.. 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:: PyObject* PyType_GetFullyQualifiedName(PyTypeObject *type)
Return the type's fully qualified name. Equivalent to
``f"{type.__module__}.{type.__qualname__}"``, or ``type.__qualname__`` if
``type.__module__`` is not a string or is equal to ``"builtins"``.
.. versionadded:: 3.13
.. c:function:: PyObject* PyType_GetModuleName(PyTypeObject *type)
Return the type's module name. Equivalent to getting the ``type.__module__``
attribute.
.. versionadded:: 3.13
.. 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``.
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