cpython/Doc/c-api/type.rst

.. highlight:: c

.. _typeobjects:

Type Objects
------------

.. index:: object: type


.. c:type:: PyTypeObject

   The C structure of the objects used to describe built-in types.


.. c:var:: PyObject* PyType_Type

   This is the type object for type objects; it is the same object as
   :class:`type` in the Python layer.


.. c:function:: int PyType_Check(PyObject *o)

   Return true if the object *o* is a type object, including instances of types
   derived from the standard type object.  Return false in all other cases.


.. c:function:: int PyType_CheckExact(PyObject *o)

   Return true if the object *o* is a type object, but not a subtype of the
   standard type object.  Return false in all other cases.


.. c:function:: unsigned int PyType_ClearCache()

   Clear the internal lookup cache. Return the current version tag.

.. c:function:: unsigned long PyType_GetFlags(PyTypeObject* type)

   Return the :c:member:`~PyTypeObject.tp_flags` member of *type*. This function is primarily
   meant for use with `Py_LIMITED_API`; the individual flag bits are
   guaranteed to be stable across Python releases, but access to
   :c:member:`~PyTypeObject.tp_flags` itself is not part of the limited API.

   .. versionadded:: 3.2

   .. versionchanged:: 3.4
      The return type is now ``unsigned long`` rather than ``long``.


.. c:function:: void PyType_Modified(PyTypeObject *type)

   Invalidate the internal lookup cache for the type and all of its
   subtypes.  This function must be called after any manual
   modification of the attributes or base classes of the type.


.. c:function:: int PyType_HasFeature(PyTypeObject *o, int feature)

   Return true if the type object *o* sets the feature *feature*.  Type features
   are denoted by single bit flags.


.. c:function:: int PyType_IS_GC(PyTypeObject *o)

   Return true if the type object includes support for the cycle detector; this
   tests the type flag :const:`Py_TPFLAGS_HAVE_GC`.


.. c:function:: int PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b)

   Return true if *a* is a subtype of *b*.

   This function only checks for actual subtypes, which means that
   :meth:`~class.__subclasscheck__` is not called on *b*.  Call
   :c:func:`PyObject_IsSubclass` to do the same check that :func:`issubclass`
   would do.


.. c:function:: PyObject* PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)

   Generic handler for the :c:member:`~PyTypeObject.tp_alloc` slot of a type object.  Use
   Python's default memory allocation mechanism to allocate a new instance and
   initialize all its contents to ``NULL``.

.. c:function:: PyObject* PyType_GenericNew(PyTypeObject *type, PyObject *args, PyObject *kwds)

   Generic handler for the :c:member:`~PyTypeObject.tp_new` slot of a type object.  Create a
   new instance using the type's :c:member:`~PyTypeObject.tp_alloc` slot.

.. c:function:: int PyType_Ready(PyTypeObject *type)

   Finalize a type object.  This should be called on all type objects to finish
   their initialization.  This function is responsible for adding inherited slots
   from a type's base class.  Return ``0`` on success, or return ``-1`` and sets an
   exception on error.

.. c:function:: void* PyType_GetSlot(PyTypeObject *type, int slot)

   Return the function pointer stored in the given slot. If the
   result is ``NULL``, this indicates that either the slot is ``NULL``,
   or that the function was called with invalid parameters.
   Callers will typically cast the result pointer into the appropriate
   function type.

   See :c:member:`PyType_Slot.slot` for possible values of the *slot* argument.

   An exception is raised if *type* is not a heap type.

   .. versionadded:: 3.4


Creating Heap-Allocated Types
.............................

The following functions and structs are used to create
:ref:`heap types <heap-types>`.

.. c:function:: PyObject* PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)

   Creates and returns a heap type object from the *spec*
   (:const:`Py_TPFLAGS_HEAPTYPE`).

   If *bases* is a tuple, the created heap type contains all types contained
   in it as base types.

   If *bases* is ``NULL``, the *Py_tp_base* slot is used instead.
   If that also is ``NULL``, the new type derives from :class:`object`.

   This function calls :c:func:`PyType_Ready` on the new type.

   .. versionadded:: 3.3

.. c:function:: PyObject* PyType_FromSpec(PyType_Spec *spec)

   Equivalent to ``PyType_FromSpecWithBases(spec, NULL)``.

.. c:type:: PyType_Spec

   Structure defining a type's behavior.

   .. c:member:: const char* PyType_Spec.name

      Name of the type, used to set :c:member:`PyTypeObject.tp_name`.

   .. c:member:: int PyType_Spec.basicsize
   .. c:member:: int PyType_Spec.itemsize

      Size of the instance in bytes, used to set
      :c:member:`PyTypeObject.tp_basicsize` and
      :c:member:`PyTypeObject.tp_itemsize`.

   .. c:member:: int PyType_Spec.flags

      Type flags, used to set :c:member:`PyTypeObject.tp_flags`.

      If the ``Py_TPFLAGS_HEAPTYPE`` flag is not set,
      :c:func:`PyType_FromSpecWithBases` sets it automatically.

   .. c:member:: PyType_Slot *PyType_Spec.slots

      Array of :c:type:`PyType_Slot` structures.
      Terminated by the special slot value ``{0, NULL}``.

.. c:type:: PyType_Slot

   Structure defining optional functionality of a type, containing a slot ID
   and a value pointer.

   .. c:member:: int PyType_Slot.slot

      A slot ID.

      Slot IDs are named like the field names of the structures
      :c:type:`PyTypeObject`, :c:type:`PyNumberMethods`,
      :c:type:`PySequenceMethods`, :c:type:`PyMappingMethods` and
      :c:type:`PyAsyncMethods` with an added ``Py_`` prefix.
      For example, use:

      * ``Py_tp_dealloc`` to set :c:member:`PyTypeObject.tp_dealloc`
      * ``Py_nb_add`` to set :c:member:`PyNumberMethods.nb_add`
      * ``Py_sq_length`` to set :c:member:`PySequenceMethods.sq_length`

      The following fields cannot be set using :c:type:`PyType_Spec` and :c:type:`PyType_Slot`:

      * :c:member:`~PyTypeObject.tp_dict`
      * :c:member:`~PyTypeObject.tp_mro`
      * :c:member:`~PyTypeObject.tp_cache`
      * :c:member:`~PyTypeObject.tp_subclasses`
      * :c:member:`~PyTypeObject.tp_weaklist`
      * :c:member:`~PyTypeObject.tp_vectorcall`
      * :c:member:`~PyTypeObject.tp_weaklistoffset`
        (see :ref:`PyMemberDef <pymemberdef-offsets>`)
      * :c:member:`~PyTypeObject.tp_dictoffset`
        (see :ref:`PyMemberDef <pymemberdef-offsets>`)
      * :c:member:`~PyTypeObject.tp_vectorcall_offset`
      * :c:member:`~PyBufferProcs.bf_getbuffer`
      * :c:member:`~PyBufferProcs.bf_releasebuffer`

      Setting :c:data:`Py_tp_bases` may be problematic on some platforms.
      To avoid issues, use the *bases* argument of
      :py:func:`PyType_FromSpecWithBases` instead.

   .. c:member:: void *PyType_Slot.pfunc

      The desired value of the slot. In most cases, this is a pointer
      to a function.

      May not be ``NULL``.