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: 20-Jun-2018, 22-Jun-2018, 16-Jul-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.

For more background and motivation, see PEP 579.


Basic idea
==========

Currently, CPython has multiple optimizations for fast calling
for a few specific function classes.
Calling instances of these classes using a plain ``tp_call`` is slower
than using the optimizations.
The basic idea of this PEP is to allow user-defined extension types
(not Python classes) to use these optimizations also,
both as caller and as callee.

The existing class ``builtin_function_or_method`` and a few others
use a ``PyMethodDef`` structure for describing the underlying C function and its signature.
The first concrete change is that this is replaced by a new structure ``PyCCallDef``.
This stores some of the same information as a ``PyMethodDef``,
but with one important addition:
the "parent" of the function (the class or module where it is defined).
Note that ``PyMethodDef`` arrays are still used to construct
functions/methods but no longer for calling them.

Second, we want that every class can use such a ``PyCCallDef`` for optimizing calls,
so the ``PyTypeObject`` structure gains a ``tp_ccalloffset`` field
giving an offset to a ``PyCCallDef *`` in the object structure
and a flag ``Py_TPFLAGS_HAVE_CCALL`` indicating that ``tp_ccalloffset`` is valid.

Third, since we want to deal efficiently with unbound and bound methods too
(as opposed to only plain functions), we need to handle ``__self__`` too:
after the ``PyCCallDef *`` in the object structure,
there is a ``PyObject *self`` field.
These two fields together are referred to as a ``PyCCallRoot`` structure.

The new protocol for efficiently calling objects using these new structures
is called the "C call protocol".


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 ``PyCCallRoot`` structure appears::

    typedef struct {
        PyCCallDef *cr_ccall;
        PyObject   *cr_self;     /* __self__ argument for methods */
    } PyCCallRoot;

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

    typedef struct {
        uint32_t  cc_flags;
        PyCFunc   cc_func;    /* C function to call */
        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.

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__``.
The new C API function ``PyCCall_GenericGetQualname()`` does exactly that.

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.

Using tp_print
--------------

We propose to replace the existing unused field ``tp_print``
by ``tp_ccalloffset``.
Since ``Py_TPFLAGS_HAVE_CCALL`` would *not* be added to
``Py_TPFLAGS_DEFAULT``, this 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``.


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_NULLARG``: ``cc_func(PyObject *self, PyObject *null)``
  (the function takes no arguments but a ``NULL`` is passed to the C function)

- ``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**: in the case of bound methods, it is currently unspecified
whether the "function object" in the paragraph above refers
to the bound method or the original function (which is wrapped by the bound method).
In the reference implementation, the bound method is passed.
In the future, this may change to the wrapped function.
Despite this ambiguity, the implementation of bound methods
guarantees that ``PyCCall_CCALLDEF(func)``
points to the ``PyCCallDef`` of the original function.

**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.
There are also no guarantees of binary compatibility
between Python versions for these flags.

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

If the ``CCALL_OBJCLASS`` flag is set and if ``cr_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 ``cr_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 ``cr_self``.

If ``cr_self`` is NULL and the flag ``CCALL_SLICE_SELF`` is set,
then the first positional argument is removed from
``args`` and instead passed as first argument to the C function.
Effectively, the first positional argument is treated as ``__self__``.
If there are no positional arguments, ``TypeError`` is raised.

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

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

Descriptor behavior
-------------------

Classes supporting the C call protocol
must implement the descriptor protocol in a specific way.
This is required for an efficient implementation of bound methods:
it allows sharing the ``PyCCallDef`` structure between bound and unbound methods.
It is also needed for a correct implementation of ``_PyObject_GetMethod``
which is used by the ``LOAD_METHOD``/``CALL_METHOD`` optimization.
First of all, if ``func`` supports the C call protocol,
then ``func.__set__`` must not be implemented.

Second, ``func.__get__`` must behave as follows:

- If ``cr_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 ``cr_self`` is NULL, then ``func.__get__(obj, cls)(*args, **kwds)``
  (with ``obj`` not None)
  must be equivalent to ``func(obj, *args, **kwds)``.
  In particular, ``__get__`` must be implemented in this case.
  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 ``cr_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 ``cr_self = Py_None``
(or any other non-NULL value).

__name__ attribute
------------------

The C call protocol requires that the function has a ``__name__``
attribute which is of type ``str`` (not a subclass).

Furthermore, this must be idempotent in the sense
that getting the ``__name__`` attribute twice in a row must return
exactly the same Python object.
This implies that it cannot be a temporary object, it must be stored somewhere.
This is required because ``PyEval_GetFuncName`` and ``PyEval_GetFuncDesc``
use borrowed references to the ``__name__`` attribute.

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

This section lists the new public API functions dealing with the C call protocol.

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

All the functions and macros below
apply to any instance supporting the C call protocol.
In other words, ``PyCCall_Check(func)`` must be true.

- ``PyObject * PyCCall_Call(PyObject *func, PyObject *args, PyObject *kwds)``:
  call ``func`` 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`` 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]``.

Macros to access the ``PyCCallRoot`` and ``PyCCallDef`` structures:

- ``PyCCallRoot * PyCCall_CCALLROOT(PyObject *func)``:
  pointer to the ``PyCCallRoot`` structure inside ``func``.

- ``PyCCallDef * PyCCall_CCALLDEF(PyObject *func)``:
  shorthand for ``PyCCall_CCALLROOT(func)->cr_ccall``.

- ``PyCCallDef * PyCCall_FLAGS(PyObject *func)``:
  shorthand for ``PyCCall_CCALLROOT(func)->cr_ccall->cc_flags``.

- ``PyObject * PyCCall_SELF(PyOject *func)``:
  shorthand for ``PyCCall_CCALLROOT(func)->cr_self``.

Generic getters, meant to be put into the ``tp_getset`` array:

- ``PyObject * PyCCall_GenericGetParent(PyObject *func, void *closure)``:
  return ``cc_parent``.
  Raise ``AttributeError`` if ``cc_parent`` is NULL.

- ``PyObject * PyCCall_GenericGetQualname(PyObject *func, void *closure)``:
  return a string suitable for using as ``__qualname__``.
  This uses the ``__qualname__`` of ``cc_parent`` if possible.
  It also uses the ``__name__`` attribute.

- ``PyObject * PyCCall_GenericGetSelf(PyObject *func, void *closure)``:
  return ``cr_self``.
  Raise ``AttributeError`` if ``cr_self`` is NULL.

Profiling
---------

The profiling events
``c_call``, ``c_return`` and ``c_exception`` are only generated
when calling actual instances of ``builtin_function_or_method`` or ``method_descriptor``.
This is done for simplicity and also for backwards compatibility
(such that the profile function does not receive objects that it does not recognize).
In a future PEP, we may extend C-level profiling to arbitrary classes
implementing the C call protocol.


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 fact, those two classes are almost merged:
the implementation becomes very similar, but they remain separate classes
(mostly for backwards compatibility).
The ``PyCCallDef`` structure is simply stored
as part of the object structure.
Both classes use ``PyCFunctionObject`` as object structure.
This is the new layout::

    typedef struct {
        PyObject_HEAD
        PyCCallDef  *m_ccall;
        PyObject    *m_self;         /* Passed as 'self' arg to the C function */
        PyCCallDef   _ccalldef;      /* Storage for m_ccall */
        PyObject    *m_name;         /* __name__; str object (not NULL) */
        PyObject    *m_module;       /* __module__; can be anything */
        const char  *m_doc;          /* __text_signature__ and __doc__ */
        PyObject    *m_weakreflist;  /* List of weak references */
    } PyCFunctionObject;

For functions of a module and for unbound methods of extension types,
``m_ccall`` points to the ``_ccalldef`` field.
For bound methods, ``m_ccall`` points to the ``PyCCallDef``
of the unbound method.

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

C API functions
---------------

The following function is added (also to the stable ABI [#pep384]_):

- ``PyObject * PyCFunction_ClsNew(PyTypeObject *cls, PyMethodDef *ml, PyObject *self, PyObject *module, PyObject *parent)``:
  create a new object with object structure ``PyCFunctionObject`` and class ``cls``.
  This is called in turn by ``PyCFunction_NewEx`` and ``PyDescr_NewMethod``.

The undocumented functions ``PyCFunction_GetFlags``
and ``PyCFunction_GET_FLAGS``
are removed because it would be non-trivial to support them
in a backwards-compatible way.


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).


Performance
===========

This PEP should not impact the performance of existing code
(in the positive or negative sense).
It is meant to allow efficient new code to be written,
not to make existing code faster.


Stable ABI
==========

None of the functions, structures or constants dealing with the C call protocol
are added to the stable ABI [#pep384]_.

There are two reasons for this:
first of all, the most useful feature of the C call protocol is probably the
``METH_FASTCALL`` calling convention.
Given that this is not even part of the public API (see also PEP 579, issue 6),
it would be strange to add anything else from the C call protocol
to the stable ABI.

Second, we want the C call protocol to be extensible in the future.
By not adding anything to the stable ABI,
we are free to do that without restrictions.


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).

The removed function ``PyCFunction_GetFlags``,
is officially part of the stable ABI [#pep384]_.
However, this is probably an oversight:
first of all, it is not even documented.
Second, the flag ``METH_FASTCALL``
is not part of the stable ABI but it is very common
(because of Argument Clinic).
So, if one cannot support ``METH_FASTCALL``,
it is hard to imagine a use case for ``PyCFunction_GetFlags``.
The fact that ``PyCFunction_GET_FLAGS`` and ``PyCFunction_GetFlags``
are not used at all by CPython outside of ``Objects/call.c``
further shows that these functions are not particularly useful.

Concluding: the only potential breakage is with C code
which accesses 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.

Note that 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.

Why CCALL_OBJCLASS?
-------------------

The flag ``CCALL_OBJCLASS`` is meant to support various cases
where the class of a ``self`` argument must be checked, such as::

    >>> list.append({}, None)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: append() requires a 'list' object but received a 'dict'

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

    >>> float.__dict__["fromhex"](list, "0xff")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: descriptor 'fromhex' for type 'float' doesn't apply to type 'list'

In the reference implementation, only the first of these uses the new code.
The other examples show that these kind of checks appear
in multiple places, so it makes sense to add generic support for them.

Why CCALL_SLICE_SELF?
---------------------

The flag ``CCALL_SLICE_SELF`` and the concept of self slicing
are needed to support methods:
the C function should not care
whether it is called as unbound method or as bound method.
In both cases, there should be a ``self`` argument
and this is simply the first positional argument of an unbound method call.

For example, ``list.append`` is a ``METH_O`` method.
Both the calls ``list.append([], 42)`` and ``[].append(42)`` should
translate to the C call ``list_append([], 42)``.

Thanks to the proposed C call protocol, we can support this in such a way
that both the unbound and the bound method share a ``PyCCallDef``
structure (with the ``CCALL_SLICE_SELF`` flag set).

Concluding, ``CCALL_SLICE_SELF`` has two advantages:
there is no extra layer of indirection for calling
and constructing bound methods does not require setting up a ``PyCCallDef`` structure.

Replacing tp_print
------------------

We repurpose ``tp_print`` as ``tp_ccalloffset`` because this makes
it easier for external projects to backport the C call protocol
to earlier Python versions.
In particular, the Cython project has shown interest in doing that
(see https://mail.python.org/pipermail/python-dev/2018-June/153927.html).


Alternative suggestions
=======================

PEP 576 is an alternative approach to solving the same problem as this PEP.
See https://mail.python.org/pipermail/python-dev/2018-July/154238.html
for comments on the difference between PEP 576 and PEP 580.


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

The reference implementation can be found at
https://github.com/jdemeyer/cpython/tree/pep580


References
==========

.. [#pep384] Löwis, PEP 384 – Defining a Stable ABI,
             https://www.python.org/dev/peps/pep-0384/

.. [#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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