ContextQMD
Back to Cpython

Type Objects

Doc/c-api/type.rst

3.15.0a825.5 KB
Original Source

.. 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 (:attr:cls.__dict__ <type.__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.

In free-threaded builds, :c:func:PyType_AddWatcher is not thread-safe, so it must be called at start up (before spawning the first thread).

.. 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:function:: int PyType_Unwatch(int watcher_id, PyObject *type)

Mark type as not watched. This undoes a previous call to :c:func:PyType_Watch. type must not be NULL.

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

On success, this function returns 0. On failure, this function returns -1 with an exception set.

.. 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_FastSubclass(PyTypeObject *type, int flag)

Return non-zero if the type object type sets the subclass flag flag. Subclass flags are denoted by :c:macro:Py_TPFLAGS_*_SUBCLASS <Py_TPFLAGS_LONG_SUBCLASS>. This function is used by many _Check functions for common types.

.. seealso:: :c:func:PyObject_TypeCheck, which is used as a slower alternative in _Check functions for types that don't come with subclass 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:~type.__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. Uses Python's default memory allocation mechanism to allocate memory for a new instance, zeros the memory, then initializes the memory as if by calling :c:func:PyObject_Init or :c:func:PyObject_InitVar.

Do not call this directly to allocate memory for an object; call the type's :c:member:~PyTypeObject.tp_alloc slot instead.

For types that support garbage collection (i.e., the :c:macro:Py_TPFLAGS_HAVE_GC flag is set), this function behaves like :c:macro:PyObject_GC_New or :c:macro:PyObject_GC_NewVar (except the memory is guaranteed to be zeroed before initialization), and should be paired with :c:func:PyObject_GC_Del in :c:member:~PyTypeObject.tp_free. Otherwise, it behaves like :c:macro:PyObject_New or :c:macro:PyObject_NewVar (except the memory is guaranteed to be zeroed before initialization) and should be paired with :c:func:PyObject_Free in :c:member:~PyTypeObject.tp_free.

.. 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. Creates a new instance using the type's :c:member:~PyTypeObject.tp_alloc slot and returns the resulting object.

.. 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 :attr:~type.__name__ attribute.

.. versionadded:: 3.11

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

Return the type's qualified name. Equivalent to getting the type's :attr:~type.__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 :attr:type.__qualname__ if :attr: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 :attr: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.

The returned reference is :term:borrowed <borrowed reference> from type, and will be valid as long as you hold a reference to type. Do not release it with :c:func:Py_DECREF or similar.

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_GetModuleByToken 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_GetModuleByToken(PyTypeObject *type, const void *mod_token)

Find the first superclass whose module has the given :ref:module token <ext-module-token>, 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:: 3.15

.. 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, or whose :ref:module token <ext-module-token> is equal to def, and return that module.

Note that modules created from a :c:type:PyModuleDef always have their token set to the :c:type:PyModuleDef's address. In other words, this function is equivalent to :c:func:PyType_GetModuleByToken, except that it:

  • returns a borrowed reference, and
  • has a non-void* argument type (which is a cosmetic difference in C).

The returned reference is :term:borrowed <borrowed reference> from type, and will be valid as long as you hold a reference to type. Do not release it with :c:func:Py_DECREF or similar.

.. versionadded:: 3.11

.. c:function:: int PyType_GetBaseByToken(PyTypeObject *type, void *tp_token, PyTypeObject **result)

Find the first superclass in type's :term:method resolution order whose :c:macro:Py_tp_token token is equal to tp_token.

  • If found, set *result to a new :term:strong reference to it and return 1.
  • If not found, set *result to NULL and return 0.
  • On error, set *result to NULL and return -1 with an exception set.

The result argument may be NULL, in which case *result is not set. Use this if you need only the return value.

The tp_token argument may not be NULL.

.. versionadded:: 3.14

.. 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

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

Return true if instances of type support creating weak references, false otherwise. This function always succeeds. type must not be NULL.

.. seealso:: * :ref:weakrefobjects * :py:mod:weakref

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.

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 :c:data:Py_tp_bases slot is used instead. If that also is NULL, the :c:data: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.

.. versionchanged:: 3.14

  Creating classes whose metaclass overrides
  :c:member:`~PyTypeObject.tp_new` is 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.

.. versionchanged:: 3.14

  Creating classes whose metaclass overrides
  :c:member:`~PyTypeObject.tp_new` is 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.

.. versionchanged:: 3.14

  Creating classes whose metaclass overrides
  :c:member:`~PyTypeObject.tp_new` is no longer allowed.

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

Make a type immutable: set the :c:macro:Py_TPFLAGS_IMMUTABLETYPE flag.

All base classes of type must be immutable.

On success, return 0. On error, set an exception and return -1.

The type must not be used before it's made immutable. For example, type instances must not be created before the type is made immutable.

.. versionadded:: 3.14

.. raw:: html

<!-- Keep old URL fragments working (see gh-97908) -->

<span id='c.PyType_Spec.PyType_Spec.name'></span> <span id='c.PyType_Spec.PyType_Spec.basicsize'></span> <span id='c.PyType_Spec.PyType_Spec.itemsize'></span> <span id='c.PyType_Spec.PyType_Spec.flags'></span> <span id='c.PyType_Spec.PyType_Spec.slots'></span>

.. 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.
  For negative :c:member:`!basicsize`, Python will insert padding when
  needed to meet :c:member:`~PyTypeObject.tp_basicsize`'s alignment
  requirements.

  .. 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.

.. raw:: html

<!-- Keep old URL fragments working (see gh-97908) -->

<span id='c.PyType_Slot.PyType_Slot.slot'></span> <span id='c.PyType_Slot.PyType_Slot.pfunc'></span>

.. 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:

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

  An additional slot is supported that does not correspond to a
  :c:type:`!PyTypeObject` struct field:

  * :c:data:`Py_tp_token`

  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:data:`Py_tp_members`.
  See :ref:`PyMemberDef documentation <pymemberdef-offsets>`
  for details.

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

  * :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>`.

  .. versionchanged:: 3.14
     The field :c:member:`~PyTypeObject.tp_vectorcall` can now be set
     using :c:data:`Py_tp_vectorcall`.  See the field's documentation
     for details.

.. c:member:: void *pfunc

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

  *pfunc* values may not be ``NULL``, except for the following slots:

  * :c:data:`Py_tp_doc`
  * :c:data:`Py_tp_token` (for clarity, prefer :c:data:`Py_TP_USE_SPEC`
    rather than ``NULL``)

.. c:macro:: Py_tp_token

A :c:member:~PyType_Slot.slot that records a static memory layout ID for a class.

If the :c:type:PyType_Spec of the class is statically allocated, the token can be set to the spec using the special value :c:data:Py_TP_USE_SPEC:

.. code-block:: c

  static PyType_Slot foo_slots[] = {
     {Py_tp_token, Py_TP_USE_SPEC},

It can also be set to an arbitrary pointer, but you must ensure that:

  • The pointer outlives the class, so it's not reused for something else while the class exists.
  • It "belongs" to the extension module where the class lives, so it will not clash with other extensions.

Use :c:func:PyType_GetBaseByToken to check if a class's superclass has a given token -- that is, check whether the memory layout is compatible.

To get the token for a given class (without considering superclasses), use :c:func:PyType_GetSlot with Py_tp_token.

.. versionadded:: 3.14

.. c:namespace:: NULL

.. c:macro:: Py_TP_USE_SPEC

  Used as a value with :c:data:`Py_tp_token` to set the token to the
  class's :c:type:`PyType_Spec`.
  Expands to ``NULL``.

  .. versionadded:: 3.14