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 thatâs 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 typeâs 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.
PyTypeObject Slot [1]
special methods/attrs
Info [2]
O
T
D
I
<R> tp_name
const char *
__name__
X
X
X
X
X
X
X
X
X
X
X
X
__getattribute__, __getattr__
G
__setattr__, __delattr__
G
%
__repr__
X
X
X
%
%
%
__hash__
X
G
__call__
X
X
__str__
X
X
__getattribute__, __getattr__
X
X
G
__setattr__, __delattr__
X
X
G
%
unsigned long
X
X
?
const char *
__doc__
X
X
X
G
X
G
__lt__, __le__, __eq__, __ne__, __gt__, __ge__
X
G
X
?
__iter__
X
__next__
X
PyMethodDef []
X
X
PyMemberDef []
X
PyGetSetDef []
X
X
__base__
X
PyObject *
__dict__
?
__get__
X
__set__, __delete__
X
X
?
__init__
X
X
X
X
?
?
__new__
X
X
?
?
X
X
?
?
X
X
<tp_bases>
PyObject *
__bases__
~
<tp_mro>
PyObject *
__mro__
~
[tp_cache]
PyObject *
void *
__subclasses__
PyObject *
(tp_del)
unsigned int
__del__
X
unsigned char
sub-slots¶ slot typedefs¶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.
The type objectâs reference count is initialized to
1by thePyObject_HEAD_INITmacro. Note that for statically allocated type objects, the typeâs instances (objects whoseob_typepoints 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.
PyVarObject Slots¶This is the typeâs type, in other words its metatype. It is initialized by the argument to the
PyObject_HEAD_INITmacro, 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 passNULLto thePyObject_HEAD_INITmacro and to initialize this field explicitly at the start of the moduleâs 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 ifob_typeisNULL, and if so, initializes it to theob_typefield of the base class.PyType_Ready()will not change this field if it is non-zero.Inheritance:
This field is 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 âDefaultâ section. (Note that many fields set on PyBaseObject_Type and PyType_Type effectively act as defaults.)
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.
These fields allow calculating the size in bytes of instances of the type.
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 âlengthâ of the object.
Functions like PyObject_NewVar() will take the value of N as an argument, and store in the instanceâs 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().
Also, the presence of an ob_size field in the instance layout doesnât 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 listsâ ob_size directly. Call PyList_Size() instead.)
The tp_basicsize includes size needed for data of the typeâs 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 baseâs 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 typeâs 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).
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 typeâs 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 donât 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.
An optional offset to a per-instance function that implements calling the object using the vectorcall protocol, a more efficient alternative of the simpler tp_call.
This field is only used if the flag Py_TPFLAGS_HAVE_VECTORCALL is set. If so, this must be a positive integer containing the offset in the instance of a vectorcallfunc pointer.
The vectorcallfunc pointer may be NULL, in which case the instance behaves as if Py_TPFLAGS_HAVE_VECTORCALL was not set: calling the instance falls back to tp_call.
Any class that sets Py_TPFLAGS_HAVE_VECTORCALL must also set tp_call and make sure its behaviour is consistent with the vectorcallfunc function. This can be done by setting tp_call to PyVectorcall_Call().
Changed in version 3.8: Before version 3.8, this slot was named tp_print. In Python 2.x, it was used for printing to a file. In Python 3.0 to 3.7, it was unused.
Changed in version 3.12: Before version 3.12, it was not recommended for mutable heap types to implement the vectorcall protocol. When a user sets __call__ in Python code, only tp_call is updated, likely making it inconsistent with the vectorcall function. Since 3.12, setting __call__ will disable vectorcall optimization by clearing the Py_TPFLAGS_HAVE_VECTORCALL flag.
Inheritance:
This field is always inherited. However, the Py_TPFLAGS_HAVE_VECTORCALL flag is not always inherited. If itâs not set, then the subclass wonât use vectorcall, except when PyVectorcall_Call() is explicitly called.
An optional pointer to the get-attribute-string function.
This field is deprecated. When it is defined, it should point to a function that acts the same as the tp_getattro function, but taking a C string instead of a Python string object to give the attribute name.
Inheritance:
Group: tp_getattr, tp_getattro
This field is inherited by subtypes together with tp_getattro: a subtype inherits both tp_getattr and tp_getattro from its base type when the subtypeâs tp_getattr and tp_getattro are both NULL.
An optional pointer to the function for setting and deleting attributes.
This field is deprecated. When it is defined, it should point to a function that acts the same as the tp_setattro function, but taking a C string instead of a Python string object to give the attribute name.
Inheritance:
Group: tp_setattr, tp_setattro
This field is inherited by subtypes together with tp_setattro: a subtype inherits both tp_setattr and tp_setattro from its base type when the subtypeâs tp_setattr and tp_setattro are both NULL.
Pointer to an additional structure that contains fields relevant only to objects which implement awaitable and asynchronous iterator protocols at the C-level. See Async Object Structures for details.
Added in version 3.5: Formerly known as tp_compare and tp_reserved.
Inheritance:
The tp_as_async field is not inherited, but the contained fields are inherited individually.
An optional pointer to a function that implements the built-in function repr().
The signature is the same as for PyObject_Repr():
PyObject *tp_repr(PyObject *self);
The function must return a string or a Unicode object. Ideally, this function should return a string that, when passed to eval(), given a suitable environment, returns an object with the same value. If this is not feasible, it should return a string starting with '<' and ending with '>' from which both the type and the value of the object can be deduced.
Inheritance:
This field is inherited by subtypes.
Default:
When this field is not set, a string of the form <%s object at %p> is returned, where %s is replaced by the type name, and %p by the objectâs memory address.
Pointer to an additional structure that contains fields relevant only to objects which implement the number protocol. These fields are documented in Number Object Structures.
Inheritance:
The tp_as_number field is not inherited, but the contained fields are inherited individually.
Pointer to an additional structure that contains fields relevant only to objects which implement the sequence protocol. These fields are documented in Sequence Object Structures.
Inheritance:
The tp_as_sequence field is not inherited, but the contained fields are inherited individually.
Pointer to an additional structure that contains fields relevant only to objects which implement the mapping protocol. These fields are documented in Mapping Object Structures.
Inheritance:
The tp_as_mapping field is not inherited, but the contained fields are inherited individually.
An optional pointer to a function that implements the built-in function hash().
The signature is the same as for PyObject_Hash():
Py_hash_t tp_hash(PyObject *);
The value -1 should not be returned as a normal return value; when an error occurs during the computation of the hash value, the function should set an exception and return -1.
When this field is not set (and tp_richcompare is not set), an attempt to take the hash of the object raises TypeError. This is the same as setting it to PyObject_HashNotImplemented().
This field can be set explicitly to PyObject_HashNotImplemented() to block inheritance of the hash method from a parent type. This is interpreted as the equivalent of __hash__ = None at the Python level, causing isinstance(o, collections.Hashable) to correctly return False. Note that the converse is also true - setting __hash__ = None on a class at the Python level will result in the tp_hash slot being set to PyObject_HashNotImplemented().
Inheritance:
Group: tp_hash, tp_richcompare
This field is inherited by subtypes together with tp_richcompare: a subtype inherits both of tp_richcompare and tp_hash, when the subtypeâs tp_richcompare and tp_hash are both NULL.
Default:
An optional pointer to a function that implements calling the object. This should be NULL if the object is not callable. The signature is the same as for PyObject_Call():
PyObject *tp_call(PyObject *self, PyObject *args, PyObject *kwargs);
Inheritance:
This field is inherited by subtypes.
An optional pointer to a function that implements the built-in operation str(). (Note that str is a type now, and str() calls the constructor for that type. This constructor calls PyObject_Str() to do the actual work, and PyObject_Str() will call this handler.)
The signature is the same as for PyObject_Str():
PyObject *tp_str(PyObject *self);
The function must return a string or a Unicode object. It should be a âfriendlyâ string representation of the object, as this is the representation that will be used, among other things, by the print() function.
Inheritance:
This field is inherited by subtypes.
Default:
When this field is not set, PyObject_Repr() is called to return a string representation.
An optional pointer to the get-attribute function.
The signature is the same as for PyObject_GetAttr():
PyObject *tp_getattro(PyObject *self, PyObject *attr);
It is usually convenient to set this field to PyObject_GenericGetAttr(), which implements the normal way of looking for object attributes.
Inheritance:
Group: tp_getattr, tp_getattro
This field is inherited by subtypes together with tp_getattr: a subtype inherits both tp_getattr and tp_getattro from its base type when the subtypeâs tp_getattr and tp_getattro are both NULL.
Default:
An optional pointer to the function for setting and deleting attributes.
The signature is the same as for PyObject_SetAttr():
int tp_setattro(PyObject *self, PyObject *attr, PyObject *value);
In addition, setting value to NULL to delete an attribute must be supported. It is usually convenient to set this field to PyObject_GenericSetAttr(), which implements the normal way of setting object attributes.
Inheritance:
Group: tp_setattr, tp_setattro
This field is inherited by subtypes together with tp_setattr: a subtype inherits both tp_setattr and tp_setattro from its base type when the subtypeâs tp_setattr and tp_setattro are both NULL.
Default:
Pointer to an additional structure that contains fields relevant only to objects which implement the buffer interface. These fields are documented in Buffer Object Structures.
Inheritance:
The tp_as_buffer field is not inherited, but the contained fields are inherited individually.
This field is a bit mask of various flags. Some flags indicate variant semantics for certain situations; others are used to indicate that certain fields in the type object (or in the extension structures referenced via tp_as_number, tp_as_sequence, tp_as_mapping, and tp_as_buffer) that were historically not always present are valid; if such a flag bit is clear, the type fields it guards must not be accessed and must be considered to have a zero or NULL value instead.
Inheritance:
Inheritance of this field is complicated. Most flag bits are inherited individually, i.e. if the base type has a flag bit set, the subtype inherits this flag bit. The flag bits that pertain to extension structures are strictly inherited if the extension structure is inherited, i.e. the base typeâs value of the flag bit is copied into the subtype together with a pointer to the extension structure. The Py_TPFLAGS_HAVE_GC flag bit is inherited together with the tp_traverse and tp_clear fields, i.e. if the Py_TPFLAGS_HAVE_GC flag bit is clear in the subtype and the tp_traverse and tp_clear fields in the subtype exist and have NULL values.
Default:
PyBaseObject_Type uses Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE.
Bit Masks:
The following bit masks are currently defined; these can be ORed together using the | operator to form the value of the tp_flags field. The macro PyType_HasFeature() takes a type and a flags value, tp and f, and checks whether tp->tp_flags & f is non-zero.
This bit is set when the type object itself is allocated on the heap, for example, types created dynamically using PyType_FromSpec(). In this case, the ob_type field of its instances is considered a reference to the type, and the type object is INCREFâed when a new instance is created, and DECREFâed when an instance is destroyed (this does not apply to instances of subtypes; only the type referenced by the instanceâs ob_type gets INCREFâed or DECREFâed). Heap types should also support garbage collection as they can form a reference cycle with their own module object.
Inheritance:
???
This bit is set when the type can be used as the base type of another type. If this bit is clear, the type cannot be subtyped (similar to a âfinalâ class in Java).
Inheritance:
???
This bit is set when the type object has been fully initialized by PyType_Ready().
Inheritance:
???
This bit is set while PyType_Ready() is in the process of initializing the type object.
Inheritance:
???
This bit is set when the object supports garbage collection. If this bit is set, memory for new instances (see tp_alloc) must be allocated using PyObject_GC_New or PyType_GenericAlloc() and deallocated (see tp_free) using PyObject_GC_Del(). More information in section Supporting Cyclic Garbage Collection.
Inheritance:
Group: Py_TPFLAGS_HAVE_GC, tp_traverse, tp_clear
The Py_TPFLAGS_HAVE_GC flag bit is inherited together with the tp_traverse and tp_clear fields, i.e. if the Py_TPFLAGS_HAVE_GC flag bit is clear in the subtype and the tp_traverse and tp_clear fields in the subtype exist and have NULL values.
This is a bitmask of all the bits that pertain to the existence of certain fields in the type object and its extension structures. Currently, it includes the following bits: Py_TPFLAGS_HAVE_STACKLESS_EXTENSION.
Inheritance:
???
This bit indicates that objects behave like unbound methods.
If this flag is set for type(meth), then:
meth.__get__(obj, cls)(*args, **kwds) (with obj not None) must be equivalent to meth(obj, *args, **kwds).
meth.__get__(None, cls)(*args, **kwds) must be equivalent to meth(*args, **kwds).
This flag enables an optimization for typical method calls like obj.meth(): it avoids creating a temporary âbound methodâ object for obj.meth.
Added in version 3.8.
Inheritance:
This flag is never inherited by types without the Py_TPFLAGS_IMMUTABLETYPE flag set. For extension types, it is inherited whenever tp_descr_get is inherited.
This bit indicates that instances of the class have a __dict__ attribute, and that the space for the dictionary is managed by the VM.
If this flag is set, Py_TPFLAGS_HAVE_GC should also be set.
The type traverse function must call PyObject_VisitManagedDict() and its clear function must call PyObject_ClearManagedDict().
Added in version 3.12.
Inheritance:
This flag is inherited unless the tp_dictoffset field is set in a superclass.
This bit indicates that instances of the class should be weakly referenceable.
Added in version 3.12.
Inheritance:
This flag is inherited unless the tp_weaklistoffset field is set in a superclass.
Only usable with variable-size types, i.e. ones with non-zero tp_itemsize.
Indicates that the variable-sized portion of an instance of this type is at the end of the instanceâs memory area, at an offset of Py_TYPE(obj)->tp_basicsize (which may be different in each subclass).
When setting this flag, be sure that all superclasses either use this memory layout, or are not variable-sized. Python does not check this.
Added in version 3.12.
Inheritance:
This flag is inherited.
These flags are used by functions such as PyLong_Check() to quickly determine if a type is a subclass of a built-in type; such specific checks are faster than a generic check, like PyObject_IsInstance(). Custom types that inherit from built-ins should have their tp_flags set appropriately, or the code that interacts with such types will behave differently depending on what kind of check is used.
This bit is set when the tp_finalize slot is present in the type structure.
Added in version 3.4.
Deprecated since version 3.8: This flag isnât necessary anymore, as the interpreter assumes the tp_finalize slot is always present in the type structure.
This bit is set when the class implements the vectorcall protocol. See tp_vectorcall_offset for details.
Inheritance:
This bit is inherited if tp_call is also inherited.
Added in version 3.9.
Changed in version 3.12: This flag is now removed from a class when the classâs __call__() method is reassigned.
This flag can now be inherited by mutable classes.
This bit is set for type objects that are immutable: type attributes cannot be set nor deleted.
PyType_Ready() automatically applies this flag to static types.
Inheritance:
This flag is not inherited.
Added in version 3.10.
Disallow creating instances of the type: set tp_new to NULL and donât create the __new__ key in the type dictionary.
The flag must be set before creating the type, not after. For example, it must be set before PyType_Ready() is called on the type.
The flag is set automatically on static types if tp_base is NULL or &PyBaseObject_Type and tp_new is NULL.
Inheritance:
This flag is not inherited. However, subclasses will not be instantiable unless they provide a non-NULL tp_new (which is only possible via the C API).
Note
To disallow instantiating a class directly but allow instantiating its subclasses (e.g. for an abstract base class), do not use this flag. Instead, make tp_new only succeed for subclasses.
Added in version 3.10.
This bit indicates that instances of the class may match mapping patterns when used as the subject of a match block. It is automatically set when registering or subclassing collections.abc.Mapping, and unset when registering collections.abc.Sequence.
Inheritance:
This flag is inherited by types that do not already set Py_TPFLAGS_SEQUENCE.
See also
PEP 634 â Structural Pattern Matching: Specification
Added in version 3.10.
This bit indicates that instances of the class may match sequence patterns when used as the subject of a match block. It is automatically set when registering or subclassing collections.abc.Sequence, and unset when registering collections.abc.Mapping.
Inheritance:
This flag is inherited by types that do not already set Py_TPFLAGS_MAPPING.
See also
PEP 634 â Structural Pattern Matching: Specification
Added in version 3.10.
Internal. Do not set or unset this flag. To indicate that a class has changed call PyType_Modified()
Warning
This flag is present in header files, but is not be used. It will be removed in a future version of CPython
An optional pointer to a NUL-terminated C string giving the docstring for this type object. This is exposed as the __doc__ attribute on the type and instances of the type.
Inheritance:
This field is not inherited by subtypes.
An optional pointer to a traversal function for the garbage collector. This is only used if the Py_TPFLAGS_HAVE_GC flag bit is set. The signature is:
int tp_traverse(PyObject *self, visitproc visit, void *arg);
More information about Pythonâs garbage collection scheme can be found in section Supporting Cyclic Garbage Collection.
The tp_traverse pointer is used by the garbage collector to detect reference cycles. A typical implementation of a tp_traverse function simply calls Py_VISIT() on each of the instanceâs members that are Python objects that the instance owns. For example, this is function local_traverse() from the _thread extension module:
static int
local_traverse(PyObject *op, visitproc visit, void *arg)
{
localobject *self = (localobject *) op;
Py_VISIT(self->args);
Py_VISIT(self->kw);
Py_VISIT(self->dict);
return 0;
}
Note that Py_VISIT() is called only on those members that can participate in reference cycles. Although there is also a self->key member, it can only be NULL or a Python string and therefore cannot be part of a reference cycle.
On the other hand, even if you know a member can never be part of a cycle, as a debugging aid you may want to visit it anyway just so the gc moduleâs get_referents() function will include it.
Heap types (Py_TPFLAGS_HEAPTYPE) must visit their type with:
It is only needed since Python 3.9. To support Python 3.8 and older, this line must be conditional:
#if PY_VERSION_HEX >= 0x03090000
Py_VISIT(Py_TYPE(self));
#endif
If the Py_TPFLAGS_MANAGED_DICT bit is set in the tp_flags field, the traverse function must call PyObject_VisitManagedDict() like this:
PyObject_VisitManagedDict((PyObject*)self, visit, arg);
Warning
When implementing tp_traverse, only the members that the instance owns (by having strong references to them) must be visited. For instance, if an object supports weak references via the tp_weaklist slot, the pointer supporting the linked list (what tp_weaklist points to) must not be visited as the instance does not directly own the weak references to itself (the weakreference list is there to support the weak reference machinery, but the instance has no strong reference to the elements inside it, as they are allowed to be removed even if the instance is still alive).
Note that Py_VISIT() requires the visit and arg parameters to local_traverse() to have these specific names; donât name them just anything.
Instances of heap-allocated types hold a reference to their type. Their traversal function must therefore either visit Py_TYPE(self), or delegate this responsibility by calling tp_traverse of another heap-allocated type (such as a heap-allocated superclass). If they do not, the type object may not be garbage-collected.
Note
The tp_traverse function can be called from any thread.
Changed in version 3.9: Heap-allocated types are expected to visit Py_TYPE(self) in tp_traverse. In earlier versions of Python, due to bug 40217, doing this may lead to crashes in subclasses.
Inheritance:
Group: Py_TPFLAGS_HAVE_GC, tp_traverse, tp_clear
This field is inherited by subtypes together with tp_clear and the Py_TPFLAGS_HAVE_GC flag bit: the flag bit, tp_traverse, and tp_clear are all inherited from the base type if they are all zero in the subtype.
An optional pointer to a clear function. The signature is:
int tp_clear(PyObject *);
The purpose of this function is to break reference cycles that are causing a cyclic isolate so that the objects can be safely destroyed. A cleared object is a partially destroyed object; the object is not obligated to satisfy design invariants held during normal use.
tp_clear does not need to delete references to objects that canât participate in reference cycles, such as Python strings or Python integers. However, it may be convenient to clear all references, and write the typeâs tp_dealloc function to invoke tp_clear to avoid code duplication. (Beware that tp_clear might have already been called. Prefer calling idempotent functions like Py_CLEAR().)
Any non-trivial cleanup should be performed in tp_finalize instead of tp_clear.
Note
If tp_clear fails to break a reference cycle then the objects in the cyclic isolate may remain indefinitely uncollectable (âleakâ). See gc.garbage.
Note
Referents (direct and indirect) might have already been cleared; they are not guaranteed to be in a consistent state.
Note
The tp_clear function can be called from any thread.
Note
An object is not guaranteed to be automatically cleared before its destructor (tp_dealloc) is called.
This function differs from the destructor (tp_dealloc) in the following ways:
The purpose of clearing an object is to remove references to other objects that might participate in a reference cycle. The purpose of the destructor, on the other hand, is a superset: it must release all resources it owns, including references to objects that cannot participate in a reference cycle (e.g., integers) as well as the objectâs own memory (by calling tp_free).
When tp_clear is called, other objects might still hold references to the object being cleared. Because of this, tp_clear must not deallocate the objectâs own memory (tp_free). The destructor, on the other hand, is only called when no (strong) references exist, and as such, must safely destroy the object itself by deallocating it.
tp_clear might never be automatically called. An objectâs destructor, on the other hand, will be automatically called some time after the object becomes unreachable (i.e., either there are no references to the object or the object is a member of a cyclic isolate).
No guarantees are made about when, if, or how often Python automatically clears an object, except:
Python will not automatically clear an object if it is reachable, i.e., there is a reference to it and it is not a member of a cyclic isolate.
Python will not automatically clear an object if it has not been automatically finalized (see tp_finalize). (If the finalizer resurrected the object, the object may or may not be automatically finalized again before it is cleared.)
If an object is a member of a cyclic isolate, Python will not automatically clear it if any member of the cyclic isolate has not yet been automatically finalized (tp_finalize).
Python will not destroy an object until after any automatic calls to its tp_clear function have returned. This ensures that the act of breaking a reference cycle does not invalidate the self pointer while tp_clear is still executing.
Python will not automatically call tp_clear multiple times concurrently.
CPython currently only automatically clears objects as needed to break reference cycles in a cyclic isolate, but future versions might clear objects regularly before their destruction.
Taken together, all tp_clear functions in the system must combine to break all reference cycles. This is subtle, and if in any doubt supply a tp_clear function. For example, the tuple type does not implement a tp_clear function, because itâs possible to prove that no reference cycle can be composed entirely of tuples. Therefore the tp_clear functions of other types are responsible for breaking any cycle containing a tuple. This isnât immediately obvious, and thereâs rarely a good reason to avoid implementing tp_clear.
Implementations of tp_clear should drop the instanceâs references to those of its members that may be Python objects, and set its pointers to those members to NULL, as in the following example:
static int
local_clear(PyObject *op)
{
localobject *self = (localobject *) op;
Py_CLEAR(self->key);
Py_CLEAR(self->args);
Py_CLEAR(self->kw);
Py_CLEAR(self->dict);
return 0;
}
The Py_CLEAR() macro should be used, because clearing references is delicate: the reference to the contained object must not be released (via Py_DECREF()) until after the pointer to the contained object is set to NULL. This is because releasing the reference may cause the contained object to become trash, triggering a chain of reclamation activity that may include invoking arbitrary Python code (due to finalizers, or weakref callbacks, associated with the contained object). If itâs possible for such code to reference self again, itâs important that the pointer to the contained object be NULL at that time, so that self knows the contained object can no longer be used. The Py_CLEAR() macro performs the operations in a safe order.
If the Py_TPFLAGS_MANAGED_DICT bit is set in the tp_flags field, the traverse function must call PyObject_ClearManagedDict() like this:
PyObject_ClearManagedDict((PyObject*)self);
More information about Pythonâs garbage collection scheme can be found in section Supporting Cyclic Garbage Collection.
Inheritance:
Group: Py_TPFLAGS_HAVE_GC, tp_traverse, tp_clear
This field is inherited by subtypes together with tp_traverse and the Py_TPFLAGS_HAVE_GC flag bit: the flag bit, tp_traverse, and tp_clear are all inherited from the base type if they are all zero in the subtype.
An optional pointer to the rich comparison function, whose signature is:
PyObject *tp_richcompare(PyObject *self, PyObject *other, int op);
The first parameter is guaranteed to be an instance of the type that is defined by PyTypeObject.
The function should return the result of the comparison (usually Py_True or Py_False). If the comparison is undefined, it must return Py_NotImplemented, if another error occurred it must return NULL and set an exception condition.
The following constants are defined to be used as the third argument for tp_richcompare and for PyObject_RichCompare():
Constant
Comparison
<
<=
==
!=
>
>=
The following macro is defined to ease writing rich comparison functions:
Return Py_True or Py_False from the function, depending on the result of a comparison. VAL_A and VAL_B must be orderable by C comparison operators (for example, they may be C ints or floats). The third argument specifies the requested operation, as for PyObject_RichCompare().
The returned value is a new strong reference.
On error, sets an exception and returns NULL from the function.
Added in version 3.7.
Inheritance:
Group: tp_hash, tp_richcompare
This field is inherited by subtypes together with tp_hash: a subtype inherits tp_richcompare and tp_hash when the subtypeâs tp_richcompare and tp_hash are both NULL.
Default:
PyBaseObject_Type provides a tp_richcompare implementation, which may be inherited. However, if only tp_hash is defined, not even the inherited function is used and instances of the type will not be able to participate in any comparisons.
While this field is still supported, Py_TPFLAGS_MANAGED_WEAKREF should be used instead, if at all possible.
If the instances of this type are weakly referenceable, this field is greater than zero and contains the offset in the instance structure of the weak reference list head (ignoring the GC header, if present); this offset is used by PyObject_ClearWeakRefs() and the PyWeakref_* functions. The instance structure needs to include a field of type PyObject* which is initialized to NULL.
Do not confuse this field with tp_weaklist; that is the list head for weak references to the type object itself.
It is an error to set both the Py_TPFLAGS_MANAGED_WEAKREF bit and tp_weaklistoffset.
Inheritance:
This field is inherited by subtypes, but see the rules listed below. A subtype may override this offset; this means that the subtype uses a different weak reference list head than the base type. Since the list head is always found via tp_weaklistoffset, this should not be a problem.
Default:
If the Py_TPFLAGS_MANAGED_WEAKREF bit is set in the tp_flags field, then tp_weaklistoffset will be set to a negative value, to indicate that it is unsafe to use this field.
An optional pointer to a function that returns an iterator for the object. Its presence normally signals that the instances of this type are iterable (although sequences may be iterable without this function).
This function has the same signature as PyObject_GetIter():
PyObject *tp_iter(PyObject *self);
Inheritance:
This field is inherited by subtypes.
An optional pointer to a function that returns the next item in an iterator. The signature is:
PyObject *tp_iternext(PyObject *self);
When the iterator is exhausted, it must return NULL; a StopIteration exception may or may not be set. When another error occurs, it must return NULL too. Its presence signals that the instances of this type are iterators.
Iterator types should also define the tp_iter function, and that function should return the iterator instance itself (not a new iterator instance).
This function has the same signature as PyIter_Next().
Inheritance:
This field is inherited by subtypes.
An optional pointer to a static NULL-terminated array of PyMethodDef structures, declaring regular methods of this type.
For each entry in the array, an entry is added to the typeâs dictionary (see tp_dict below) containing a method descriptor.
Inheritance:
This field is not inherited by subtypes (methods are inherited through a different mechanism).
An optional pointer to a static NULL-terminated array of PyMemberDef structures, declaring regular data members (fields or slots) of instances of this type.
For each entry in the array, an entry is added to the typeâs dictionary (see tp_dict below) containing a member descriptor.
Inheritance:
This field is not inherited by subtypes (members are inherited through a different mechanism).
An optional pointer to a static NULL-terminated array of PyGetSetDef structures, declaring computed attributes of instances of this type.
For each entry in the array, an entry is added to the typeâs dictionary (see tp_dict below) containing a getset descriptor.
Inheritance:
This field is not inherited by subtypes (computed attributes are inherited through a different mechanism).
An optional pointer to a base type from which type properties are inherited. At this level, only single inheritance is supported; multiple inheritance require dynamically creating a type object by calling the metatype.
Note
Slot initialization is subject to the rules of initializing globals. C99 requires the initializers to be âaddress constantsâ. Function designators like PyType_GenericNew(), with implicit conversion to a pointer, are valid C99 address constants.
However, the unary â&â operator applied to a non-static variable like PyBaseObject_Type is not required to produce an address constant. Compilers may support this (gcc does), MSVC does not. Both compilers are strictly standard conforming in this particular behavior.
Consequently, tp_base should be set in the extension moduleâs init function.
Inheritance:
This field is not inherited by subtypes (obviously).
Default:
This field defaults to &PyBaseObject_Type (which to Python programmers is known as the type object).
The typeâs dictionary is stored here by PyType_Ready().
This field should normally be initialized to NULL before PyType_Ready is called; it may also be initialized to a dictionary containing initial attributes for the type. Once PyType_Ready() has initialized the type, extra attributes for the type may be added to this dictionary only if they donât correspond to overloaded operations (like __add__()). Once initialization for the type has finished, this field should be treated as read-only.
Some types may not store their dictionary in this slot. Use PyType_GetDict() to retrieve the dictionary for an arbitrary type.
Changed in version 3.12: Internals detail: For static builtin types, this is always NULL. Instead, the dict for such types is stored on PyInterpreterState. Use PyType_GetDict() to get the dict for an arbitrary type.
Inheritance:
This field is not inherited by subtypes (though the attributes defined in here are inherited through a different mechanism).
Default:
If this field is NULL, PyType_Ready() will assign a new dictionary to it.
An optional pointer to a âdescriptor getâ function.
The function signature is:
PyObject * tp_descr_get(PyObject *self, PyObject *obj, PyObject *type);
Inheritance:
This field is inherited by subtypes.
An optional pointer to a function for setting and deleting a descriptorâs value.
The function signature is:
int tp_descr_set(PyObject *self, PyObject *obj, PyObject *value);
The value argument is set to NULL to delete the value.
Inheritance:
This field is inherited by subtypes.
While this field is still supported, Py_TPFLAGS_MANAGED_DICT should be used instead, if at all possible.
If the instances of this type have a dictionary containing instance variables, this field is non-zero and contains the offset in the instances of the type of the instance variable dictionary; this offset is used by PyObject_GenericGetAttr().
Do not confuse this field with tp_dict; that is the dictionary for attributes of the type object itself.
The value specifies the offset of the dictionary from the start of the instance structure.
The tp_dictoffset should be regarded as write-only. To get the pointer to the dictionary call PyObject_GenericGetDict(). Calling PyObject_GenericGetDict() may need to allocate memory for the dictionary, so it is may be more efficient to call PyObject_GetAttr() when accessing an attribute on the object.
It is an error to set both the Py_TPFLAGS_MANAGED_DICT bit and tp_dictoffset.
Inheritance:
This field is inherited by subtypes. A subtype should not override this offset; doing so could be unsafe, if C code tries to access the dictionary at the previous offset. To properly support inheritance, use Py_TPFLAGS_MANAGED_DICT.
Default:
This slot has no default. For static types, if the field is NULL then no __dict__ gets created for instances.
If the Py_TPFLAGS_MANAGED_DICT bit is set in the tp_flags field, then tp_dictoffset will be set to -1, to indicate that it is unsafe to use this field.
An optional pointer to an instance initialization function.
This function corresponds to the __init__() method of classes. Like __init__(), it is possible to create an instance without calling __init__(), and it is possible to reinitialize an instance by calling its __init__() method again.
The function signature is:
int tp_init(PyObject *self, PyObject *args, PyObject *kwds);
The self argument is the instance to be initialized; the args and kwds arguments represent positional and keyword arguments of the call to __init__().
The tp_init function, if not NULL, is called when an instance is created normally by calling its type, after the typeâs tp_new function has returned an instance of the type. If the tp_new function returns an instance of some other type that is not a subtype of the original type, no tp_init function is called; if tp_new returns an instance of a subtype of the original type, the subtypeâs tp_init is called.
Returns 0 on success, -1 and sets an exception on error.
Inheritance:
This field is inherited by subtypes.
Default:
For static types this field does not have a default.
An optional pointer to an instance allocation function.
The function signature is:
PyObject *tp_alloc(PyTypeObject *self, Py_ssize_t nitems);
Inheritance:
Static subtypes inherit this slot, which will be PyType_GenericAlloc() if inherited from object.
Heap subtypes do not inherit this slot.
Default:
For heap subtypes, this field is always set to PyType_GenericAlloc().
For static subtypes, this slot is inherited (see above).
An optional pointer to an instance creation function.
The function signature is:
PyObject *tp_new(PyTypeObject *subtype, PyObject *args, PyObject *kwds);
The subtype argument is the type of the object being created; the args and kwds arguments represent positional and keyword arguments of the call to the type. Note that subtype doesnât have to equal the type whose tp_new function is called; it may be a subtype of that type (but not an unrelated type).
The tp_new function should call subtype->tp_alloc(subtype, nitems) to allocate space for the object, and then do only as much further initialization as is absolutely necessary. Initialization that can safely be ignored or repeated should be placed in the tp_init handler. A good rule of thumb is that for immutable types, all initialization should take place in tp_new, while for mutable types, most initialization should be deferred to tp_init.
Set the Py_TPFLAGS_DISALLOW_INSTANTIATION flag to disallow creating instances of the type in Python.
Inheritance:
This field is inherited by subtypes, except it is not inherited by static types whose tp_base is NULL or &PyBaseObject_Type.
Default:
For static types this field has no default. This means if the slot is defined as NULL, the type cannot be called to create new instances; presumably there is some other way to create instances, like a factory function.
An optional pointer to an instance deallocation function. Its signature is:
void tp_free(void *self);
This function must free the memory allocated by tp_alloc.
Inheritance:
Static subtypes inherit this slot, which will be PyObject_Free() if inherited from object. Exception: If the type supports garbage collection (i.e., the Py_TPFLAGS_HAVE_GC flag is set in tp_flags) and it would inherit PyObject_Free(), then this slot is not inherited but instead defaults to PyObject_GC_Del().
Heap subtypes do not inherit this slot.
Default:
For heap subtypes, this slot defaults to a deallocator suitable to match PyType_GenericAlloc() and the value of the Py_TPFLAGS_HAVE_GC flag.
For static subtypes, this slot is inherited (see above).
An optional pointer to a function called by the garbage collector.
The garbage collector needs to know whether a particular object is collectible or not. Normally, it is sufficient to look at the objectâs typeâs tp_flags field, and check the Py_TPFLAGS_HAVE_GC flag bit. But some types have a mixture of statically and dynamically allocated instances, and the statically allocated instances are not collectible. Such types should define this function; it should return 1 for a collectible instance, and 0 for a non-collectible instance. The signature is:
int tp_is_gc(PyObject *self);
(The only example of this are types themselves. The metatype, PyType_Type, defines this function to distinguish between statically and dynamically allocated types.)
Inheritance:
This field is inherited by subtypes.
Default:
This slot has no default. If this field is NULL, Py_TPFLAGS_HAVE_GC is used as the functional equivalent.
Tuple of base types.
This field should be set to NULL and treated as read-only. Python will fill it in when the type is initialized.
For dynamically created classes, the Py_tp_bases slot can be used instead of the bases argument of PyType_FromSpecWithBases(). The argument form is preferred.
Warning
Multiple inheritance does not work well for statically defined types. If you set tp_bases to a tuple, Python will not raise an error, but some slots will only be inherited from the first base.
Inheritance:
This field is not inherited.
Tuple containing the expanded set of base types, starting with the type itself and ending with object, in Method Resolution Order.
This field should be set to NULL and treated as read-only. Python will fill it in when the type is initialized.
Inheritance:
This field is not inherited; it is calculated fresh by PyType_Ready().
Unused. Internal use only.
Inheritance:
This field is not inherited.
A collection of subclasses. Internal use only. May be an invalid pointer.
To get a list of subclasses, call the Python method __subclasses__().
Changed in version 3.12: For some types, this field does not hold a valid PyObject*. The type was changed to void* to indicate this.
Inheritance:
This field is not inherited.
Weak reference list head, for weak references to this type object. Not inherited. Internal use only.
Changed in version 3.12: Internals detail: For the static builtin types this is always NULL, even if weakrefs are added. Instead, the weakrefs for each are stored on PyInterpreterState. Use the public C-API or the internal _PyObject_GET_WEAKREFS_LISTPTR() macro to avoid the distinction.
Inheritance:
This field is not inherited.
This field is deprecated. Use tp_finalize instead.
Used to index into the method cache. Internal use only.
Inheritance:
This field is not inherited.
An optional pointer to an instance finalization function. This is the C implementation of the __del__() special method. Its signature is:
void tp_finalize(PyObject *self);
The primary purpose of finalization is to perform any non-trivial cleanup that must be performed before the object is destroyed, while the object and any other objects it directly or indirectly references are still in a consistent state. The finalizer is allowed to execute arbitrary Python code.
Before Python automatically finalizes an object, some of the objectâs direct or indirect referents might have themselves been automatically finalized. However, none of the referents will have been automatically cleared (tp_clear) yet.
Other non-finalized objects might still be using a finalized object, so the finalizer must leave the object in a sane state (e.g., invariants are still met).
Note
After Python automatically finalizes an object, Python might start automatically clearing (tp_clear) the object and its referents (direct and indirect). Cleared objects are not guaranteed to be in a consistent state; a finalized object must be able to tolerate cleared referents.
Note
An object is not guaranteed to be automatically finalized before its destructor (tp_dealloc) is called. It is recommended to call PyObject_CallFinalizerFromDealloc() at the beginning of tp_dealloc to guarantee that the object is always finalized before destruction.
Note
The tp_finalize function can be called from any thread, although the GIL will be held.
Note
The tp_finalize function can be called during shutdown, after some global variables have been deleted. See the documentation of the __del__() method for details.
When Python finalizes an object, it behaves like the following algorithm:
Python might mark the object as finalized. Currently, Python always marks objects whose type supports garbage collection (i.e., the Py_TPFLAGS_HAVE_GC flag is set in tp_flags) and never marks other types of objects; this might change in a future version.
If the object is not marked as finalized and its tp_finalize finalizer function is non-NULL, the finalizer function is called.
If the finalizer function was called and the finalizer made the object reachable (i.e., there is a reference to the object and it is not a member of a cyclic isolate), then the finalizer is said to have resurrected the object. It is unspecified whether the finalizer can also resurrect the object by adding a new reference to the object that does not make it reachable, i.e., the object is (still) a member of a cyclic isolate.
If the finalizer resurrected the object, the objectâs pending destruction is canceled and the objectâs finalized mark might be removed if present. Currently, Python never removes the finalized mark; this might change in a future version.
Automatic finalization refers to any finalization performed by Python except via calls to PyObject_CallFinalizer() or PyObject_CallFinalizerFromDealloc(). No guarantees are made about when, if, or how often an object is automatically finalized, except:
Python will not automatically finalize an object if it is reachable, i.e., there is a reference to it and it is not a member of a cyclic isolate.
Python will not automatically finalize an object if finalizing it would not mark the object as finalized. Currently, this applies to objects whose type does not support garbage collection, i.e., the Py_TPFLAGS_HAVE_GC flag is not set. Such objects can still be manually finalized by calling PyObject_CallFinalizer() or PyObject_CallFinalizerFromDealloc().
Python will not automatically finalize any two members of a cyclic isolate concurrently.
Python will not automatically finalize an object after it has automatically cleared (tp_clear) the object.
If an object is a member of a cyclic isolate, Python will not automatically finalize it after automatically clearing (see tp_clear) any other member.
Python will automatically finalize every member of a cyclic isolate before it automatically clears (see tp_clear) any of them.
If Python is going to automatically clear an object (tp_clear), it will automatically finalize the object first.
Python currently only automatically finalizes objects that are members of a cyclic isolate, but future versions might finalize objects regularly before their destruction.
To manually finalize an object, do not call this function directly; call PyObject_CallFinalizer() or PyObject_CallFinalizerFromDealloc() instead.
tp_finalize should leave the current exception status unchanged. The recommended way to write a non-trivial finalizer is to back up the exception at the beginning by calling PyErr_GetRaisedException() and restore the exception at the end by calling PyErr_SetRaisedException(). If an exception is encountered in the middle of the finalizer, log and clear it with PyErr_WriteUnraisable() or PyErr_FormatUnraisable(). For example:
static void
foo_finalize(PyObject *self)
{
// Save the current exception, if any.
PyObject *exc = PyErr_GetRaisedException();
// ...
if (do_something_that_might_raise() != success_indicator) {
PyErr_WriteUnraisable(self);
goto done;
}
done:
// Restore the saved exception. This silently discards any exception
// raised above, so be sure to call PyErr_WriteUnraisable first if
// necessary.
PyErr_SetRaisedException(exc);
}
Inheritance:
This field is inherited by subtypes.
Added in version 3.4.
Changed in version 3.8: Before version 3.8 it was necessary to set the Py_TPFLAGS_HAVE_FINALIZE flags bit in order for this field to be used. This is no longer required.
A vectorcall function to use for calls of this type object (rather than instances). In other words, tp_vectorcall can be used to optimize type.__call__, which typically returns a new instance of type.
As with any vectorcall function, if tp_vectorcall is NULL, the tp_call protocol (Py_TYPE(type)->tp_call) is used instead.
Note
The vectorcall protocol requires that the vectorcall function has the same behavior as the corresponding tp_call. This means that type->tp_vectorcall must match the behavior of Py_TYPE(type)->tp_call.
Specifically, if type uses the default metaclass, type->tp_vectorcall must behave the same as PyType_Type->tp_call, which:
calls type->tp_new,
if the result is a subclass of type, calls type->tp_init on the result of tp_new, and
returns the result of tp_new.
Typically, tp_vectorcall is overridden to optimize this process for specific tp_new and tp_init. When doing this for user-subclassable types, note that both can be overridden (using __new__() and __init__(), respectively).
Inheritance:
This field is never inherited.
Added in version 3.9: (the field exists since 3.8 but itâs only used since 3.9)
Internal. Do not use.
Added in version 3.12.
Traditionally, types defined in C code are static, that is, a static PyTypeObject structure is defined directly in code and initialized using PyType_Ready().
This results in types that are limited relative to types defined in Python:
Static types are limited to one base, i.e. they cannot use multiple inheritance.
Static type objects (but not necessarily their instances) are immutable. It is not possible to add or modify the type objectâs attributes from Python.
Static type objects are shared across sub-interpreters, so they should not include any subinterpreter-specific state.
Also, since PyTypeObject is only part of the Limited API as an opaque struct, any extension modules using static types must be compiled for a specific Python minor version.
An alternative to static types is heap-allocated types, or heap types for short, which correspond closely to classes created by Pythonâs class statement. Heap types have the Py_TPFLAGS_HEAPTYPE flag set.
This is done by filling a PyType_Spec structure and calling PyType_FromSpec(), PyType_FromSpecWithBases(), PyType_FromModuleAndSpec(), or PyType_FromMetaclass().
This structure holds pointers to the functions which an object uses to implement the number protocol. Each function is used by the function of similar name documented in the Number Protocol section.
Here is the structure definition:
typedef struct {
binaryfunc nb_add;
binaryfunc nb_subtract;
binaryfunc nb_multiply;
binaryfunc nb_remainder;
binaryfunc nb_divmod;
ternaryfunc nb_power;
unaryfunc nb_negative;
unaryfunc nb_positive;
unaryfunc nb_absolute;
inquiry nb_bool;
unaryfunc nb_invert;
binaryfunc nb_lshift;
binaryfunc nb_rshift;
binaryfunc nb_and;
binaryfunc nb_xor;
binaryfunc nb_or;
unaryfunc nb_int;
void *nb_reserved;
unaryfunc nb_float;
binaryfunc nb_inplace_add;
binaryfunc nb_inplace_subtract;
binaryfunc nb_inplace_multiply;
binaryfunc nb_inplace_remainder;
ternaryfunc nb_inplace_power;
binaryfunc nb_inplace_lshift;
binaryfunc nb_inplace_rshift;
binaryfunc nb_inplace_and;
binaryfunc nb_inplace_xor;
binaryfunc nb_inplace_or;
binaryfunc nb_floor_divide;
binaryfunc nb_true_divide;
binaryfunc nb_inplace_floor_divide;
binaryfunc nb_inplace_true_divide;
unaryfunc nb_index;
binaryfunc nb_matrix_multiply;
binaryfunc nb_inplace_matrix_multiply;
} PyNumberMethods;
Note
Binary and ternary functions must check the type of all their operands, and implement the necessary conversions (at least one of the operands is an instance of the defined type). If the operation is not defined for the given operands, binary and ternary functions must return Py_NotImplemented, if another error occurred they must return NULL and set an exception.
Note
The nb_reserved field should always be NULL. It was previously called nb_long, and was renamed in Python 3.0.1.
This structure holds pointers to the functions which an object uses to implement the mapping protocol. It has three members:
This function is used by PyMapping_Size() and PyObject_Size(), and has the same signature. This slot may be set to NULL if the object has no defined length.
This function is used by PyObject_GetItem() and PySequence_GetSlice(), and has the same signature as PyObject_GetItem(). This slot must be filled for the PyMapping_Check() function to return 1, it can be NULL otherwise.
This function is used by PyObject_SetItem(), PyObject_DelItem(), PySequence_SetSlice() and PySequence_DelSlice(). It has the same signature as PyObject_SetItem(), but v can also be set to NULL to delete an item. If this slot is NULL, the object does not support item assignment and deletion.
This structure holds pointers to the functions which an object uses to implement the sequence protocol.
This function is used by PySequence_Size() and PyObject_Size(), and has the same signature. It is also used for handling negative indices via the sq_item and the sq_ass_item slots.
This function is used by PySequence_Concat() and has the same signature. It is also used by the + operator, after trying the numeric addition via the nb_add slot.
This function is used by PySequence_Repeat() and has the same signature. It is also used by the * operator, after trying numeric multiplication via the nb_multiply slot.
This function is used by PySequence_GetItem() and has the same signature. It is also used by PyObject_GetItem(), after trying the subscription via the mp_subscript slot. This slot must be filled for the PySequence_Check() function to return 1, it can be NULL otherwise.
Negative indexes are handled as follows: if the sq_length slot is filled, it is called and the sequence length is used to compute a positive index which is passed to sq_item. If sq_length is NULL, the index is passed as is to the function.
This function is used by PySequence_SetItem() and has the same signature. It is also used by PyObject_SetItem() and PyObject_DelItem(), after trying the item assignment and deletion via the mp_ass_subscript slot. This slot may be left to NULL if the object does not support item assignment and deletion.
This function may be used by PySequence_Contains() and has the same signature. This slot may be left to NULL, in this case PySequence_Contains() simply traverses the sequence until it finds a match.
This function is used by PySequence_InPlaceConcat() and has the same signature. It should modify its first operand, and return it. This slot may be left to NULL, in this case PySequence_InPlaceConcat() will fall back to PySequence_Concat(). It is also used by the augmented assignment +=, after trying numeric in-place addition via the nb_inplace_add slot.
This function is used by PySequence_InPlaceRepeat() and has the same signature. It should modify its first operand, and return it. This slot may be left to NULL, in this case PySequence_InPlaceRepeat() will fall back to PySequence_Repeat(). It is also used by the augmented assignment *=, after trying numeric in-place multiplication via the nb_inplace_multiply slot.
This structure holds pointers to the functions required by the Buffer protocol. The protocol defines how an exporter object can expose its internal data to consumer objects.
The signature of this function is:
int (PyObject *exporter, Py_buffer *view, int flags);
Handle a request to exporter to fill in view as specified by flags. Except for point (3), an implementation of this function MUST take these steps:
Check if the request can be met. If not, raise BufferError, set view->obj to NULL and return -1.
Fill in the requested fields.
Increment an internal counter for the number of exports.
Set view->obj to exporter and increment view->obj.
Return 0.
If exporter is part of a chain or tree of buffer providers, two main schemes can be used:
Re-export: Each member of the tree acts as the exporting object and sets view->obj to a new reference to itself.
Redirect: The buffer request is redirected to the root object of the tree. Here, view->obj will be a new reference to the root object.
The individual fields of view are described in section Buffer structure, the rules how an exporter must react to specific requests are in section Buffer request types.
All memory pointed to in the Py_buffer structure belongs to the exporter and must remain valid until there are no consumers left. format, shape, strides, suboffsets and internal are read-only for the consumer.
PyBuffer_FillInfo() provides an easy way of exposing a simple bytes buffer while dealing correctly with all request types.
PyObject_GetBuffer() is the interface for the consumer that wraps this function.
The signature of this function is:
void (PyObject *exporter, Py_buffer *view);
Handle a request to release the resources of the buffer. If no resources need to be released, PyBufferProcs.bf_releasebuffer may be NULL. Otherwise, a standard implementation of this function will take these optional steps:
Decrement an internal counter for the number of exports.
If the counter is 0, free all memory associated with view.
The exporter MUST use the internal field to keep track of buffer-specific resources. This field is guaranteed to remain constant, while a consumer MAY pass a copy of the original buffer as the view argument.
This function MUST NOT decrement view->obj, since that is done automatically in PyBuffer_Release() (this scheme is useful for breaking reference cycles).
PyBuffer_Release() is the interface for the consumer that wraps this function.
Added in version 3.5.
This structure holds pointers to the functions required to implement awaitable and asynchronous iterator objects.
Here is the structure definition:
typedef struct {
unaryfunc am_await;
unaryfunc am_aiter;
unaryfunc am_anext;
sendfunc am_send;
} PyAsyncMethods;
The signature of this function is:
PyObject *am_await(PyObject *self);
The returned object must be an iterator, i.e. PyIter_Check() must return 1 for it.
This slot may be set to NULL if an object is not an awaitable.
The signature of this function is:
PyObject *am_aiter(PyObject *self);
Must return an asynchronous iterator object. See __anext__() for details.
This slot may be set to NULL if an object does not implement asynchronous iteration protocol.
The signature of this function is:
PyObject *am_anext(PyObject *self);
Must return an awaitable object. See __anext__() for details. This slot may be set to NULL.
The signature of this function is:
PySendResult am_send(PyObject *self, PyObject *arg, PyObject **result);
See PyIter_Send() for details. This slot may be set to NULL.
Added in version 3.10.
The purpose of this function is to separate memory allocation from memory initialization. It should return a pointer to a block of memory of adequate length for the instance, suitably aligned, and initialized to zeros, but with ob_refcnt set to 1 and ob_type set to the type argument. If the typeâs tp_itemsize is non-zero, the objectâs ob_size field should be initialized to nitems and the length of the allocated memory block should be tp_basicsize + nitems*tp_itemsize, rounded up to a multiple of sizeof(void*); otherwise, nitems is not used and the length of the block should be tp_basicsize.
This function should not do any other instance initialization, not even to allocate additional memory; that should be done by tp_new.
See tp_free.
See tp_new.
See tp_init.
See tp_repr.
Return the value of the named attribute for the object.
Set the value of the named attribute for the object. The value argument is set to NULL to delete the attribute.
Return the value of the named attribute for the object.
See tp_getattro.
Set the value of the named attribute for the object. The value argument is set to NULL to delete the attribute.
See tp_setattro.
See tp_descr_get.
See tp_descr_set.
See tp_hash.
See tp_richcompare.
See tp_iter.
See tp_iternext.
See am_send.
The following are simple examples of Python type definitions. They include common usage you may encounter. Some demonstrate tricky corner cases. For more examples, practical info, and a tutorial, see Defining Extension Types: Tutorial and Defining Extension Types: Assorted Topics.
A basic static type:
typedef struct {
PyObject_HEAD
const char *data;
} MyObject;
static PyTypeObject MyObject_Type = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "mymod.MyObject",
.tp_basicsize = sizeof(MyObject),
.tp_doc = PyDoc_STR("My objects"),
.tp_new = myobj_new,
.tp_dealloc = (destructor)myobj_dealloc,
.tp_repr = (reprfunc)myobj_repr,
};
You may also find older code (especially in the CPython code base) with a more verbose initializer:
static PyTypeObject MyObject_Type = {
PyVarObject_HEAD_INIT(NULL, 0)
"mymod.MyObject", /* tp_name */
sizeof(MyObject), /* tp_basicsize */
0, /* tp_itemsize */
(destructor)myobj_dealloc, /* tp_dealloc */
0, /* tp_vectorcall_offset */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_as_async */
(reprfunc)myobj_repr, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
0, /* tp_flags */
PyDoc_STR("My objects"), /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
0, /* tp_methods */
0, /* tp_members */
0, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
0, /* tp_init */
0, /* tp_alloc */
myobj_new, /* tp_new */
};
A type that supports weakrefs, instance dicts, and hashing:
typedef struct {
PyObject_HEAD
const char *data;
} MyObject;
static PyTypeObject MyObject_Type = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "mymod.MyObject",
.tp_basicsize = sizeof(MyObject),
.tp_doc = PyDoc_STR("My objects"),
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE |
Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_MANAGED_DICT |
Py_TPFLAGS_MANAGED_WEAKREF,
.tp_new = myobj_new,
.tp_traverse = (traverseproc)myobj_traverse,
.tp_clear = (inquiry)myobj_clear,
.tp_alloc = PyType_GenericNew,
.tp_dealloc = (destructor)myobj_dealloc,
.tp_repr = (reprfunc)myobj_repr,
.tp_hash = (hashfunc)myobj_hash,
.tp_richcompare = PyBaseObject_Type.tp_richcompare,
};
A str subclass that cannot be subclassed and cannot be called to create instances (e.g. uses a separate factory func) using Py_TPFLAGS_DISALLOW_INSTANTIATION flag:
typedef struct {
PyUnicodeObject raw;
char *extra;
} MyStr;
static PyTypeObject MyStr_Type = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "mymod.MyStr",
.tp_basicsize = sizeof(MyStr),
.tp_base = NULL, // set to &PyUnicode_Type in module init
.tp_doc = PyDoc_STR("my custom str"),
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
.tp_repr = (reprfunc)myobj_repr,
};
The simplest static type with fixed-length instances:
typedef struct {
PyObject_HEAD
} MyObject;
static PyTypeObject MyObject_Type = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "mymod.MyObject",
};
The simplest static type with variable-length instances:
typedef struct {
PyObject_VAR_HEAD
const char *data[1];
} MyObject;
static PyTypeObject MyObject_Type = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "mymod.MyObject",
.tp_basicsize = sizeof(MyObject) - sizeof(char *),
.tp_itemsize = sizeof(char *),
};
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