a new ``PyType_Slot`` can specify the desired alignment, to
reduce space requirements for instances. reduce space requirements for instances.
- Alternatively, it might be possible to align ``tp_basicsize`` up at class
creation/readying time. Other layouts for variable-size types
-------------------------------------
A flag like ``Py_TPFLAGS_ITEMS_AT_END`` could be added to signal the
“tuple-like” layout described in :ref:`697-var-sized`, and all mechanisms
this PEP proposes could be adapted to support it.
Other layouts could be added as well.
However, it seems there'd be very little practical benefit,
so it's just a theoretical possibility.
Rejected Ideas Rejected Ideas