Call Protocol — Python 3.9.21 documentation

# 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 another 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:

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.

  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)

  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.

  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.

  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.

  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.

  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.

  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)

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