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.. highlight:: c
.. _common-structs:
Common Object Structures
========================
There are a large number of structures which are used in the definition of
object types for Python. This section describes these structures and how they
are used.
Base object types and macros
----------------------------
All Python objects ultimately share a small number of fields at the beginning
of the object's representation in memory. These are represented by the
:c:type:`PyObject` and :c:type:`PyVarObject` types, which are defined, in turn,
by the expansions of some macros also used, whether directly or indirectly, in
the definition of all other Python objects. Additional macros can be found
under :ref:`reference counting <countingrefs>`.
.. c:type:: PyObject
All object types are extensions of this type. This is a type which
contains the information Python needs to treat a pointer to an object as an
object. In a normal "release" build, it contains only the object's
reference count and a pointer to the corresponding type object.
Nothing is actually declared to be a :c:type:`PyObject`, but every pointer
to a Python object can be cast to a :c:expr:`PyObject*`. Access to the
members must be done by using the macros :c:macro:`Py_REFCNT` and
:c:macro:`Py_TYPE`.
.. c:type:: PyVarObject
This is an extension of :c:type:`PyObject` that adds the :attr:`ob_size`
field. This is only used for objects that have some notion of *length*.
This type does not often appear in the Python/C API.
Access to the members must be done by using the macros
:c:macro:`Py_REFCNT`, :c:macro:`Py_TYPE`, and :c:macro:`Py_SIZE`.
.. c:macro:: PyObject_HEAD
This is a macro used when declaring new types which represent objects
without a varying length. The PyObject_HEAD macro expands to::
PyObject ob_base;
See documentation of :c:type:`PyObject` above.
.. c:macro:: PyObject_VAR_HEAD
This is a macro used when declaring new types which represent objects
with a length that varies from instance to instance.
The PyObject_VAR_HEAD macro expands to::
PyVarObject ob_base;
See documentation of :c:type:`PyVarObject` above.
.. c:function:: int Py_Is(PyObject *x, PyObject *y)
Test if the *x* object is the *y* object, the same as ``x is y`` in Python.
.. versionadded:: 3.10
.. c:function:: int Py_IsNone(PyObject *x)
Test if an object is the ``None`` singleton,
the same as ``x is None`` in Python.
.. versionadded:: 3.10
.. c:function:: int Py_IsTrue(PyObject *x)
Test if an object is the ``True`` singleton,
the same as ``x is True`` in Python.
.. versionadded:: 3.10
.. c:function:: int Py_IsFalse(PyObject *x)
Test if an object is the ``False`` singleton,
the same as ``x is False`` in Python.
.. versionadded:: 3.10
.. c:function:: PyTypeObject* Py_TYPE(PyObject *o)
Get the type of the Python object *o*.
Return a :term:`borrowed reference`.
Use the :c:func:`Py_SET_TYPE` function to set an object type.
.. versionchanged:: 3.11
:c:func:`Py_TYPE()` is changed to an inline static function.
The parameter type is no longer :c:expr:`const PyObject*`.
.. c:function:: int Py_IS_TYPE(PyObject *o, PyTypeObject *type)
Return non-zero if the object *o* type is *type*. Return zero otherwise.
Equivalent to: ``Py_TYPE(o) == type``.
.. versionadded:: 3.9
.. c:function:: void Py_SET_TYPE(PyObject *o, PyTypeObject *type)
Set the object *o* type to *type*.
.. versionadded:: 3.9
.. c:function:: Py_ssize_t Py_SIZE(PyVarObject *o)
Get the size of the Python object *o*.
Use the :c:func:`Py_SET_SIZE` function to set an object size.
.. versionchanged:: 3.11
:c:func:`Py_SIZE()` is changed to an inline static function.
The parameter type is no longer :c:expr:`const PyVarObject*`.
.. c:function:: void Py_SET_SIZE(PyVarObject *o, Py_ssize_t size)
Set the object *o* size to *size*.
.. versionadded:: 3.9
.. c:macro:: PyObject_HEAD_INIT(type)
This is a macro which expands to initialization values for a new
:c:type:`PyObject` type. This macro expands to::
_PyObject_EXTRA_INIT
1, type,
.. c:macro:: PyVarObject_HEAD_INIT(type, size)
This is a macro which expands to initialization values for a new
:c:type:`PyVarObject` type, including the :attr:`ob_size` field.
This macro expands to::
_PyObject_EXTRA_INIT
1, type, size,
Implementing functions and methods
----------------------------------
.. c:type:: PyCFunction
Type of the functions used to implement most Python callables in C.
Functions of this type take two :c:expr:`PyObject*` parameters and return
one such value. If the return value is ``NULL``, an exception shall have
been set. If not ``NULL``, the return value is interpreted as the return
value of the function as exposed in Python. The function must return a new
reference.
The function signature is::
PyObject *PyCFunction(PyObject *self,
PyObject *args);
.. c:type:: PyCFunctionWithKeywords
Type of the functions used to implement Python callables in C
with signature :const:`METH_VARARGS | METH_KEYWORDS`.
The function signature is::
PyObject *PyCFunctionWithKeywords(PyObject *self,
PyObject *args,
PyObject *kwargs);
.. c:type:: _PyCFunctionFast
Type of the functions used to implement Python callables in C
with signature :const:`METH_FASTCALL`.
The function signature is::
PyObject *_PyCFunctionFast(PyObject *self,
PyObject *const *args,
Py_ssize_t nargs);
.. c:type:: _PyCFunctionFastWithKeywords
Type of the functions used to implement Python callables in C
with signature :const:`METH_FASTCALL | METH_KEYWORDS`.
The function signature is::
PyObject *_PyCFunctionFastWithKeywords(PyObject *self,
PyObject *const *args,
Py_ssize_t nargs,
PyObject *kwnames);
.. c:type:: PyCMethod
Type of the functions used to implement Python callables in C
with signature :const:`METH_METHOD | METH_FASTCALL | METH_KEYWORDS`.
The function signature is::
PyObject *PyCMethod(PyObject *self,
PyTypeObject *defining_class,
PyObject *const *args,
Py_ssize_t nargs,
PyObject *kwnames)
.. versionadded:: 3.9
.. c:type:: PyMethodDef
Structure used to describe a method of an extension type. This structure has
four fields:
.. c:member:: const char* ml_name
name of the method
.. c:member:: PyCFunction ml_meth
pointer to the C implementation
.. c:member:: int ml_flags
flags bits indicating how the call should be constructed
.. c:member:: const char* ml_doc
points to the contents of the docstring
The :c:member:`ml_meth` is a C function pointer. The functions may be of different
types, but they always return :c:expr:`PyObject*`. If the function is not of
the :c:type:`PyCFunction`, the compiler will require a cast in the method table.
Even though :c:type:`PyCFunction` defines the first parameter as
:c:expr:`PyObject*`, it is common that the method implementation uses the
specific C type of the *self* object.
The :c:member:`ml_flags` field is a bitfield which can include the following flags.
The individual flags indicate either a calling convention or a binding
convention.
There are these calling conventions:
.. data:: METH_VARARGS
This is the typical calling convention, where the methods have the type
:c:type:`PyCFunction`. The function expects two :c:expr:`PyObject*` values.
The first one is the *self* object for methods; for module functions, it is
the module object. The second parameter (often called *args*) is a tuple
object representing all arguments. This parameter is typically processed
using :c:func:`PyArg_ParseTuple` or :c:func:`PyArg_UnpackTuple`.
.. data:: METH_VARARGS | METH_KEYWORDS
Methods with these flags must be of type :c:type:`PyCFunctionWithKeywords`.
The function expects three parameters: *self*, *args*, *kwargs* where
*kwargs* is a dictionary of all the keyword arguments or possibly ``NULL``
if there are no keyword arguments. The parameters are typically processed
using :c:func:`PyArg_ParseTupleAndKeywords`.
.. data:: METH_FASTCALL
Fast calling convention supporting only positional arguments.
The methods have the type :c:type:`_PyCFunctionFast`.
The first parameter is *self*, the second parameter is a C array
of :c:expr:`PyObject*` values indicating the arguments and the third
parameter is the number of arguments (the length of the array).
.. versionadded:: 3.7
.. versionchanged:: 3.10
``METH_FASTCALL`` is now part of the stable ABI.
.. data:: METH_FASTCALL | METH_KEYWORDS
Extension of :const:`METH_FASTCALL` supporting also keyword arguments,
with methods of type :c:type:`_PyCFunctionFastWithKeywords`.
Keyword arguments are passed the same way as in the
:ref:`vectorcall protocol <vectorcall>`:
there is an additional fourth :c:expr:`PyObject*` parameter
which is a tuple representing the names of the keyword arguments
(which are guaranteed to be strings)
or possibly ``NULL`` if there are no keywords. The values of the keyword
arguments are stored in the *args* array, after the positional arguments.
.. versionadded:: 3.7
.. data:: METH_METHOD | METH_FASTCALL | METH_KEYWORDS
Extension of :const:`METH_FASTCALL | METH_KEYWORDS` supporting the *defining
class*, that is, the class that contains the method in question.
The defining class might be a superclass of ``Py_TYPE(self)``.
The method needs to be of type :c:type:`PyCMethod`, the same as for
``METH_FASTCALL | METH_KEYWORDS`` with ``defining_class`` argument added after
``self``.
.. versionadded:: 3.9
.. data:: METH_NOARGS
Methods without parameters don't need to check whether arguments are given if
they are listed with the :const:`METH_NOARGS` flag. They need to be of type
:c:type:`PyCFunction`. The first parameter is typically named *self* and will
hold a reference to the module or object instance. In all cases the second
parameter will be ``NULL``.
The function must have 2 parameters. Since the second parameter is unused,
:c:macro:`Py_UNUSED` can be used to prevent a compiler warning.
.. data:: METH_O
Methods with a single object argument can be listed with the :const:`METH_O`
flag, instead of invoking :c:func:`PyArg_ParseTuple` with a ``"O"`` argument.
They have the type :c:type:`PyCFunction`, with the *self* parameter, and a
:c:expr:`PyObject*` parameter representing the single argument.
These two constants are not used to indicate the calling convention but the
binding when use with methods of classes. These may not be used for functions
defined for modules. At most one of these flags may be set for any given
method.
.. data:: METH_CLASS
.. index:: pair: built-in function; classmethod
The method will be passed the type object as the first parameter rather
than an instance of the type. This is used to create *class methods*,
similar to what is created when using the :func:`classmethod` built-in
function.
.. data:: METH_STATIC
.. index:: pair: built-in function; staticmethod
The method will be passed ``NULL`` as the first parameter rather than an
instance of the type. This is used to create *static methods*, similar to
what is created when using the :func:`staticmethod` built-in function.
One other constant controls whether a method is loaded in place of another
definition with the same method name.
.. data:: METH_COEXIST
The method will be loaded in place of existing definitions. Without
*METH_COEXIST*, the default is to skip repeated definitions. Since slot
wrappers are loaded before the method table, the existence of a
*sq_contains* slot, for example, would generate a wrapped method named
:meth:`__contains__` and preclude the loading of a corresponding
PyCFunction with the same name. With the flag defined, the PyCFunction
will be loaded in place of the wrapper object and will co-exist with the
slot. This is helpful because calls to PyCFunctions are optimized more
than wrapper object calls.
Accessing attributes of extension types
---------------------------------------
.. c:type:: PyMemberDef
Structure which describes an attribute of a type which corresponds to a C
struct member. Its fields are, in order:
.. c:member:: const char* name
Name of the member.
A NULL value marks the end of a ``PyMemberDef[]`` array.
The string should be static, no copy is made of it.
.. c:member:: Py_ssize_t offset
The offset in bytes that the member is located on the type’s object struct.
.. c:member:: int type
The type of the member in the C struct.
See :ref:`PyMemberDef-types` for the possible values.
.. c:member:: int flags
Zero or more of the :ref:`PyMemberDef-flags`, combined using bitwise OR.
.. c:member:: const char* doc
The docstring, or NULL.
The string should be static, no copy is made of it.
Typically, it is defined using :c:macro:`PyDoc_STR`.
By default (when :c:member:`flags` is ``0``), members allow
both read and write access.
Use the :c:macro:`Py_READONLY` flag for read-only access.
Certain types, like :c:macro:`Py_T_STRING`, imply :c:macro:`Py_READONLY`.
Only :c:macro:`Py_T_OBJECT_EX` (and legacy :c:macro:`T_OBJECT`) members can
be deleted.
.. _pymemberdef-offsets:
For heap-allocated types (created using :c:func:`PyType_FromSpec` or similar),
``PyMemberDef`` may contain a definition for the special member
``"__vectorcalloffset__"``, corresponding to
:c:member:`~PyTypeObject.tp_vectorcall_offset` in type objects.
These must be defined with ``Py_T_PYSSIZET`` and ``Py_READONLY``, for example::
static PyMemberDef spam_type_members[] = {
{"__vectorcalloffset__", Py_T_PYSSIZET,
offsetof(Spam_object, vectorcall), Py_READONLY},
{NULL} /* Sentinel */
};
(You may need to ``#include <stddef.h>`` for :c:func:`!offsetof`.)
The legacy offsets :c:member:`~PyTypeObject.tp_dictoffset` and
:c:member:`~PyTypeObject.tp_weaklistoffset` can be defined similarly using
``"__dictoffset__"`` and ``"__weaklistoffset__"`` members, but extensions
are strongly encouraged to use :const:`Py_TPFLAGS_MANAGED_DICT` and
:const:`Py_TPFLAGS_MANAGED_WEAKREF` instead.
.. versionchanged:: 3.12
``PyMemberDef`` is always available.
Previously, it required including ``"structmember.h"``.
.. c:function:: PyObject* PyMember_GetOne(const char *obj_addr, struct PyMemberDef *m)
Get an attribute belonging to the object at address *obj_addr*. The
attribute is described by ``PyMemberDef`` *m*. Returns ``NULL``
on error.
.. versionchanged:: 3.12
``PyMember_GetOne`` is always available.
Previously, it required including ``"structmember.h"``.
.. c:function:: int PyMember_SetOne(char *obj_addr, struct PyMemberDef *m, PyObject *o)
Set an attribute belonging to the object at address *obj_addr* to object *o*.
The attribute to set is described by ``PyMemberDef`` *m*. Returns ``0``
if successful and a negative value on failure.
.. versionchanged:: 3.12
``PyMember_SetOne`` is always available.
Previously, it required including ``"structmember.h"``.
.. _PyMemberDef-flags:
Member flags
^^^^^^^^^^^^
The following flags can be used with :c:member:`PyMemberDef.flags`:
.. c:macro:: Py_READONLY
Not writable.
.. c:macro:: Py_AUDIT_READ
Emit an ``object.__getattr__`` :ref:`audit event <audit-events>`
before reading.
.. c:macro:: Py_RELATIVE_OFFSET
Indicates that the :c:member:`~PyMemberDef.offset` of this ``PyMemberDef``
entry indicates an offset from the subclass-specific data, rather than
from ``PyObject``.
Can only be used as part of :c:member:`Py_tp_members <PyTypeObject.tp_members>`
:c:type:`slot <PyTypeSlot>` when creating a class using negative
:c:member:`~PyTypeDef.basicsize`.
It is mandatory in that case.
This flag is only used in :c:type:`PyTypeSlot`.
When setting :c:member:`~PyTypeObject.tp_members` during
class creation, Python clears it and sets
:c:member:`PyMemberDef.offset` to the offset from the ``PyObject`` struct.
.. index::
single: READ_RESTRICTED
single: WRITE_RESTRICTED
single: RESTRICTED
.. versionchanged:: 3.10
The :const:`!RESTRICTED`, :const:`!READ_RESTRICTED` and
:const:`!WRITE_RESTRICTED` macros available with
``#include "structmember.h"`` are deprecated.
:const:`!READ_RESTRICTED` and :const:`!RESTRICTED` are equivalent to
:const:`Py_AUDIT_READ`; :const:`!WRITE_RESTRICTED` does nothing.
.. index::
single: READONLY
.. versionchanged:: 3.12
The :const:`!READONLY` macro was renamed to :const:`Py_READONLY`.
The :const:`!PY_AUDIT_READ` macro was renamed with the ``Py_`` prefix.
The new names are now always available.
Previously, these required ``#include "structmember.h"``.
The header is still available and it provides the old names.
.. _PyMemberDef-types:
Member types
^^^^^^^^^^^^
:c:member:`PyMemberDef.type` can be one of the following macros corresponding
to various C types.
When the member is accessed in Python, it will be converted to the
equivalent Python type.
When it is set from Python, it will be converted back to the C type.
If that is not possible, an exception such as :exc:`TypeError` or
:exc:`ValueError` is raised.
Unless marked (D), attributes defined this way cannot be deleted
using e.g. :keyword:`del` or :py:func:`delattr`.
================================ ============================= ======================
Macro name C type Python type
================================ ============================= ======================
.. c:macro:: Py_T_BYTE :c:expr:`char` :py:class:`int`
.. c:macro:: Py_T_SHORT :c:expr:`short` :py:class:`int`
.. c:macro:: Py_T_INT :c:expr:`int` :py:class:`int`
.. c:macro:: Py_T_LONG :c:expr:`long` :py:class:`int`
.. c:macro:: Py_T_LONGLONG :c:expr:`long long` :py:class:`int`
.. c:macro:: Py_T_UBYTE :c:expr:`unsigned char` :py:class:`int`
.. c:macro:: Py_T_UINT :c:expr:`unsigned int` :py:class:`int`
.. c:macro:: Py_T_USHORT :c:expr:`unsigned short` :py:class:`int`
.. c:macro:: Py_T_ULONG :c:expr:`unsigned long` :py:class:`int`
.. c:macro:: Py_T_ULONGLONG :c:expr:`unsigned long long` :py:class:`int`
.. c:macro:: Py_T_PYSSIZET :c:expr:`Py_ssize_t` :py:class:`int`
.. c:macro:: Py_T_FLOAT :c:expr:`float` :py:class:`float`
.. c:macro:: Py_T_DOUBLE :c:expr:`double` :py:class:`float`
.. c:macro:: Py_T_BOOL :c:expr:`char` :py:class:`bool`
(written as 0 or 1)
.. c:macro:: Py_T_STRING :c:expr:`const char *` (*) :py:class:`str` (RO)
.. c:macro:: Py_T_STRING_INPLACE :c:expr:`const char[]` (*) :py:class:`str` (RO)
.. c:macro:: Py_T_CHAR :c:expr:`char` (0-127) :py:class:`str` (**)
.. c:macro:: Py_T_OBJECT_EX :c:expr:`PyObject *` :py:class:`object` (D)
================================ ============================= ======================
(*): Zero-terminated, UTF8-encoded C string.
With :c:macro:`!Py_T_STRING` the C representation is a pointer;
with :c:macro:`!Py_T_STRING_INLINE` the string is stored directly
in the structure.
(**): String of length 1. Only ASCII is accepted.
(RO): Implies :c:macro:`Py_READONLY`.
(D): Can be deleted, in which case the pointer is set to ``NULL``.
Reading a ``NULL`` pointer raises :py:exc:`AttributeError`.
.. index::
single: T_BYTE
single: T_SHORT
single: T_INT
single: T_LONG
single: T_LONGLONG
single: T_UBYTE
single: T_USHORT
single: T_UINT
single: T_ULONG
single: T_ULONGULONG
single: T_PYSSIZET
single: T_FLOAT
single: T_DOUBLE
single: T_BOOL
single: T_CHAR
single: T_STRING
single: T_STRING_INPLACE
single: T_OBJECT_EX
single: structmember.h
.. versionadded:: 3.12
In previous versions, the macros were only available with
``#include "structmember.h"`` and were named without the ``Py_`` prefix
(e.g. as ``T_INT``).
The header is still available and contains the old names, along with
the following deprecated types:
.. c:macro:: T_OBJECT
Like ``Py_T_OBJECT_EX``, but ``NULL`` is converted to ``None``.
This results in surprising behavior in Python: deleting the attribute
effectively sets it to ``None``.
.. c:macro:: T_NONE
Always ``None``. Must be used with :c:macro:`Py_READONLY`.
Defining Getters and Setters
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. c:type:: PyGetSetDef
Structure to define property-like access for a type. See also description of
the :c:member:`PyTypeObject.tp_getset` slot.
.. c:member:: const char* name
attribute name
.. c:member:: getter get
C function to get the attribute.
.. c:member:: setter set
Optional C function to set or delete the attribute, if omitted the attribute is readonly.
.. c:member:: const char* doc
optional docstring
.. c:member:: void* closure
Optional function pointer, providing additional data for getter and setter.
The ``get`` function takes one :c:expr:`PyObject*` parameter (the
instance) and a function pointer (the associated ``closure``)::
typedef PyObject *(*getter)(PyObject *, void *);
It should return a new reference on success or ``NULL`` with a set exception
on failure.
``set`` functions take two :c:expr:`PyObject*` parameters (the instance and
the value to be set) and a function pointer (the associated ``closure``)::
typedef int (*setter)(PyObject *, PyObject *, void *);
In case the attribute should be deleted the second parameter is ``NULL``.
Should return ``0`` on success or ``-1`` with a set exception on failure.
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