cpython/Doc/c-api/type.rst

.. highlight:: c

.. _typeobjects:

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

.. index:: pair: object; type


.. c:type:: PyTypeObject

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


.. c:var:: PyTypeObject 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 non-zero if the object *o* is a type object, including instances of
   types derived from the standard type object.  Return 0 in all other cases.
   This function always succeeds.


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

   Return non-zero if the object *o* is a type object, but not a subtype of
   the standard type object.  Return 0 in all other cases.  This function
   always succeeds.


.. 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 :ref:`limited API <limited-c-api>`.

   .. versionadded:: 3.2

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


.. c:function:: PyObject* PyType_GetDict(PyTypeObject* type)

   Return the type object's internal namespace, which is otherwise only
   exposed via a read-only proxy (``cls.__dict__``).  This is a
   replacement for accessing :c:member:`~PyTypeObject.tp_dict` directly.
   The returned dictionary must be treated as read-only.

   This function is meant for specific embedding and language-binding cases,
   where direct access to the dict is necessary and indirect access
   (e.g. via the proxy or :c:func:`PyObject_GetAttr`) isn't adequate.

   Extension modules should continue to use ``tp_dict``,
   directly or indirectly, when setting up their own types.

   .. versionadded:: 3.12


.. 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_AddWatcher(PyType_WatchCallback callback)

   Register *callback* as a type watcher. Return a non-negative integer ID
   which must be passed to future calls to :c:func:`PyType_Watch`. In case of
   error (e.g. no more watcher IDs available), return ``-1`` and set an
   exception.

   .. versionadded:: 3.12


.. c:function:: int PyType_ClearWatcher(int watcher_id)

   Clear watcher identified by *watcher_id* (previously returned from
   :c:func:`PyType_AddWatcher`). Return ``0`` on success, ``-1`` on error (e.g.
   if *watcher_id* was never registered.)

   An extension should never call ``PyType_ClearWatcher`` with a *watcher_id*
   that was not returned to it by a previous call to
   :c:func:`PyType_AddWatcher`.

   .. versionadded:: 3.12


.. c:function:: int PyType_Watch(int watcher_id, PyObject *type)

   Mark *type* as watched. The callback granted *watcher_id* by
   :c:func:`PyType_AddWatcher` will be called whenever
   :c:func:`PyType_Modified` reports a change to *type*. (The callback may be
   called only once for a series of consecutive modifications to *type*, if
   :c:func:`!_PyType_Lookup` is not called on *type* between the modifications;
   this is an implementation detail and subject to change.)

   An extension should never call ``PyType_Watch`` with a *watcher_id* that was
   not returned to it by a previous call to :c:func:`PyType_AddWatcher`.

   .. versionadded:: 3.12


.. c:type:: int (*PyType_WatchCallback)(PyObject *type)

   Type of a type-watcher callback function.

   The callback must not modify *type* or cause :c:func:`PyType_Modified` to be
   called on *type* or any type in its MRO; violating this rule could cause
   infinite recursion.

   .. versionadded:: 3.12


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

   Return non-zero 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 :c:macro:`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.

   .. note::
       If some of the base classes implements the GC protocol and the provided
       type does not include the :c:macro:`Py_TPFLAGS_HAVE_GC` in its flags, then
       the GC protocol will be automatically implemented from its parents. On
       the contrary, if the type being created does include
       :c:macro:`Py_TPFLAGS_HAVE_GC` in its flags then it **must** implement the
       GC protocol itself by at least implementing the
       :c:member:`~PyTypeObject.tp_traverse` handle.

.. c:function:: PyObject* PyType_GetName(PyTypeObject *type)

   Return the type's name. Equivalent to getting the type's ``__name__`` attribute.

   .. versionadded:: 3.11

.. c:function:: PyObject* PyType_GetQualName(PyTypeObject *type)

   Return the type's qualified name. Equivalent to getting the
   type's ``__qualname__`` attribute.

   .. versionadded:: 3.11

.. 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.

   .. versionadded:: 3.4

   .. versionchanged:: 3.10
      :c:func:`PyType_GetSlot` can now accept all types.
      Previously, it was limited to :ref:`heap types <heap-types>`.

.. c:function:: PyObject* PyType_GetModule(PyTypeObject *type)

   Return the module object associated with the given type when the type was
   created using :c:func:`PyType_FromModuleAndSpec`.

   If no module is associated with the given type, sets :py:class:`TypeError`
   and returns ``NULL``.

   This function is usually used to get the module in which a method is defined.
   Note that in such a method, ``PyType_GetModule(Py_TYPE(self))``
   may not return the intended result.
   ``Py_TYPE(self)`` may be a *subclass* of the intended class, and subclasses
   are not necessarily defined in the same module as their superclass.
   See :c:type:`PyCMethod` to get the class that defines the method.
   See :c:func:`PyType_GetModuleByDef` for cases when :c:type:`!PyCMethod` cannot
   be used.

   .. versionadded:: 3.9

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

   Return the state of the module object associated with the given type.
   This is a shortcut for calling :c:func:`PyModule_GetState()` on the result
   of :c:func:`PyType_GetModule`.

   If no module is associated with the given type, sets :py:class:`TypeError`
   and returns ``NULL``.

   If the *type* has an associated module but its state is ``NULL``,
   returns ``NULL`` without setting an exception.

   .. versionadded:: 3.9

.. c:function:: PyObject* PyType_GetModuleByDef(PyTypeObject *type, struct PyModuleDef *def)

   Find the first superclass whose module was created from
   the given :c:type:`PyModuleDef` *def*, and return that module.

   If no module is found, raises a :py:class:`TypeError` and returns ``NULL``.

   This function is intended to be used together with
   :c:func:`PyModule_GetState()` to get module state from slot methods (such as
   :c:member:`~PyTypeObject.tp_init` or :c:member:`~PyNumberMethods.nb_add`)
   and other places where a method's defining class cannot be passed using the
   :c:type:`PyCMethod` calling convention.

   .. versionadded:: 3.11

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

   Attempt to assign a version tag to the given type.

   Returns 1 if the type already had a valid version tag or a new one was
   assigned, or 0 if a new tag could not be assigned.

   .. versionadded:: 3.12


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

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

.. c:function:: PyObject* PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module, PyType_Spec *spec, PyObject *bases)

   Create and return a :ref:`heap type <heap-types>` from the *spec*
   (see :c:macro:`Py_TPFLAGS_HEAPTYPE`).

   The metaclass *metaclass* is used to construct the resulting type object.
   When *metaclass* is ``NULL``, the metaclass is derived from *bases*
   (or *Py_tp_base[s]* slots if *bases* is ``NULL``, see below).

   Metaclasses that override :c:member:`~PyTypeObject.tp_new` are not
   supported, except if ``tp_new`` is ``NULL``.
   (For backwards compatibility, other ``PyType_From*`` functions allow
   such metaclasses. They ignore ``tp_new``, which may result in incomplete
   initialization. This is deprecated and in Python 3.14+ such metaclasses will
   not be supported.)

   The *bases* argument can be used to specify base classes; it can either
   be only one class or a tuple of classes.
   If *bases* is ``NULL``, the *Py_tp_bases* slot is used instead.
   If that also is ``NULL``, the *Py_tp_base* slot is used instead.
   If that also is ``NULL``, the new type derives from :class:`object`.

   The *module* argument can be used to record the module in which the new
   class is defined. It must be a module object or ``NULL``.
   If not ``NULL``, the module is associated with the new type and can later be
   retrieved with :c:func:`PyType_GetModule`.
   The associated module is not inherited by subclasses; it must be specified
   for each class individually.

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

   Note that this function does *not* fully match the behavior of
   calling :py:class:`type() <type>` or using the :keyword:`class` statement.
   With user-provided base types or metaclasses, prefer
   :ref:`calling <capi-call>` :py:class:`type` (or the metaclass)
   over ``PyType_From*`` functions.
   Specifically:

   * :py:meth:`~object.__new__` is not called on the new class
     (and it must be set to ``type.__new__``).
   * :py:meth:`~object.__init__` is not called on the new class.
   * :py:meth:`~object.__init_subclass__` is not called on any bases.
   * :py:meth:`~object.__set_name__` is not called on new descriptors.

   .. versionadded:: 3.12

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

   Equivalent to ``PyType_FromMetaclass(NULL, module, spec, bases)``.

   .. versionadded:: 3.9

   .. versionchanged:: 3.10

      The function now accepts a single class as the *bases* argument and
      ``NULL`` as the ``tp_doc`` slot.

   .. versionchanged:: 3.12

      The function now finds and uses a metaclass corresponding to the provided
      base classes.  Previously, only :class:`type` instances were returned.

      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
      which may result in incomplete initialization.
      Creating classes whose metaclass overrides
      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
      will be no longer allowed.

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

   Equivalent to ``PyType_FromMetaclass(NULL, NULL, spec, bases)``.

   .. versionadded:: 3.3

   .. versionchanged:: 3.12

      The function now finds and uses a metaclass corresponding to the provided
      base classes.  Previously, only :class:`type` instances were returned.

      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
      which may result in incomplete initialization.
      Creating classes whose metaclass overrides
      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
      will be no longer allowed.

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

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

   .. versionchanged:: 3.12

      The function now finds and uses a metaclass corresponding to the
      base classes provided in *Py_tp_base[s]* slots.
      Previously, only :class:`type` instances were returned.

      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
      which may result in incomplete initialization.
      Creating classes whose metaclass overrides
      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
      will be no longer allowed.

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.. c:type:: PyType_Spec

   Structure defining a type's behavior.

   .. c:member:: const char* name

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

   .. c:member:: int basicsize

      If positive, specifies the size of the instance in bytes.
      It is used to set :c:member:`PyTypeObject.tp_basicsize`.

      If zero, specifies that :c:member:`~PyTypeObject.tp_basicsize`
      should be inherited.

      If negative, the absolute value specifies how much space instances of the
      class need *in addition* to the superclass.
      Use :c:func:`PyObject_GetTypeData` to get a pointer to subclass-specific
      memory reserved this way.

      .. versionchanged:: 3.12

         Previously, this field could not be negative.

   .. c:member:: int itemsize

      Size of one element of a variable-size type, in bytes.
      Used to set :c:member:`PyTypeObject.tp_itemsize`.
      See ``tp_itemsize`` documentation for caveats.

      If zero, :c:member:`~PyTypeObject.tp_itemsize` is inherited.
      Extending arbitrary variable-sized classes is dangerous,
      since some types use a fixed offset for variable-sized memory,
      which can then overlap fixed-sized memory used by a subclass.
      To help prevent mistakes, inheriting ``itemsize`` is only possible
      in the following situations:

      - The base is not variable-sized (its
        :c:member:`~PyTypeObject.tp_itemsize`).
      - The requested :c:member:`PyType_Spec.basicsize` is positive,
        suggesting that the memory layout of the base class is known.
      - The requested :c:member:`PyType_Spec.basicsize` is zero,
        suggesting that the subclass does not access the instance's memory
        directly.
      - With the :c:macro:`Py_TPFLAGS_ITEMS_AT_END` flag.

   .. c:member:: unsigned int 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 *slots

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

      Each slot ID should be specified at most once.

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.. c:type:: PyType_Slot

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

   .. c:member:: int 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 “offset” fields cannot be set using :c:type:`PyType_Slot`:

         * :c:member:`~PyTypeObject.tp_weaklistoffset`
           (use :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF` instead if possible)
         * :c:member:`~PyTypeObject.tp_dictoffset`
           (use :c:macro:`Py_TPFLAGS_MANAGED_DICT` instead if possible)
         * :c:member:`~PyTypeObject.tp_vectorcall_offset`
           (use ``"__vectorcalloffset__"`` in
           :ref:`PyMemberDef <pymemberdef-offsets>`)

         If it is not possible to switch to a ``MANAGED`` flag (for example,
         for vectorcall or to support Python older than 3.12), specify the
         offset in :c:member:`Py_tp_members <PyTypeObject.tp_members>`.
         See :ref:`PyMemberDef documentation <pymemberdef-offsets>`
         for details.

      The following fields cannot be set at all when creating a heap type:

      * :c:member:`~PyTypeObject.tp_vectorcall`
        (use :c:member:`~PyTypeObject.tp_new` and/or
        :c:member:`~PyTypeObject.tp_init`)

      * Internal fields:
        :c:member:`~PyTypeObject.tp_dict`,
        :c:member:`~PyTypeObject.tp_mro`,
        :c:member:`~PyTypeObject.tp_cache`,
        :c:member:`~PyTypeObject.tp_subclasses`, and
        :c:member:`~PyTypeObject.tp_weaklist`.

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

     .. versionchanged:: 3.9

        Slots in :c:type:`PyBufferProcs` may be set in the unlimited API.

     .. versionchanged:: 3.11
        :c:member:`~PyBufferProcs.bf_getbuffer` and
        :c:member:`~PyBufferProcs.bf_releasebuffer` are now available
        under the :ref:`limited API <limited-c-api>`.

   .. c:member:: void *pfunc

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

      Slots other than ``Py_tp_doc`` may not be ``NULL``.