python-peps/pep-0580.rst

PEP: 580
Title: The C call protocol
Author: Jeroen Demeyer <J.Demeyer@UGent.be>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 14-Jun-2018
Python-Version: 3.8
Post-History: 19-Jun-2018


Abstract
========

A new "C call" protocol is proposed.
It is meant for classes representing functions or methods
which need to implement fast calling.
The goal is to generalize existing optimizations for built-in functions
to arbitrary extension types.

In the reference implementation,
this new protocol is used for the existing classes
``builtin_function_or_method`` and ``method_descriptor``.
However, in the future, more classes may implement it.

**NOTE**: This PEP deals only with CPython implementation details,
it does not affect the Python language or standard library.


Motivation
==========

Currently, the Python bytecode interpreter has various optimizations
for calling instances of ``builtin_function_or_method``,
``method_descriptor``, ``method`` and ``function``.
However, none of these classes is subclassable.
Therefore, these optimizations are not available to
user-defined extension types.

If this PEP is implemented, then the checks
for ``builtin_function_or_method`` and ``method_descriptor``
could be replaced by simply checking for and using the C call protocol.
This simplifies existing code.

We also design the C call protocol such that it can easily
be extended with new features in the future.

This protocol replaces the use of ``PyMethodDef`` pointers
in instances of ``builtin_function_or_method`` for example.
However, ``PyMethodDef`` arrays are still used to construct
functions/methods but no longer for calling them.

For more background and motivation, see PEP 579.


New data structures
===================

The ``PyTypeObject`` structure gains a new field ``Py_ssize_t tp_ccalloffset``
and a new flag ``Py_TPFLAGS_HAVE_CCALL``.
If this flag is set, then ``tp_ccalloffset`` is assumed to be a valid
offset inside the object structure (similar to ``tp_weaklistoffset``).
It must be a strictly positive integer.
At that offset, a ``PyCMethodDef`` structure appears::

    typedef struct {
        PyCCallDef *cm_ccall;
        PyObject   *cm_self;     /* __self__ argument for methods */
    } PyCMethodDef;

The ``PyCCallDef`` structure contains everything needed to describe how
the function can be called::

    typedef struct {
        uint32_t     cc_flags;
        PyCFunction  cc_func;    /* C function to call */
        PyObject    *cc_name;    /* str object */
        PyObject    *cc_parent;  /* class or module */
    } PyCCallDef;

The reason for putting ``__self__`` outside of ``PyCCallDef``
is that ``PyCCallDef`` is not meant to be changed after creating the function.
A single ``PyCCallDef`` can be shared
by an unbound method and multiple bound methods.
This wouldn't work if we would put ``__self__`` inside that structure.

**NOTE**: unlike ``tp_dictoffset`` we do not allow negative numbers
for ``tp_ccalloffset`` to mean counting from the end.
There does not seem to be a use case for it and it would only complicate
the implementation.

**NOTE**: in the reference implementation, ``tp_ccalloffset`` actually
replaces ``tp_print`` and ``Py_TPFLAGS_HAVE_CCALL`` is *not*
added to ``Py_TPFLAGS_DEFAULT``.
The latter ensures full backwards compatibility for existing
extension modules setting ``tp_print``.
It also means that we can require that ``tp_ccalloffset`` is a valid
offset when ``Py_TPFLAGS_HAVE_CCALL`` is specified:
we do not need to check ``tp_ccalloffset != 0``.
In future Python versions, we may decide that ``tp_print``
becomes ``tp_ccalloffset`` unconditionally,
drop the ``Py_TPFLAGS_HAVE_CCALL`` flag and instead check for
``tp_ccalloffset != 0``.

Parent
------

The ``cc_parent`` field (accessed for example by a ``__parent__``
or ``__objclass__`` descriptor from Python code) can be any Python object.
For methods of extension types, this is set to the class.
For functions of modules, this is set to the module.

The parent serves multiple purposes: for methods of extension types,
it is used for type checks like the following::

    >>> list.append({}, "x")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: descriptor 'append' requires a 'list' object but received a 'dict'

PEP 573 specifies that every function should have access to the
module in which it is defined.
For functions of a module, this is given by the parent.
For methods, this works indirectly through the class,
assuming that the class has a pointer to the module.

The parent would also typically be used to implement ``__qualname__``.

Custom classes are free to set ``cc_parent`` to whatever they want.
It is only used by the C call protocol if the ``CCALL_OBJCLASS`` flag is set.


The C call protocol
===================

We say that a class implements the C call protocol
if it has the ``Py_TPFLAGS_HAVE_CCALL`` flag set
(as explained above, it must then set ``tp_ccalloffset > 0``).
Such a class must implement ``__call__`` as described in this section
(in practice, this just means setting ``tp_call`` to ``PyCCall_Call``).

The ``cc_func`` field is a C function pointer.
Its precise signature depends on flags.
Below are the possible values for ``cc_flags & CCALL_SIGNATURE``
together with the arguments that the C function takes.
The return value is always ``PyObject *``.
The following are completely analogous to the existing ``PyMethodDef``
signature flags:

- ``CCALL_VARARGS``: ``cc_func(PyObject *self, PyObject *args)``

- ``CCALL_VARARGS | CCALL_KEYWORDS``: ``cc_func(PyObject *self, PyObject *args, PyObject *kwds)``

- ``CCALL_FASTCALL``: ``cc_func(PyObject *self, PyObject *const *args, Py_ssize_t nargs)``

- ``CCALL_FASTCALL | CCALL_KEYWORDS``: ``cc_func(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)``

- ``CCALL_NOARGS``: ``cc_func(PyObject *self, PyObject *unused)``

- ``CCALL_O``: ``cc_func(PyObject *self, PyObject *arg)``

The flag ``CCALL_FUNCARG`` may be combined with any of these.
If so, the C function takes an additional argument as first argument
which is the function object (the ``self`` in ``__call__``).
For example, we have the following signature:

- ``CCALL_FUNCARG | CCALL_VARARGS``: ``cc_func(PyObject *func, PyObject *self, PyObject *args)``

**NOTE**: unlike the existing ``METH_...`` flags,
the ``CCALL_...`` constants do not necessarily represent single bits.
So checking ``cc_flags & CCALL_VARARGS != 0`` is not a valid way
for checking the signature.

Checking __objclass__
---------------------

If the ``CCALL_OBJCLASS`` flag is set and if ``cm_self`` is NULL
(this is the case for unbound methods of extension types),
then a type check is done:
the function must be called with at least one positional argument
and the first (typically called ``self``) must be an instance of
``cc_parent`` (which must be a class).
If not, a ``TypeError`` is raised.

Self slicing
------------

If ``cm_self`` is not NULL or if the flag ``CCALL_SLICE_SELF``
is not set in ``cc_flags``, then the argument passed as ``self``
is simply ``cm_self``.

If ``cm_self`` is NULL and the flag ``CCALL_SLICE_SELF`` is set,
then the first positional argument (if any) is removed from
``args`` and instead passed as first argument to the C function.
Effectively, the first positional argument is treated as ``__self__``.
This is meant to support unbound methods such that the C function does
not see the difference between bound and unbound method calls.
This does not affect keyword arguments in any way.

This process is called self slicing and a function is said to have self
slicing if ``cm_self`` is NULL and ``CCALL_SLICE_SELF`` is set.

Note that a ``METH_NOARGS`` function with self slicing effectively has
one argument, namely ``self``.
Analogously, a ``METH_O`` function with self slicing has two arguments.

Supporting the LOAD_METHOD/CALL_METHOD opcodes
----------------------------------------------

Classes supporting the C call protocol
must implement ``__get__`` in a specific way.
This is required to correctly deal with the ``LOAD_METHOD``/``CALL_METHOD`` optimization.
If ``func`` supports the C call protocol, then ``func.__get__``
must behave as follows:

- If ``cm_self`` is not NULL, then ``__get__`` must be a no-op
  in the sense that ``func.__get__(obj, cls)(*args, **kwds)``
  behaves exactly the same as ``func(*args, **kwds)``.
  It is also allowed for ``__get__`` to be not implemented at all.

- If ``cm_self`` is NULL, then ``func.__get__(obj, cls)(*args, **kwds)``
  (with ``obj`` not None)
  must be equivalent to ``func(obj, *args, **kwds)``.
  Note that this is unrelated to self slicing: ``obj`` may be passed
  as ``self`` argument to the C function or it may be the first positional argument.

- If ``cm_self`` is NULL, then ``func.__get__(None, cls)(*args, **kwds)``
  must be equivalent to ``func(*args, **kwds)``.

There are no restrictions on the object ``func.__get__(obj, cls)``.
The latter is not required to implement the C call protocol for example.
It only specifies what ``func.__get__(obj, cls).__call__`` does.

For classes that do not care about ``__self__`` and ``__get__`` at all,
the easiest solution is to assign ``cm_self = Py_None``
(or any other non-NULL value).

Generic API functions
---------------------

The following C API functions are added:

- ``int PyCCall_Check(PyObject *op)``:
  return true if ``op`` implements the C call protocol.

- ``PyObject * PyCCall_Call(PyObject *func, PyObject *args, PyObject *kwds)``:
  call ``func`` (which must implement the C call protocol)
  with positional arguments ``args`` and keyword arguments ``kwds``
  (``kwds`` may be NULL).
  This function is meant to be put in the ``tp_call`` slot.

- ``PyObject * PyCCall_FastCall(PyObject *func, PyObject *const *args, Py_ssize_t nargs, PyObject *kwds)``:
  call ``func`` (which must implement the C call protocol)
  with ``nargs`` positional arguments given by ``args[0]``, …, ``args[nargs-1]``.
  The parameter ``kwds`` can be NULL (no keyword arguments),
  a dict with ``name:value`` items or a tuple with keyword names.
  In the latter case, the keyword values are stored in the ``args``
  array, starting at ``args[nargs]``.

Profiling
---------

A flag ``CCALL_PROFILE`` is added to control profiling [#setprofile]_.
If this flag is set, then the profiling events
``c_call``, ``c_return`` and ``c_exception`` are generated.
When an unbound method is called
(``cm_self`` is NULL and ``CCALL_SLICE_SELF`` is set),
the argument to the profiling function is the corresponding bound method
(obtained by calling ``__get__``).
This is meant for backwards compatibility and to simplify
the implementation of the profiling function.


Changes to built-in functions and methods
=========================================

The reference implementation of this PEP changes
the existing classes ``builtin_function_or_method`` and ``method_descriptor``
to use the C call protocol.
In both cases, the ``PyCCallDef`` structure is simply stored
as part of the object structure.
So, these are the new layouts of ``PyCFunctionObject`` and ``PyMethodDescrObject``::

    typedef struct {
        PyObject_HEAD
        PyCCallDef  *m_ccall;
        PyObject    *m_self;
        PyObject    *m_module;
        PyObject    *m_weakreflist;
        PyCCallDef   _ccalldef;
    } PyCFunctionObject;

    typedef struct {
        PyObject_HEAD
        PyCCallDef  *md_ccall;
        PyObject    *md_self;       /* Always NULL */
        PyObject    *md_qualname;
        PyCCallDef   _ccalldef;
    } PyMethodDescrObject;

For functions of a module, ``m_ccall`` would point to the ``_ccalldef``
field.
For bound methods, ``m_ccall`` would point to the ``PyCCallDef``
of the unbound method.

**NOTE**: the new layout of ``PyMethodDescrObject`` changes it
such that it no longer starts with ``PyDescr_COMMON``.
This is really an implementation detail and it should cause few (if any)
compatibility problems.


Inheritance
===========

Extension types inherit the type flag ``Py_TPFLAGS_HAVE_CCALL``
and the value ``tp_ccalloffset`` from the base class,
provided that they implement ``tp_call`` and ``tp_descr_get``
the same way as the base class.
Heap types never inherit the C call protocol because
that would not be safe (heap types can be changed dynamically).


Backwards compatibility
=======================

There should be no difference at all for the Python interface,
and neither for the documented C API
(in the sense that all functions remain supported with the same functionality).

So the only potential breakage is with C code accessing the
internals of ``PyCFunctionObject`` and ``PyMethodDescrObject``.
We expect very few problems because of this.


Rationale
=========

Why is this better than PEP 575?
--------------------------------

One of the major complaints of PEP 575 was that is was coupling
functionality (the calling and introspection protocol)
with the class hierarchy:
a class could only benefit from the new features
if it was a subclass of ``base_function``.
It may be difficult for existing classes to do that
because they may have other constraints on the layout of the C object structure,
coming from an existing base class or implementation details.
For example, ``functools.lru_cache`` cannot implement PEP 575 as-is.

It also complicated the implementation precisely because changes
were needed both in the implementation details and in the class hierarchy.

The current PEP does not have these problems.

Why store the function pointer in the instance?
-----------------------------------------------

The actual information needed for calling an object
is stored in the instance (in the ``PyCCallDef`` structure)
instead of the class.
This is different from the ``tp_call`` slot or earlier attempts
at implementing a ``tp_fastcall`` slot [#bpo29259]_.

The main use case is built-in functions and methods.
For those, the C function to be called does depend on the instance.

However, the current protocol makes it easy to support the case
where the same C function is called for all instances:
just use a single static ``PyCCallDef`` structure for every instance.


Reference implementation
========================

Work in progress.


References
==========

.. [#setprofile] ``sys.setprofile`` documentation,
                 https://docs.python.org/3.8/library/sys.html#sys.setprofile

.. [#bpo29259] Add tp_fastcall to PyTypeObject: support FASTCALL calling convention for all callable objects,
               https://bugs.python.org/issue29259

Copyright
=========

This document has been placed in the public domain.



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