425 lines
17 KiB
ReStructuredText
425 lines
17 KiB
ReStructuredText
.. highlight:: c
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.. _call:
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Call Protocol
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=============
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CPython supports two different calling protocols:
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*tp_call* and vectorcall.
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The *tp_call* Protocol
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----------------------
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Instances of classes that set :c:member:`~PyTypeObject.tp_call` are callable.
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The signature of the slot is::
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PyObject *tp_call(PyObject *callable, PyObject *args, PyObject *kwargs);
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A call is made using a tuple for the positional arguments
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and a dict for the keyword arguments, similarly to
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``callable(*args, **kwargs)`` in Python code.
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*args* must be non-NULL (use an empty tuple if there are no arguments)
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but *kwargs* may be *NULL* if there are no keyword arguments.
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This convention is not only used by *tp_call*:
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:c:member:`~PyTypeObject.tp_new` and :c:member:`~PyTypeObject.tp_init`
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also pass arguments this way.
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To call an object, use :c:func:`PyObject_Call` or other
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:ref:`call API <capi-call>`.
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.. _vectorcall:
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The Vectorcall Protocol
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-----------------------
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.. versionadded:: 3.9
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The vectorcall protocol was introduced in :pep:`590` as an additional protocol
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for making calls more efficient.
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As rule of thumb, CPython will prefer the vectorcall for internal calls
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if the callable supports it. However, this is not a hard rule.
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Additionally, some third-party extensions use *tp_call* directly
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(rather than using :c:func:`PyObject_Call`).
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Therefore, a class supporting vectorcall must also implement
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:c:member:`~PyTypeObject.tp_call`.
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Moreover, the callable must behave the same
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regardless of which protocol is used.
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The recommended way to achieve this is by setting
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:c:member:`~PyTypeObject.tp_call` to :c:func:`PyVectorcall_Call`.
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This bears repeating:
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.. warning::
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A class supporting vectorcall **must** also implement
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:c:member:`~PyTypeObject.tp_call` with the same semantics.
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A class should not implement vectorcall if that would be slower
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than *tp_call*. For example, if the callee needs to convert
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the arguments to an args tuple and kwargs dict anyway, then there is no point
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in implementing vectorcall.
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Classes can implement the vectorcall protocol by enabling the
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:const:`Py_TPFLAGS_HAVE_VECTORCALL` flag and setting
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:c:member:`~PyTypeObject.tp_vectorcall_offset` to the offset inside the
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object structure where a *vectorcallfunc* appears.
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This is a pointer to a function with the following signature:
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.. c:type:: PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwnames)
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- *callable* is the object being called.
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- *args* is a C array consisting of the positional arguments followed by the
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values of the keyword arguments.
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This can be *NULL* if there are no arguments.
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- *nargsf* is the number of positional arguments plus possibly the
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:const:`PY_VECTORCALL_ARGUMENTS_OFFSET` flag.
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To get the actual number of positional arguments from *nargsf*,
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use :c:func:`PyVectorcall_NARGS`.
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- *kwnames* is a tuple containing the names of the keyword arguments;
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in other words, the keys of the kwargs dict.
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These names must be strings (instances of ``str`` or a subclass)
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and they must be unique.
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If there are no keyword arguments, then *kwnames* can instead be *NULL*.
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.. c:var:: PY_VECTORCALL_ARGUMENTS_OFFSET
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If this flag is set in a vectorcall *nargsf* argument, the callee is allowed
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to temporarily change ``args[-1]``. In other words, *args* points to
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argument 1 (not 0) in the allocated vector.
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The callee must restore the value of ``args[-1]`` before returning.
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For :c:func:`PyObject_VectorcallMethod`, this flag means instead that
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``args[0]`` may be changed.
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Whenever they can do so cheaply (without additional allocation), callers
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are encouraged to use :const:`PY_VECTORCALL_ARGUMENTS_OFFSET`.
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Doing so will allow callables such as bound methods to make their onward
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calls (which include a prepended *self* argument) very efficiently.
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To call an object that implements vectorcall, use a :ref:`call API <capi-call>`
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function as with any other callable.
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:c:func:`PyObject_Vectorcall` will usually be most efficient.
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.. note::
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In CPython 3.8, the vectorcall API and related functions were available
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provisionally under names with a leading underscore:
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``_PyObject_Vectorcall``, ``_Py_TPFLAGS_HAVE_VECTORCALL``,
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``_PyObject_VectorcallMethod``, ``_PyVectorcall_Function``,
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``_PyObject_CallOneArg``, ``_PyObject_CallMethodNoArgs``,
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``_PyObject_CallMethodOneArg``.
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Additionally, ``PyObject_VectorcallDict`` was available as
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``_PyObject_FastCallDict``.
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The old names are still defined as aliases of the new, non-underscored names.
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Recursion Control
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.................
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When using *tp_call*, callees do not need to worry about
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:ref:`recursion <recursion>`: CPython uses
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:c:func:`Py_EnterRecursiveCall` and :c:func:`Py_LeaveRecursiveCall`
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for calls made using *tp_call*.
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For efficiency, this is not the case for calls done using vectorcall:
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the callee should use *Py_EnterRecursiveCall* and *Py_LeaveRecursiveCall*
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if needed.
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Vectorcall Support API
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......................
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.. c:function:: Py_ssize_t PyVectorcall_NARGS(size_t nargsf)
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Given a vectorcall *nargsf* argument, return the actual number of
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arguments.
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Currently equivalent to::
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(Py_ssize_t)(nargsf & ~PY_VECTORCALL_ARGUMENTS_OFFSET)
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However, the function ``PyVectorcall_NARGS`` should be used to allow
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for future extensions.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.8
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.. c:function:: vectorcallfunc PyVectorcall_Function(PyObject *op)
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If *op* does not support the vectorcall protocol (either because the type
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does not or because the specific instance does not), return *NULL*.
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Otherwise, return the vectorcall function pointer stored in *op*.
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This function never raises an exception.
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This is mostly useful to check whether or not *op* supports vectorcall,
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which can be done by checking ``PyVectorcall_Function(op) != NULL``.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.8
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.. c:function:: PyObject* PyVectorcall_Call(PyObject *callable, PyObject *tuple, PyObject *dict)
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Call *callable*'s :c:type:`vectorcallfunc` with positional and keyword
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arguments given in a tuple and dict, respectively.
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This is a specialized function, intended to be put in the
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:c:member:`~PyTypeObject.tp_call` slot or be used in an implementation of ``tp_call``.
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It does not check the :const:`Py_TPFLAGS_HAVE_VECTORCALL` flag
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and it does not fall back to ``tp_call``.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.8
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.. _capi-call:
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Object Calling API
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------------------
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Various functions are available for calling a Python object.
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Each converts its arguments to a convention supported by the called object –
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either *tp_call* or vectorcall.
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In order to do as litle conversion as possible, pick one that best fits
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the format of data you have available.
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The following table summarizes the available functions;
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please see individual documentation for details.
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+------------------------------------------+------------------+--------------------+---------------+
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| Function | callable | args | kwargs |
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+==========================================+==================+====================+===============+
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| :c:func:`PyObject_Call` | ``PyObject *`` | tuple | dict/``NULL`` |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallNoArgs` | ``PyObject *`` | --- | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallOneArg` | ``PyObject *`` | 1 object | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallObject` | ``PyObject *`` | tuple/``NULL`` | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallFunction` | ``PyObject *`` | format | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallMethod` | obj + ``char*`` | format | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallFunctionObjArgs` | ``PyObject *`` | variadic | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallMethodObjArgs` | obj + name | variadic | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallMethodNoArgs` | obj + name | --- | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_CallMethodOneArg` | obj + name | 1 object | --- |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_Vectorcall` | ``PyObject *`` | vectorcall | vectorcall |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_VectorcallDict` | ``PyObject *`` | vectorcall | dict/``NULL`` |
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+------------------------------------------+------------------+--------------------+---------------+
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| :c:func:`PyObject_VectorcallMethod` | arg + name | vectorcall | vectorcall |
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+------------------------------------------+------------------+--------------------+---------------+
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.. c:function:: PyObject* PyObject_Call(PyObject *callable, PyObject *args, PyObject *kwargs)
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Call a callable Python object *callable*, with arguments given by the
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tuple *args*, and named arguments given by the dictionary *kwargs*.
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*args* must not be *NULL*; use an empty tuple if no arguments are needed.
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If no named arguments are needed, *kwargs* can be *NULL*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This is the equivalent of the Python expression:
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``callable(*args, **kwargs)``.
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.. c:function:: PyObject* PyObject_CallNoArgs(PyObject *callable)
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Call a callable Python object *callable* without any arguments. It is the
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most efficient way to call a callable Python object without any argument.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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.. versionadded:: 3.9
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.. c:function:: PyObject* PyObject_CallOneArg(PyObject *callable, PyObject *arg)
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Call a callable Python object *callable* with exactly 1 positional argument
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*arg* and no keyword arguments.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.9
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.. c:function:: PyObject* PyObject_CallObject(PyObject *callable, PyObject *args)
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Call a callable Python object *callable*, with arguments given by the
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tuple *args*. If no arguments are needed, then *args* can be *NULL*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This is the equivalent of the Python expression: ``callable(*args)``.
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.. c:function:: PyObject* PyObject_CallFunction(PyObject *callable, const char *format, ...)
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Call a callable Python object *callable*, with a variable number of C arguments.
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The C arguments are described using a :c:func:`Py_BuildValue` style format
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string. The format can be *NULL*, indicating that no arguments are provided.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This is the equivalent of the Python expression: ``callable(*args)``.
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Note that if you only pass :c:type:`PyObject \*` args,
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:c:func:`PyObject_CallFunctionObjArgs` is a faster alternative.
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.. versionchanged:: 3.4
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The type of *format* was changed from ``char *``.
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.. c:function:: PyObject* PyObject_CallMethod(PyObject *obj, const char *name, const char *format, ...)
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Call the method named *name* of object *obj* with a variable number of C
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arguments. The C arguments are described by a :c:func:`Py_BuildValue` format
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string that should produce a tuple.
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The format can be *NULL*, indicating that no arguments are provided.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This is the equivalent of the Python expression:
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``obj.name(arg1, arg2, ...)``.
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Note that if you only pass :c:type:`PyObject \*` args,
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:c:func:`PyObject_CallMethodObjArgs` is a faster alternative.
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.. versionchanged:: 3.4
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The types of *name* and *format* were changed from ``char *``.
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.. c:function:: PyObject* PyObject_CallFunctionObjArgs(PyObject *callable, ..., NULL)
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Call a callable Python object *callable*, with a variable number of
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:c:type:`PyObject \*` arguments. The arguments are provided as a variable number
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of parameters followed by *NULL*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This is the equivalent of the Python expression:
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``callable(arg1, arg2, ...)``.
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.. c:function:: PyObject* PyObject_CallMethodObjArgs(PyObject *obj, PyObject *name, ..., NULL)
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Call a method of the Python object *obj*, where the name of the method is given as a
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Python string object in *name*. It is called with a variable number of
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:c:type:`PyObject \*` arguments. The arguments are provided as a variable number
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of parameters followed by *NULL*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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.. c:function:: PyObject* PyObject_CallMethodNoArgs(PyObject *obj, PyObject *name)
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Call a method of the Python object *obj* without arguments,
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where the name of the method is given as a Python string object in *name*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.9
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.. c:function:: PyObject* PyObject_CallMethodOneArg(PyObject *obj, PyObject *name, PyObject *arg)
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Call a method of the Python object *obj* with a single positional argument
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*arg*, where the name of the method is given as a Python string object in
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*name*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.9
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.. c:function:: PyObject* PyObject_Vectorcall(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwnames)
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Call a callable Python object *callable*.
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The arguments are the same as for :c:type:`vectorcallfunc`.
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If *callable* supports vectorcall_, this directly calls
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the vectorcall function stored in *callable*.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.9
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.. c:function:: PyObject* PyObject_VectorcallDict(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwdict)
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Call *callable* with positional arguments passed exactly as in the vectorcall_ protocol,
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but with keyword arguments passed as a dictionary *kwdict*.
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The *args* array contains only the positional arguments.
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Regardless of which protocol is used internally,
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a conversion of arguments needs to be done.
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Therefore, this function should only be used if the caller
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already has a dictionary ready to use for the keyword arguments,
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but not a tuple for the positional arguments.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.9
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.. c:function:: PyObject* PyObject_VectorcallMethod(PyObject *name, PyObject *const *args, size_t nargsf, PyObject *kwnames)
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Call a method using the vectorcall calling convention. The name of the method
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is given as a Python string *name*. The object whose method is called is
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*args[0]*, and the *args* array starting at *args[1]* represents the arguments
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of the call. There must be at least one positional argument.
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*nargsf* is the number of positional arguments including *args[0]*,
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plus :const:`PY_VECTORCALL_ARGUMENTS_OFFSET` if the value of ``args[0]`` may
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temporarily be changed. Keyword arguments can be passed just like in
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:c:func:`PyObject_Vectorcall`.
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If the object has the :const:`Py_TPFLAGS_METHOD_DESCRIPTOR` feature,
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this will call the unbound method object with the full
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*args* vector as arguments.
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Return the result of the call on success, or raise an exception and return
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*NULL* on failure.
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This function is not part of the :ref:`limited API <stable>`.
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.. versionadded:: 3.9
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Call Support API
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----------------
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.. c:function:: int PyCallable_Check(PyObject *o)
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Determine if the object *o* is callable. Return ``1`` if the object is callable
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and ``0`` otherwise. This function always succeeds.
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