Call Protocol

CPython supports two different calling protocols: tp_call and vectorcall.

The tp_call Protocol

Instances of classes that set tp_call are callable. The signature of the slot is:

PyObject *tp_call(PyObject *callable, PyObject *args, PyObject *kwargs);

A call is made using a tuple for the positional arguments and a dict for the keyword arguments, similarly to callable(*args, **kwargs) in Python code. args must be non-NULL (use an empty tuple if there are no arguments) but kwargs may be NULL if there are no keyword arguments.

This convention is not only used by tp_call: tp_new and tp_init also pass arguments this way.

To call an object, use PyObject_Call() or other call API.

The Vectorcall Protocol

New in version 3.9.

The vectorcall protocol was introduced in PEP 590 as an additional protocol for making calls more efficient.

As rule of thumb, CPython will prefer the vectorcall for internal calls if the callable supports it. However, this is not a hard rule. Additionally, some third-party extensions use tp_call directly (rather than using PyObject_Call()). Therefore, a class supporting vectorcall must also implement tp_call. Moreover, the callable must behave the same regardless of which protocol is used. The recommended way to achieve this is by setting tp_call to PyVectorcall_Call(). This bears repeating:

Warning

A class supporting vectorcall must also implement tp_call with the same semantics.

A class should not implement vectorcall if that would be slower than tp_call. For example, if the callee needs to convert the arguments to an args tuple and kwargs dict anyway, then there is no point in implementing vectorcall.

Classes can implement the vectorcall protocol by enabling the Py_TPFLAGS_HAVE_VECTORCALL flag and setting tp_vectorcall_offset to the offset inside the object structure where a vectorcallfunc appears. This is a pointer to a function with the following signature:

typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwnames)

• callable is the object being called.

• args is a C array consisting of the positional arguments followed by the

  values of the keyword arguments. This can be NULL if there are no arguments.

• nargsf is the number of positional arguments plus possibly the

  PY_VECTORCALL_ARGUMENTS_OFFSET flag. To get the actual number of positional arguments from nargsf, use PyVectorcall_NARGS().

• kwnames is a tuple containing the names of the keyword arguments;

  in other words, the keys of the kwargs dict. These names must be strings (instances of str or a subclass) and they must be unique. If there are no keyword arguments, then kwnames can instead be NULL.

PY_VECTORCALL_ARGUMENTS_OFFSET

If this flag is set in a vectorcall nargsf argument, the callee is allowed to temporarily change args[-1]. In other words, args points to argument 1 (not 0) in the allocated vector. The callee must restore the value of args[-1] before returning.

For PyObject_VectorcallMethod(), this flag means instead that args[0] may be changed.

Whenever they can do so cheaply (without additional allocation), callers are encouraged to use PY_VECTORCALL_ARGUMENTS_OFFSET. Doing so will allow callables such as bound methods to make their onward calls (which include a prepended self argument) very efficiently.

To call an object that implements vectorcall, use a call API function as with any other callable. PyObject_Vectorcall() will usually be most efficient.

Note

In CPython 3.8, the vectorcall API and related functions were available provisionally under names with a leading underscore: _PyObject_Vectorcall, _Py_TPFLAGS_HAVE_VECTORCALL, _PyObject_VectorcallMethod, _PyVectorcall_Function, _PyObject_CallOneArg, _PyObject_CallMethodNoArgs, _PyObject_CallMethodOneArg. Additionally, PyObject_VectorcallDict was available as _PyObject_FastCallDict. The old names are still defined as aliases of the new, non-underscored names.

Recursion Control

When using tp_call, callees do not need to worry about recursion: CPython uses Py_EnterRecursiveCall() and Py_LeaveRecursiveCall() for calls made using tp_call.

For efficiency, this is not the case for calls done using vectorcall: the callee should use Py_EnterRecursiveCall and Py_LeaveRecursiveCall if needed.

Vectorcall Support API

Py_ssize_t PyVectorcall_NARGS(size_t nargsf)

Given a vectorcall nargsf argument, return the actual number of arguments. Currently equivalent to:

(Py_ssize_t)(nargsf & ~PY_VECTORCALL_ARGUMENTS_OFFSET)

However, the function PyVectorcall_NARGS should be used to allow for future extensions.

This function is not part of the limited API.

New in version 3.8.

vectorcallfunc PyVectorcall_Function(PyObject *op)

If op does not support the vectorcall protocol (either because the type does not or because the specific instance does not), return NULL. Otherwise, return the vectorcall function pointer stored in op. This function never raises an exception.

This is mostly useful to check whether or not op supports vectorcall, which can be done by checking PyVectorcall_Function(op) != NULL.

This function is not part of the limited API.

New in version 3.8.

PyObject *PyVectorcall_Call(PyObject *callable, PyObject *tuple, PyObject *dict)

Call callable’s vectorcallfunc with positional and keyword arguments given in a tuple and dict, respectively.

This is a specialized function, intended to be put in the tp_call slot or be used in an implementation of tp_call. It does not check the Py_TPFLAGS_HAVE_VECTORCALL flag and it does not fall back to tp_call.

This function is not part of the limited API.

New in version 3.8.

Object Calling API

Various functions are available for calling a Python object. Each converts its arguments to a convention supported by the called object – either tp_call or vectorcall. In order to do as little conversion as possible, pick one that best fits the format of data you have available.

The following table summarizes the available functions; please see individual documentation for details.

Function	callable	args	kwargs
PyObject_Call()	PyObject *	tuple	dict/NULL
PyObject_CallNoArgs()	PyObject *	—	—
PyObject_CallOneArg()	PyObject *	1 object	—
PyObject_CallObject()	PyObject *	tuple/NULL	—
PyObject_CallFunction()	PyObject *	format	—
PyObject_CallMethod()	obj + char*	format	—
PyObject_CallFunctionObjArgs()	PyObject *	variadic	—
PyObject_CallMethodObjArgs()	obj + name	variadic	—
PyObject_CallMethodNoArgs()	obj + name	—	—
PyObject_CallMethodOneArg()	obj + name	1 object	—
PyObject_Vectorcall()	PyObject *	vectorcall	vectorcall
PyObject_VectorcallDict()	PyObject *	vectorcall	dict/NULL
PyObject_VectorcallMethod()	arg + name	vectorcall	vectorcall

PyObject *PyObject_Call(PyObject *callable, PyObject *args, PyObject *kwargs)

Return value: New reference. Part of the Stable ABI.

Call a callable Python object callable, with arguments given by the tuple args, and named arguments given by the dictionary kwargs.

args must not be NULL; use an empty tuple if no arguments are needed. If no named arguments are needed, kwargs can be NULL.

Return the result of the call on success, or raise an exception and return NULL on failure.

This is the equivalent of the Python expression: callable(*args, **kwargs).

PyObject *PyObject_CallNoArgs(PyObject *callable)

Part of the Stable ABI since version 3.10.

Call a callable Python object callable without any arguments. It is the most efficient way to call a callable Python object without any argument.

Return the result of the call on success, or raise an exception and return NULL on failure.

New in version 3.9.

PyObject *PyObject_CallOneArg(PyObject *callable, PyObject *arg)

Call a callable Python object callable with exactly 1 positional argument arg and no keyword arguments.

Return the result of the call on success, or raise an exception and return NULL on failure.

This function is not part of the limited API.

New in version 3.9.

PyObject *PyObject_CallObject(PyObject *callable, PyObject *args)

Return value: New reference. Part of the Stable ABI.

Call a callable Python object callable, with arguments given by the tuple args. If no arguments are needed, then args can be NULL.

Return the result of the call on success, or raise an exception and return NULL on failure.

This is the equivalent of the Python expression: callable(*args).

PyObject *PyObject_CallFunction(PyObject *callable, const char *format, ...)

Return value: New reference. Part of the Stable ABI.

Call a callable Python object callable, with a variable number of C arguments. The C arguments are described using a Py_BuildValue() style format string. The format can be NULL, indicating that no arguments are provided.

Return the result of the call on success, or raise an exception and return NULL on failure.

This is the equivalent of the Python expression: callable(*args).

Note that if you only pass PyObject* args, PyObject_CallFunctionObjArgs() is a faster alternative.

Changed in version 3.4: The type of format was changed from char *.

PyObject *PyObject_CallMethod(PyObject *obj, const char *name, const char *format, ...)

Return value: New reference. Part of the Stable ABI.

Call the method named name of object obj with a variable number of C arguments. The C arguments are described by a Py_BuildValue() format string that should produce a tuple.

The format can be NULL, indicating that no arguments are provided.

Return the result of the call on success, or raise an exception and return NULL on failure.

This is the equivalent of the Python expression: obj.name(arg1, arg2, ...).

Note that if you only pass PyObject* args, PyObject_CallMethodObjArgs() is a faster alternative.

Changed in version 3.4: The types of name and format were changed from char *.

PyObject *PyObject_CallFunctionObjArgs(PyObject *callable, ...)

Return value: New reference. Part of the Stable ABI.

Call a callable Python object callable, with a variable number of PyObject* arguments. The arguments are provided as a variable number of parameters followed by NULL.

Return the result of the call on success, or raise an exception and return NULL on failure.

This is the equivalent of the Python expression: callable(arg1, arg2, ...).

PyObject *PyObject_CallMethodObjArgs(PyObject *obj, PyObject *name, ...)

Return value: New reference. Part of the Stable ABI.

Call a method of the Python object obj, where the name of the method is given as a Python string object in name. It is called with a variable number of PyObject* arguments. The arguments are provided as a variable number of parameters followed by NULL.

Return the result of the call on success, or raise an exception and return NULL on failure.

PyObject *PyObject_CallMethodNoArgs(PyObject *obj, PyObject *name)

Call a method of the Python object obj without arguments, where the name of the method is given as a Python string object in name.

Return the result of the call on success, or raise an exception and return NULL on failure.

This function is not part of the limited API.

New in version 3.9.

PyObject *PyObject_CallMethodOneArg(PyObject *obj, PyObject *name, PyObject *arg)

Call a method of the Python object obj with a single positional argument arg, where the name of the method is given as a Python string object in name.

Return the result of the call on success, or raise an exception and return NULL on failure.

This function is not part of the limited API.

New in version 3.9.

PyObject *PyObject_Vectorcall(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwnames)

Call a callable Python object callable. The arguments are the same as for vectorcallfunc. If callable supports vectorcall, this directly calls the vectorcall function stored in callable.

Return the result of the call on success, or raise an exception and return NULL on failure.

This function is not part of the limited API.

New in version 3.9.

PyObject *PyObject_VectorcallDict(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwdict)

Call callable with positional arguments passed exactly as in the vectorcall protocol, but with keyword arguments passed as a dictionary kwdict. The args array contains only the positional arguments.

Regardless of which protocol is used internally, a conversion of arguments needs to be done. Therefore, this function should only be used if the caller already has a dictionary ready to use for the keyword arguments, but not a tuple for the positional arguments.

This function is not part of the limited API.

New in version 3.9.

PyObject *PyObject_VectorcallMethod(PyObject *name, PyObject *const *args, size_t nargsf, PyObject *kwnames)

Call a method using the vectorcall calling convention. The name of the method is given as a Python string name. The object whose method is called is args[0], and the args array starting at args[1] represents the arguments of the call. There must be at least one positional argument. nargsf is the number of positional arguments including args[0], plus PY_VECTORCALL_ARGUMENTS_OFFSET if the value of args[0] may temporarily be changed. Keyword arguments can be passed just like in PyObject_Vectorcall().

If the object has the Py_TPFLAGS_METHOD_DESCRIPTOR feature, this will call the unbound method object with the full args vector as arguments.

Return the result of the call on success, or raise an exception and return NULL on failure.

This function is not part of the limited API.

New in version 3.9.

Call Support API

int PyCallable_Check(PyObject *o)

Part of the Stable ABI.

Determine if the object o is callable. Return 1 if the object is callable and 0 otherwise. This function always succeeds.