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.
The structure definition for PyTypeObject can be found in
Include/cpython/object.h. For convenience of reference, this repeats the
definition found there:
typedefstruct_typeobject{PyObject_VAR_HEADconstchar*tp_name;/* For printing, in format "<module>.<name>" */Py_ssize_ttp_basicsize,tp_itemsize;/* For allocation *//* Methods to implement standard operations */destructortp_dealloc;Py_ssize_ttp_vectorcall_offset;getattrfunctp_getattr;setattrfunctp_setattr;PyAsyncMethods*tp_as_async;/* formerly known as tp_compare (Python 2) or tp_reserved (Python 3) */reprfunctp_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) */hashfunctp_hash;ternaryfunctp_call;reprfunctp_str;getattrofunctp_getattro;setattrofunctp_setattro;/* Functions to access object as input/output buffer */PyBufferProcs*tp_as_buffer;/* Flags to define presence of optional/expanded features */unsignedlongtp_flags;constchar*tp_doc;/* Documentation string *//* Assigned meaning in release 2.0 *//* call function for all accessible objects */traverseproctp_traverse;/* delete references to contained objects */inquirytp_clear;/* Assigned meaning in release 2.1 *//* rich comparisons */richcmpfunctp_richcompare;/* weak reference enabler */Py_ssize_ttp_weaklistoffset;/* Iterators */getiterfunctp_iter;iternextfunctp_iternext;/* Attribute descriptor and subclassing stuff */structPyMethodDef*tp_methods;structPyMemberDef*tp_members;structPyGetSetDef*tp_getset;// Strong reference on a heap type, borrowed reference on a static typestruct_typeobject*tp_base;PyObject*tp_dict;descrgetfunctp_descr_get;descrsetfunctp_descr_set;Py_ssize_ttp_dictoffset;initproctp_init;allocfunctp_alloc;newfunctp_new;freefunctp_free;/* Low-level free-memory routine */inquirytp_is_gc;/* For PyObject_IS_GC */PyObject*tp_bases;PyObject*tp_mro;/* method resolution order */PyObject*tp_cache;PyObject*tp_subclasses;PyObject*tp_weaklist;destructortp_del;/* Type attribute cache version tag. Added in version 2.6 */unsignedinttp_version_tag;destructortp_finalize;vectorcallfunctp_vectorcall;/* bitset of which type-watchers care about this type */unsignedchartp_watched;}PyTypeObject;
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.
This is the type object’s reference count, 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.
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.
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.)
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).
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 “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.
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.
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).
A pointer to the instance destructor function. This function must be defined
unless the type guarantees that its instances will never be deallocated (as is
the case for the singletons None and Ellipsis). The function signature is:
voidtp_dealloc(PyObject*self);
The destructor function is called by the Py_DECREF() and
Py_XDECREF() macros when the new reference count is zero. At this point,
the instance is still in existence, but there are no references to it. The
destructor function should free all references which the instance owns, free all
memory buffers owned by the instance (using the freeing function corresponding
to the allocation function used to allocate the buffer), and call the type’s
tp_free function. If the type is not subtypable
(doesn’t have the Py_TPFLAGS_BASETYPE flag bit set), it is
permissible to call the object deallocator directly instead of via
tp_free. The object deallocator should be the one used to allocate the
instance; this is normally PyObject_Del() if the instance was allocated
using PyObject_New or PyObject_NewVar, or
PyObject_GC_Del() if the instance was allocated using
PyObject_GC_New or PyObject_GC_NewVar.
If the type supports garbage collection (has the Py_TPFLAGS_HAVE_GC
flag bit set), the destructor should call PyObject_GC_UnTrack()
before clearing any member fields.
Finally, 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. In order to avoid dangling pointers, the
recommended way to achieve this is:
staticvoidfoo_dealloc(foo_object*self){PyTypeObject*tp=Py_TYPE(self);// free references and buffers heretp->tp_free(self);Py_DECREF(tp);}
Warning
In a garbage collected Python, 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
will own the Global Interpreter Lock (GIL). However, if the object being
destroyed in turn destroys objects from some other C or 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.
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.
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.
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 subtype’s 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.
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 subtype’s tp_setattr and tp_setattro are both NULL.
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 <%sobjectat%p> is
returned, where %s is replaced by the type name, and %p by the object’s
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.
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.
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().
This field is inherited by subtypes together with
tp_richcompare: a subtype inherits both of
tp_richcompare and tp_hash, when the subtype’s
tp_richcompare and tp_hash are both NULL.
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():
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 function must return a string or a Unicode object. It should be a “friendly” 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.
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 subtype’s tp_getattr and tp_getattro are both NULL.
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.
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 subtype’s tp_setattr and tp_setattro are both NULL.
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 type’s 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.
.. XXX are most flag bits really inherited individually?
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 INCREF’ed when a new instance is created, and
DECREF’ed when an instance is destroyed (this does not apply to instances of
subtypes; only the type referenced by the instance’s ob_type gets INCREF’ed or
DECREF’ed). Heap types should also support garbage collection
as they can form a reference cycle with their own module object.
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 “final” class in
Java).
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.
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 “bound method” 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.
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 instance’s 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.
Functions such as PyLong_Check() will call PyType_FastSubclass()
with one of these flags 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 isn’t necessary anymore, as the interpreter assumes the
tp_finalize slot is always present in the
type structure.
Disallow creating instances of the type: set
tp_new to NULL and don’t 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.
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.
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.
This bit indicates that instances of this type will have an “inline values”
array (containing the object’s attributes) placed directly after the end
of the object.
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.
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 instance’s members that are Python
objects that the instance owns. For example, this is function local_traverse() from the
_thread extension module:
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 module’s
get_referents() function will include it.
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).
Warning
The traversal function must not have any side effects. It must not
modify the reference counts of any Python objects nor create or destroy
any Python objects.
Note that Py_VISIT() requires the visit and arg parameters to
local_traverse() to have these specific names; don’t 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.
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.
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 for the garbage collector. This is only
used if the Py_TPFLAGS_HAVE_GC flag bit is set. The signature is:
inttp_clear(PyObject*);
The tp_clear member function is used to break reference cycles in cyclic
garbage detected by the garbage collector. 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 it’s
possible to prove that no reference cycle can be composed entirely of tuples.
Therefore the tp_clear functions of other types must be sufficient to
break any cycle containing a tuple. This isn’t immediately obvious, and there’s
rarely a good reason to avoid implementing tp_clear.
Implementations of tp_clear should drop the instance’s 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:
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 it’s possible for such code to reference self again,
it’s 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.
Note that tp_clear is not always called
before an instance is deallocated. For example, when reference counting
is enough to determine that an object is no longer used, the cyclic garbage
collector is not involved and tp_dealloc is
called directly.
Because the goal of tp_clear functions is to break reference cycles,
it’s not necessary to clear contained objects like Python strings or Python
integers, which can’t participate in reference cycles. On the other hand, it may
be convenient to clear all contained Python objects, and write the type’s
tp_dealloc function to invoke 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.
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.
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().
This field is inherited by subtypes together with tp_hash:
a subtype inherits tp_richcompare and tp_hash when
the subtype’s 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.
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.
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).
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().
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 type’s 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 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 “address constants”. Function
designators like PyType_GenericNew(), with implicit conversion
to a pointer, are valid C99 address constants.
However, the unary ‘&’ 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 module’s 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).
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 don’t
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.
Warning
It is not safe to use PyDict_SetItem() on or otherwise modify
tp_dict with the dictionary C-API.
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.
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.
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 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 type’s 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 subtype’s 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.
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 doesn’t 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.
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 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 object’s type’s
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:
inttp_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.
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_basesslot 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.
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.
The corresponding slot IDPy_tp_finalize is part of the Stable ABI since version 3.5.
An optional pointer to an instance finalization function. This is the C
implementation of the __del__() special method. Its signature
is:
voidtp_finalize(PyObject*self);
If tp_finalize is set, the interpreter calls it once when
finalizing an instance. It is called either from the garbage
collector (if the instance is part of an isolated reference cycle) or
just before the object is deallocated. Either way, it is guaranteed
to be called before attempting to break reference cycles, ensuring
that it finds the object in a sane state.
tp_finalize should not mutate the current exception status;
therefore, a recommended way to write a non-trivial finalizer is:
staticvoidlocal_finalize(PyObject*self){/* Save the current exception, if any. */PyObject*exc=PyErr_GetRaisedException();/* ... *//* Restore the saved exception. */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.
Vectorcall function to use for calls of this type object.
In other words, it is used to implement
vectorcall for type.__call__.
If tp_vectorcall is NULL, the default call implementation
using __new__() and __init__() is used.
Inheritance:
This field is never inherited.
Added in version 3.9: (the field exists since 3.8 but it’s only used since 3.9)
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 object’s 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
Python’s class statement. Heap types have the Py_TPFLAGS_HEAPTYPE
flag set.
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.
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 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 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.
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.
The corresponding slot IDPy_sq_inplace_concat is part of the Stable ABI.
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.
The corresponding slot IDPy_sq_inplace_repeat is part of the Stable ABI.
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 corresponding slot IDPy_bf_getbuffer is part of the Stable ABI since version 3.11.
The signature of this function is:
int(PyObject*exporter,Py_buffer*view,intflags);
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 corresponding slot IDPy_bf_releasebuffer is part of the Stable ABI since version 3.11.
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.
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 type’s tp_itemsize is non-zero, the object’s 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.
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:
typedefstruct{PyUnicodeObjectraw;char*extra;}MyStr;staticPyTypeObjectMyStr_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:
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Last updated on Oct 26, 2026 (13:10 UTC).
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