Type Object Structures¶

Perhaps one of the most important structures of the Python object system is the structure that defines a new type: the PyTypeObject structure. Type objects can be handled using any of the PyObject_* or PyType_* functions, but do not offer much that’s interesting to most Python applications. These objects are fundamental to how objects behave, so they are very important to the interpreter itself and to any extension module that implements new types.

Type objects are fairly large compared to most of the standard types. The reason for the size is that each type object stores a large number of values, mostly C function pointers, each of which implements a small part of the type’s functionality. The fields of the type object are examined in detail in this section. The fields will be described in the order in which they occur in the structure.

In addition to the following quick reference, the Examples section provides at-a-glance insight into the meaning and use of PyTypeObject.

Quick Reference¶

“tp slots”¶

PyTypeObject Slot [1]	Type	special methods/attrs	Info [2]
PyTypeObject Slot [1]	Type	special methods/attrs	O	T	D	I
<R> `tp_name`	const char *	__name__	X	X
`tp_basicsize`	`Py_ssize_t`		X	X		X
`tp_itemsize`	`Py_ssize_t`			X		X
`tp_dealloc`	`destructor`		X	X		X
`tp_vectorcall_offset`	`Py_ssize_t`			X		X
(`tp_getattr`)	`getattrfunc`	__getattribute__, __getattr__				G
(`tp_setattr`)	`setattrfunc`	__setattr__, __delattr__				G
`tp_as_async`	`PyAsyncMethods` *	sub-slots				%
`tp_repr`	`reprfunc`	__repr__	X	X		X
`tp_as_number`	`PyNumberMethods` *	sub-slots				%
`tp_as_sequence`	`PySequenceMethods` *	sub-slots				%
`tp_as_mapping`	`PyMappingMethods` *	sub-slots				%
`tp_hash`	`hashfunc`	__hash__	X			G
`tp_call`	`ternaryfunc`	__call__		X		X
`tp_str`	`reprfunc`	__str__	X			X
`tp_getattro`	`getattrofunc`	__getattribute__, __getattr__	X	X		G
`tp_setattro`	`setattrofunc`	__setattr__, __delattr__	X	X		G
`tp_as_buffer`	`PyBufferProcs` *	sub-slots				%
`tp_flags`	unsigned long		X	X		?
`tp_doc`	const char *	__doc__	X	X
`tp_traverse`	`traverseproc`			X		G
`tp_clear`	`inquiry`			X		G
`tp_richcompare`	`richcmpfunc`	__lt__, __le__, __eq__, __ne__, __gt__, __ge__	X			G
(`tp_weaklistoffset`)	`Py_ssize_t`			X		?
`tp_iter`	`getiterfunc`	__iter__				X
`tp_iternext`	`iternextfunc`	__next__				X
`tp_methods`	`PyMethodDef` []		X	X
`tp_members`	`PyMemberDef` []			X
`tp_getset`	`PyGetSetDef` []		X	X
`tp_base`	`PyTypeObject` *	__base__			X
`tp_dict`	`PyObject` *	__dict__			?
`tp_descr_get`	`descrgetfunc`	__get__				X
`tp_descr_set`	`descrsetfunc`	__set__, __delete__				X
(`tp_dictoffset`)	`Py_ssize_t`			X		?
`tp_init`	`initproc`	__init__	X	X		X
`tp_alloc`	`allocfunc`		X		?	?
`tp_new`	`newfunc`	__new__	X	X	?	?
`tp_free`	`freefunc`		X	X	?	?
`tp_is_gc`	`inquiry`			X		X
<`tp_bases`>	`PyObject` *	__bases__			~
<`tp_mro`>	`PyObject` *	__mro__			~
[`tp_cache`]	`PyObject` *
[`tp_subclasses`]	void *	__subclasses__
[`tp_weaklist`]	`PyObject` *
(`tp_del`)	`destructor`
[`tp_version_tag`]	unsigned int
`tp_finalize`	`destructor`	__del__				X
`tp_vectorcall`	`vectorcallfunc`
[`tp_watched`]	unsigned char

sub-slots¶

Slot	Type	special methods
`am_await`	`unaryfunc`	__await__
`am_aiter`	`unaryfunc`	__aiter__
`am_anext`	`unaryfunc`	__anext__
`am_send`	`sendfunc`

`nb_add`	`binaryfunc`	__add__ __radd__
`nb_inplace_add`	`binaryfunc`	__iadd__
`nb_subtract`	`binaryfunc`	__sub__ __rsub__
`nb_inplace_subtract`	`binaryfunc`	__isub__
`nb_multiply`	`binaryfunc`	__mul__ __rmul__
`nb_inplace_multiply`	`binaryfunc`	__imul__
`nb_remainder`	`binaryfunc`	__mod__ __rmod__
`nb_inplace_remainder`	`binaryfunc`	__imod__
`nb_divmod`	`binaryfunc`	__divmod__ __rdivmod__
`nb_power`	`ternaryfunc`	__pow__ __rpow__
`nb_inplace_power`	`ternaryfunc`	__ipow__
`nb_negative`	`unaryfunc`	__neg__
`nb_positive`	`unaryfunc`	__pos__
`nb_absolute`	`unaryfunc`	__abs__
`nb_bool`	`inquiry`	__bool__
`nb_invert`	`unaryfunc`	__invert__
`nb_lshift`	`binaryfunc`	__lshift__ __rlshift__
`nb_inplace_lshift`	`binaryfunc`	__ilshift__
`nb_rshift`	`binaryfunc`	__rshift__ __rrshift__
`nb_inplace_rshift`	`binaryfunc`	__irshift__
`nb_and`	`binaryfunc`	__and__ __rand__
`nb_inplace_and`	`binaryfunc`	__iand__
`nb_xor`	`binaryfunc`	__xor__ __rxor__
`nb_inplace_xor`	`binaryfunc`	__ixor__
`nb_or`	`binaryfunc`	__or__ __ror__
`nb_inplace_or`	`binaryfunc`	__ior__
`nb_int`	`unaryfunc`	__int__
`nb_reserved`	void *
`nb_float`	`unaryfunc`	__float__
`nb_floor_divide`	`binaryfunc`	__floordiv__
`nb_inplace_floor_divide`	`binaryfunc`	__ifloordiv__
`nb_true_divide`	`binaryfunc`	__truediv__
`nb_inplace_true_divide`	`binaryfunc`	__itruediv__
`nb_index`	`unaryfunc`	__index__
`nb_matrix_multiply`	`binaryfunc`	__matmul__ __rmatmul__
`nb_inplace_matrix_multiply`	`binaryfunc`	__imatmul__

`mp_length`	`lenfunc`	__len__
`mp_subscript`	`binaryfunc`	__getitem__
`mp_ass_subscript`	`objobjargproc`	__setitem__, __delitem__

`sq_length`	`lenfunc`	__len__
`sq_concat`	`binaryfunc`	__add__
`sq_repeat`	`ssizeargfunc`	__mul__
`sq_item`	`ssizeargfunc`	__getitem__
`sq_ass_item`	`ssizeobjargproc`	__setitem__ __delitem__
`sq_contains`	`objobjproc`	__contains__
`sq_inplace_concat`	`binaryfunc`	__iadd__
`sq_inplace_repeat`	`ssizeargfunc`	__imul__

`bf_getbuffer`	`getbufferproc()`	__buffer__
`bf_releasebuffer`	`releasebufferproc()`	__release_buffer__

slot typedefs¶

typedef	Parameter Types	Return Type
`allocfunc`	`PyTypeObject` * `Py_ssize_t`	`PyObject` *
`destructor`	`PyObject` *	void
`freefunc`	void *	void
`traverseproc`	`PyObject` * `visitproc` void *	int
`newfunc`	`PyTypeObject` * `PyObject` * `PyObject` *	`PyObject` *
`initproc`	`PyObject` * `PyObject` * `PyObject` *	int
`reprfunc`	`PyObject` *	`PyObject` *
`getattrfunc`	`PyObject` * const char *	`PyObject` *
`setattrfunc`	`PyObject` * const char * `PyObject` *	int
`getattrofunc`	`PyObject` * `PyObject` *	`PyObject` *
`setattrofunc`	`PyObject` * `PyObject` * `PyObject` *	int
`descrgetfunc`	`PyObject` * `PyObject` * `PyObject` *	`PyObject` *
`descrsetfunc`	`PyObject` * `PyObject` * `PyObject` *	int
`hashfunc`	`PyObject` *	Py_hash_t
`richcmpfunc`	`PyObject` * `PyObject` * int	`PyObject` *
`getiterfunc`	`PyObject` *	`PyObject` *
`iternextfunc`	`PyObject` *	`PyObject` *
`lenfunc`	`PyObject` *	`Py_ssize_t`
`getbufferproc`	`PyObject` * `Py_buffer` * int	int
`releasebufferproc`	`PyObject` * `Py_buffer` *	void
`inquiry`	`PyObject` *	int
`unaryfunc`	`PyObject` *	`PyObject` *
`binaryfunc`	`PyObject` * `PyObject` *	`PyObject` *
`ternaryfunc`	`PyObject` * `PyObject` * `PyObject` *	`PyObject` *
`ssizeargfunc`	`PyObject` * `Py_ssize_t`	`PyObject` *
`ssizeobjargproc`	`PyObject` * `Py_ssize_t` `PyObject` *	int
`objobjproc`	`PyObject` * `PyObject` *	int
`objobjargproc`	`PyObject` * `PyObject` * `PyObject` *	int

See Slot Type typedefs below for more detail.

PyTypeObject Definition¶

The structure definition for PyTypeObject can be found in Include/cpython/object.h. For convenience of reference, this repeats the definition found there:

typedef struct _typeobject {
    PyObject_VAR_HEAD
    const char *tp_name; /* For printing, in format "<module>.<name>" */
    Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */

    /* Methods to implement standard operations */

    destructor tp_dealloc;
    Py_ssize_t tp_vectorcall_offset;
    getattrfunc tp_getattr;
    setattrfunc tp_setattr;
    PyAsyncMethods *tp_as_async; /* formerly known as tp_compare (Python 2)
                                    or tp_reserved (Python 3) */
    reprfunc tp_repr;

    /* Method suites for standard classes */

    PyNumberMethods *tp_as_number;
    PySequenceMethods *tp_as_sequence;
    PyMappingMethods *tp_as_mapping;

    /* More standard operations (here for binary compatibility) */

    hashfunc tp_hash;
    ternaryfunc tp_call;
    reprfunc tp_str;
    getattrofunc tp_getattro;
    setattrofunc tp_setattro;

    /* Functions to access object as input/output buffer */
    PyBufferProcs *tp_as_buffer;

    /* Flags to define presence of optional/expanded features */
    unsigned long tp_flags;

    const char *tp_doc; /* Documentation string */

    /* Assigned meaning in release 2.0 */
    /* call function for all accessible objects */
    traverseproc tp_traverse;

    /* delete references to contained objects */
    inquiry tp_clear;

    /* Assigned meaning in release 2.1 */
    /* rich comparisons */
    richcmpfunc tp_richcompare;

    /* weak reference enabler */
    Py_ssize_t tp_weaklistoffset;

    /* Iterators */
    getiterfunc tp_iter;
    iternextfunc tp_iternext;

    /* Attribute descriptor and subclassing stuff */
    PyMethodDef *tp_methods;
    PyMemberDef *tp_members;
    PyGetSetDef *tp_getset;
    // Strong reference on a heap type, borrowed reference on a static type
    PyTypeObject *tp_base;
    PyObject *tp_dict;
    descrgetfunc tp_descr_get;
    descrsetfunc tp_descr_set;
    Py_ssize_t tp_dictoffset;
    initproc tp_init;
    allocfunc tp_alloc;
    newfunc tp_new;
    freefunc tp_free; /* Low-level free-memory routine */
    inquiry tp_is_gc; /* For PyObject_IS_GC */
    PyObject *tp_bases;
    PyObject *tp_mro; /* method resolution order */
    PyObject *tp_cache; /* no longer used */
    void *tp_subclasses;  /* for static builtin types this is an index */
    PyObject *tp_weaklist; /* not used for static builtin types */
    destructor tp_del;

    /* Type attribute cache version tag. Added in version 2.6.
     * If zero, the cache is invalid and must be initialized.
     */
    unsigned int tp_version_tag;

    destructor tp_finalize;
    vectorcallfunc tp_vectorcall;

    /* bitset of which type-watchers care about this type */
    unsigned char tp_watched;

    /* Number of tp_version_tag values used.
     * Set to _Py_ATTR_CACHE_UNUSED if the attribute cache is
     * disabled for this type (e.g. due to custom MRO entries).
     * Otherwise, limited to MAX_VERSIONS_PER_CLASS (defined elsewhere).
     */
    uint16_t tp_versions_used;
} PyTypeObject;

PyObject Slots¶

The type object structure extends the PyVarObject structure. The ob_size field is used for dynamic types (created by type_new(), usually called from a class statement). Note that PyType_Type (the metatype) initializes tp_itemsize, which means that its instances (i.e. type objects) must have the ob_size field.

PyObject.ob_refcnt

The type object’s reference count is initialized to 1 by the PyObject_HEAD_INIT macro. Note that for statically allocated type objects, the type’s instances (objects whose ob_type points back to the type) do not count as references. But for dynamically allocated type objects, the instances do count as references.

Inheritance:

This field is not inherited by subtypes.

PyObject.ob_type

This is the type’s type, in other words its metatype. It is initialized by the argument to the PyObject_HEAD_INIT macro, and its value should normally be &PyType_Type. However, for dynamically loadable extension modules that must be usable on Windows (at least), the compiler complains that this is not a valid initializer. Therefore, the convention is to pass NULL to the PyObject_HEAD_INIT macro and to initialize this field explicitly at the start of the module’s initialization function, before doing anything else. This is typically done like this:
Foo_Type.ob_type = &PyType_Type;
This should be done before any instances of the type are created. PyType_Ready() checks if ob_type is NULL, and if so, initializes it to the ob_type field of the base class. PyType_Ready() will not change this field if it is non-zero.

Inheritance:

This field is inherited by subtypes.

PyVarObject Slots¶

PyVarObject.ob_size

For statically allocated type objects, this should be initialized to zero. For dynamically allocated type objects, this field has a special internal meaning.

This field should be accessed using the Py_SIZE() macro.

Inheritance:

This field is not inherited by subtypes.

PyTypeObject Slots¶

Each slot has a section describing inheritance. If PyType_Ready() may set a value when the field is set to NULL then there will also be a “Default” section. (Note that many fields set on PyBaseObject_Type and PyType_Type effectively act as defaults.)

const char *PyTypeObject.tp_name¶

See Py_tp_name for the corresponding Slot ID.

Pointer to a NUL-terminated string containing the name of the type. For types that are accessible as module globals, the string should be the full module name, followed by a dot, followed by the type name; for built-in types, it should be just the type name. If the module is a submodule of a package, the full package name is part of the full module name. For example, a type named T defined in module M in subpackage Q in package P should have the tp_name initializer "P.Q.M.T".

For dynamically allocated type objects, this should just be the type name, and the module name explicitly stored in the type dict as the value for key '__module__'.

For statically allocated type objects, the tp_name field should contain a dot. Everything before the last dot is made accessible as the __module__ attribute, and everything after the last dot is made accessible as the __name__ attribute.

If no dot is present, the entire tp_name field is made accessible as the __name__ attribute, and the __module__ attribute is undefined (unless explicitly set in the dictionary, as explained above). This means your type will be impossible to pickle. Additionally, it will not be listed in module documentations created with pydoc.

This field must not be NULL. It is the only required field in PyTypeObject() (other than potentially tp_itemsize).

Inheritance:

This field is not inherited by subtypes.

Py_ssize_t PyTypeObject.tp_basicsize¶

Py_ssize_t PyTypeObject.tp_itemsize¶

These fields allow calculating the size in bytes of instances of the type.

See Py_tp_basicsize, Py_tp_extra_basicsize and Py_tp_itemsize for the corresponding Slot IDs.

There are two kinds of types: types with fixed-length instances have a zero tp_itemsize field, types with variable-length instances have a non-zero tp_itemsize field. For a type with fixed-length instances, all instances have the same size, given in tp_basicsize. (Exceptions to this rule can be made using PyUnstable_Object_GC_NewWithExtraData().)

For a type with variable-length instances, the instances must have an ob_size field, and the instance size is tp_basicsize plus N times tp_itemsize, where N is the “length” of the object.

Functions like PyObject_NewVar() will take the value of N as an argument, and store in the instance’s ob_size field. Note that the ob_size field may later be used for other purposes. For example, int instances use the bits of ob_size in an implementation-defined way; the underlying storage and its size should be accessed using PyLong_Export().

Note

The ob_size field should be accessed using the Py_SIZE() and Py_SET_SIZE() macros.

Also, the presence of an ob_size field in the instance layout doesn’t mean that the instance structure is variable-length. For example, the list type has fixed-length instances, yet those instances have a ob_size field. (As with int, avoid reading lists’ ob_size directly. Call PyList_Size() instead.)

The tp_basicsize includes size needed for data of the type’s tp_base, plus any extra data needed by each instance.

The correct way to set tp_basicsize is to use the sizeof operator on the struct used to declare the instance layout. This struct must include the struct used to declare the base type. In other words, tp_basicsize must be greater than or equal to the base’s tp_basicsize.

Since every type is a subtype of object, this struct must include PyObject or PyVarObject (depending on whether ob_size should be included). These are usually defined by the macro PyObject_HEAD or PyObject_VAR_HEAD, respectively.

The basic size does not include the GC header size, as that header is not part of PyObject_HEAD.

For cases where struct used to declare the base type is unknown, see PyType_Spec.basicsize and PyType_FromMetaclass().

Notes about alignment:

tp_basicsize must be a multiple of _Alignof(PyObject). When using sizeof on a struct that includes PyObject_HEAD, as recommended, the compiler ensures this. When not using a C struct, or when using compiler extensions like __attribute__((packed)), it is up to you.
If the variable items require a particular alignment, tp_basicsize and tp_itemsize must each be a multiple of that alignment. For example, if a type’s variable part stores a double, it is your responsibility that both fields are a multiple of _Alignof(double).

Inheritance:

These fields are inherited separately by subtypes. (That is, if the field is set to zero, PyType_Ready() will copy the value from the base type, indicating that the instances do not need additional storage.)

If the base type has a non-zero tp_itemsize, it is generally not safe to set tp_itemsize to a different non-zero value in a subtype (though this depends on the implementation of the base type).

destructor PyTypeObject.tp_dealloc¶

The corresponding slot ID Py_tp_dealloc is part of the Stable ABI.

A pointer to the instance destructor function. The function signature is:

void tp_dealloc(PyObject *self);

The destructor function should remove all references which the instance owns (e.g., call Py_CLEAR()), free all memory buffers owned by the instance, and call the type’s tp_free function to free the object itself.

If you may call functions that may set the error indicator, you must use PyErr_GetRaisedException() and PyErr_SetRaisedException() to ensure you don’t clobber a preexisting error indicator (the deallocation could have occurred while processing a different error):

static void
foo_dealloc(foo_object *self)
{
    PyObject *et, *ev, *etb;
    PyObject *exc = PyErr_GetRaisedException();
    ...
    PyErr_SetRaisedException(exc);
}

The dealloc handler itself must not raise an exception; if it hits an error case it should call PyErr_FormatUnraisable() to log (and clear) an unraisable exception.

No guarantees are made about when an object is destroyed, except:

Python will destroy an object immediately or some time after the final reference to the object is deleted, unless its finalizer (tp_finalize) subsequently resurrects the object.
An object will not be destroyed while it is being automatically finalized (tp_finalize) or automatically cleared (tp_clear).

CPython currently destroys an object immediately from Py_DECREF() when the new reference count is zero, but this may change in a future version.

It is recommended to call PyObject_CallFinalizerFromDealloc() at the beginning of tp_dealloc to guarantee that the object is always finalized before destruction.

If the type supports garbage collection (the Py_TPFLAGS_HAVE_GC flag is set), the destructor should call PyObject_GC_UnTrack() before clearing any member fields.

It is permissible to call tp_clear from tp_dealloc to reduce code duplication and to guarantee that the object is always cleared before destruction. Beware that tp_clear might have already been called.

If the type is heap allocated (Py_TPFLAGS_HEAPTYPE), the deallocator should release the owned reference to its type object (via Py_DECREF()) after calling the type deallocator. See the example code below.:

static void
foo_dealloc(PyObject *op)
{
   foo_object *self = (foo_object *) op;
   PyObject_GC_UnTrack(self);
   Py_CLEAR(self->ref);
   Py_TYPE(self)->tp_free(self);
}

tp_dealloc must leave the exception status unchanged. If it needs to call something that might raise an exception, the exception state must be backed up first and restored later (after logging any exceptions with PyErr_WriteUnraisable()).

Example:

static void
foo_dealloc(PyObject *self)
{
    PyObject *exc = PyErr_GetRaisedException();

    if (PyObject_CallFinalizerFromDealloc(self) < 0) {
        // self was resurrected.
        goto done;
    }

    PyTypeObject *tp = Py_TYPE(self);

    if (tp->tp_flags & Py_TPFLAGS_HAVE_GC) {
        PyObject_GC_UnTrack(self);
    }

    // Optional, but convenient to avoid code duplication.
    if (tp->tp_clear && tp->tp_clear(self) < 0) {
        PyErr_WriteUnraisable(self);
    }

    // Any additional destruction goes here.

    tp->tp_free(self);
    self = NULL;  // In case PyErr_WriteUnraisable() is called below.

    if (tp->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        Py_CLEAR(tp);
    }

done:
    // Optional, if something was called that might have raised an
    // exception.
    if (PyErr_Occurred()) {
        PyErr_WriteUnraisable(self);
    }
    PyErr_SetRaisedException(exc);
}

tp_dealloc may be called from any Python thread, not just the thread which created the object (if the object becomes part of a refcount cycle, that cycle might be collected by a garbage collection on any thread). This is not a problem for Python API calls, since the thread on which tp_dealloc is called with an attached thread state. However, if the object being destroyed in turn destroys objects from some other C library, care should be taken to ensure that destroying those objects on the thread which called tp_dealloc will not violate any assumptions of the library.

Inheritance:

This field is inherited by subtypes.