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.
Typedefs: unaryfunc, binaryfunc, ternaryfunc, inquiry, intargfunc, intintargfunc, intobjargproc, intintobjargproc, objobjargproc, destructor, freefunc, printfunc, getattrfunc, getattrofunc, setattrfunc, setattrofunc, reprfunc, hashfunc
The structure definition for PyTypeObject
can be found in
Include/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;
printfunc tp_print;
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 */
/* call function for all accessible objects */
traverseproc tp_traverse;
/* delete references to contained objects */
inquiry tp_clear;
/* 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 */
struct PyMethodDef *tp_methods;
struct PyMemberDef *tp_members;
struct PyGetSetDef *tp_getset;
struct _typeobject *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;
PyObject *tp_subclasses;
PyObject *tp_weaklist;
destructor tp_del;
/* Type attribute cache version tag. Added in version 2.6 */
unsigned int tp_version_tag;
destructor tp_finalize;
} 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.
PyObject* PyObject._ob_next
PyObject* PyObject._ob_prev
These fields are only present when the macro Py_TRACE_REFS
is defined.
Their initialization to NULL is taken care of by the PyObject_HEAD_INIT
macro. For statically allocated objects, these fields always remain NULL.
For dynamically allocated objects, these two fields are used to link the object
into a doubly-linked list of all live objects on the heap. This could be used
for various debugging purposes; currently the only use is to print the objects
that are still alive at the end of a run when the environment variable
PYTHONDUMPREFS
is set.
These fields are not inherited by subtypes.
Py_ssize_t PyObject.ob_refcnt
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 field is not inherited by subtypes.
PyTypeObject* PyObject.ob_type
This is the type's type, in other words its metatype. It is initialized by the
argument to the PyObject_HEAD_INIT
macro, and its value should normally be
&PyType_Type
. However, for dynamically loadable extension modules that must
be usable on Windows (at least), the compiler complains that this is not a valid
initializer. Therefore, the convention is to pass NULL to the
PyObject_HEAD_INIT
macro and to initialize this field explicitly at the
start of the module's initialization function, before doing anything else. This
is typically done like this:
Foo_Type.ob_type = &PyType_Type;
This should be done before any instances of the type are created.
PyType_Ready()
checks if ob_type
is NULL, and if so,
initializes it to the ob_type
field of the base class.
PyType_Ready()
will not change this field if it is non-zero.
This field is inherited by subtypes.
Py_ssize_t PyVarObject.ob_size
For statically allocated type objects, this should be initialized to zero. For dynamically allocated type objects, this field has a special internal meaning.
This field is not inherited by subtypes.
const char* PyTypeObject.tp_name
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 is not inherited by subtypes.
Py_ssize_t PyTypeObject.tp_basicsize
Py_ssize_t PyTypeObject.tp_itemsize
These fields allow calculating the size in bytes of instances of the type.
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
.
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. The value of
N is typically stored in the instance's ob_size
field. There are
exceptions: for example, ints use a negative ob_size
to indicate a
negative number, and N is abs(ob_size)
there. 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 structure for the list type has
fixed-length instances, yet those instances have a meaningful ob_size
field).
The basic size includes the fields in the instance declared by the macro
PyObject_HEAD
or PyObject_VAR_HEAD
(whichever is used to
declare the instance struct) and this in turn includes the _ob_prev
and
_ob_next
fields if they are present. This means that the only correct
way to get an initializer for the tp_basicsize
is to use the
sizeof
operator on the struct used to declare the instance layout.
The basic size does not include the GC header size.
These fields are inherited separately by subtypes. 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 note about alignment: if the variable items require a particular alignment,
this should be taken care of by the value of tp_basicsize
. Example:
suppose a type implements an array of double
. tp_itemsize
is
sizeof(double)
. It is the programmer's responsibility that
tp_basicsize
is a multiple of sizeof(double)
(assuming this is the
alignment requirement for double
).
destructor PyTypeObject.tp_dealloc
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 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 finally (as its
last action) 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_VarNew()
, or
PyObject_GC_Del()
if the instance was allocated using
PyObject_GC_New()
or PyObject_GC_NewVar()
.
This field is inherited by subtypes.
printfunc PyTypeObject.tp_print
Reserved slot, formerly used for print formatting in Python 2.x.
getattrfunc PyTypeObject.tp_getattr
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. The signature is
PyObject * tp_getattr(PyObject *o, char *attr_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.
setattrfunc PyTypeObject.tp_setattr
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. The signature is
PyObject * tp_setattr(PyObject *o, char *attr_name, PyObject *v);
The v argument is set to NULL to delete the attribute.
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.
PyAsyncMethods* tp_as_async
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.
New in version 3.5: Formerly known as tp_compare
and tp_reserved
.
reprfunc PyTypeObject.tp_repr
An optional pointer to a function that implements the built-in function
repr()
.
The signature is the same as for PyObject_Repr()
; it 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.
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 object's
memory address.
This field is inherited by subtypes.
PyNumberMethods* tp_as_number
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.
The tp_as_number
field is not inherited, but the contained fields are
inherited individually.
PySequenceMethods* tp_as_sequence
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.
The tp_as_sequence
field is not inherited, but the contained fields
are inherited individually.
PyMappingMethods* tp_as_mapping
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.
The tp_as_mapping
field is not inherited, but the contained fields
are inherited individually.
hashfunc PyTypeObject.tp_hash
An optional pointer to a function that implements the built-in function
hash()
.
The signature is the same as for PyObject_Hash()
; it must return a
value of the type Py_hash_t. 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()
.
When this field is not set, an attempt to take the hash of the
object raises TypeError
.
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.
ternaryfunc PyTypeObject.tp_call
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()
.
This field is inherited by subtypes.
reprfunc PyTypeObject.tp_str
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()
; it must return a string
or a Unicode object. This function should return a "friendly" string
representation of the object, as this is the representation that will be used,
among other things, by the print()
function.
When this field is not set, PyObject_Repr()
is called to return a string
representation.
This field is inherited by subtypes.
getattrofunc PyTypeObject.tp_getattro
An optional pointer to the get-attribute function.
The signature is the same as for PyObject_GetAttr()
. It is usually
convenient to set this field to PyObject_GenericGetAttr()
, which
implements the normal way of looking for object attributes.
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.
setattrofunc PyTypeObject.tp_setattro
An optional pointer to the function for setting and deleting attributes.
The signature is the same as for PyObject_SetAttr()
, but setting
v 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.
PyBufferProcs* PyTypeObject.tp_as_buffer
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.
The tp_as_buffer
field is not inherited, but the contained fields are
inherited individually.
unsigned long PyTypeObject.tp_flags
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 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.
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.
Py_TPFLAGS_HEAPTYPE
This bit is set when the type object itself is allocated on the heap. 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).
Py_TPFLAGS_BASETYPE
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).
Py_TPFLAGS_READY
This bit is set when the type object has been fully initialized by
PyType_Ready()
.
Py_TPFLAGS_READYING
This bit is set while PyType_Ready()
is in the process of initializing
the type object.
Py_TPFLAGS_HAVE_GC
This bit is set when the object supports garbage collection. If this bit
is set, instances must be created using PyObject_GC_New()
and
destroyed using PyObject_GC_Del()
. More information in section
Supporting Cyclic Garbage Collection. This bit also implies that the
GC-related fields tp_traverse
and tp_clear
are present in
the type object.
Py_TPFLAGS_DEFAULT
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
,
Py_TPFLAGS_HAVE_VERSION_TAG
.
Py_TPFLAGS_LONG_SUBCLASS
Py_TPFLAGS_LIST_SUBCLASS
Py_TPFLAGS_TUPLE_SUBCLASS
Py_TPFLAGS_BYTES_SUBCLASS
Py_TPFLAGS_UNICODE_SUBCLASS
Py_TPFLAGS_DICT_SUBCLASS
Py_TPFLAGS_BASE_EXC_SUBCLASS
Py_TPFLAGS_TYPE_SUBCLASS
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.
Py_TPFLAGS_HAVE_FINALIZE
This bit is set when the tp_finalize
slot is present in the
type structure.
New in version 3.4.
const char* PyTypeObject.tp_doc
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.
This field is not inherited by subtypes.
traverseproc PyTypeObject.tp_traverse
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. More information
about Python's 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 instance's members that are Python
objects. For example, this is function local_traverse()
from the
_thread
extension module:
static int
local_traverse(localobject *self, visitproc visit, void *arg)
{
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
module's
get_referents()
function will include it.
Note that Py_VISIT()
requires the visit and arg parameters to
local_traverse()
to have these specific names; don't name them just
anything.
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.
inquiry PyTypeObject.tp_clear
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 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:
static int
local_clear(localobject *self)
{
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 decremented until
after the pointer to the contained object is set to NULL. This is because
decrementing the reference count 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.
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
.
More information about Python's garbage collection scheme can be found in section Supporting Cyclic Garbage Collection.
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.
richcmpfunc PyTypeObject.tp_richcompare
An optional pointer to the rich comparison function, whose signature is
PyObject *tp_richcompare(PyObject *a, PyObject *b, 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.
Note
If you want to implement a type for which only a limited set of
comparisons makes sense (e.g. ==
and !=
, but not <
and
friends), directly raise TypeError
in the rich comparison function.
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.
The following constants are defined to be used as the third argument for
tp_richcompare
and for PyObject_RichCompare()
:
Constant | Comparison |
---|---|
Py_LT | < |
Py_LE | <= |
Py_EQ | == |
Py_NE | != |
Py_GT | > |
Py_GE | >= |
Py_ssize_t PyTypeObject.tp_weaklistoffset
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.
When a type defined by a class statement has no __slots__
declaration,
and none of its base types are weakly referenceable, the type is made weakly
referenceable by adding a weak reference list head slot to the instance layout
and setting the tp_weaklistoffset
of that slot's offset.
When a type's __slots__
declaration contains a slot named
__weakref__
, that slot becomes the weak reference list head for
instances of the type, and the slot's offset is stored in the type's
tp_weaklistoffset
.
When a type's __slots__
declaration does not contain a slot named
__weakref__
, the type inherits its tp_weaklistoffset
from its
base type.
getiterfunc PyTypeObject.tp_iter
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()
.
This field is inherited by subtypes.
iternextfunc PyTypeObject.tp_iternext
An optional pointer to a function that returns the next item in an iterator.
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()
.
This field is inherited by subtypes.
struct PyMethodDef* PyTypeObject.tp_methods
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 type's dictionary (see
tp_dict
below) containing a method descriptor.
This field is not inherited by subtypes (methods are inherited through a different mechanism).
struct PyMemberDef* PyTypeObject.tp_members
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.
This field is not inherited by subtypes (members are inherited through a different mechanism).
struct PyGetSetDef* PyTypeObject.tp_getset
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 type's dictionary (see
tp_dict
below) containing a getset descriptor.
This field is not inherited by subtypes (computed attributes are inherited through a different mechanism).
PyTypeObject* PyTypeObject.tp_base
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.
This field is not inherited by subtypes (obviously), but it defaults to
&PyBaseObject_Type
(which to Python programmers is known as the type
object
).
PyObject* PyTypeObject.tp_dict
The type's 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 don't
correspond to overloaded operations (like __add__()
).
This field is not inherited by subtypes (though the attributes defined in here are inherited through a different mechanism).
Warning
It is not safe to use PyDict_SetItem()
on or otherwise modify
tp_dict
with the dictionary C-API.
descrgetfunc PyTypeObject.tp_descr_get
An optional pointer to a "descriptor get" function.
The function signature is
PyObject * tp_descr_get(PyObject *self, PyObject *obj, PyObject *type);
This field is inherited by subtypes.
descrsetfunc PyTypeObject.tp_descr_set
An optional pointer to a function for setting and deleting a descriptor's 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. This field is inherited by subtypes.
Py_ssize_t PyTypeObject.tp_dictoffset
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.
If the value of this field is greater than zero, it specifies the offset from
the start of the instance structure. If the value is less than zero, it
specifies the offset from the end of the instance structure. A negative
offset is more expensive to use, and should only be used when the instance
structure contains a variable-length part. This is used for example to add an
instance variable dictionary to subtypes of str
or tuple
. Note
that the tp_basicsize
field should account for the dictionary added to
the end in that case, even though the dictionary is not included in the basic
object layout. On a system with a pointer size of 4 bytes,
tp_dictoffset
should be set to -4
to indicate that the dictionary is
at the very end of the structure.
The real dictionary offset in an instance can be computed from a negative
tp_dictoffset
as follows:
dictoffset = tp_basicsize + abs(ob_size)*tp_itemsize + tp_dictoffset
if dictoffset is not aligned on sizeof(void*):
round up to sizeof(void*)
where tp_basicsize
, tp_itemsize
and tp_dictoffset
are
taken from the type object, and ob_size
is taken from the instance. The
absolute value is taken because ints use the sign of ob_size
to
store the sign of the number. (There's never a need to do this calculation
yourself; it is done for you by _PyObject_GetDictPtr()
.)
This field is inherited by subtypes, but see the rules listed below. A subtype
may override this offset; this means that the subtype instances store the
dictionary at a difference offset than the base type. Since the dictionary is
always found via tp_dictoffset
, this should not be a problem.
When a type defined by a class statement has no __slots__
declaration,
and none of its base types has an instance variable dictionary, a dictionary
slot is added to the instance layout and the tp_dictoffset
is set to
that slot's offset.
When a type defined by a class statement has a __slots__
declaration,
the type inherits its tp_dictoffset
from its base type.
(Adding a slot named __dict__
to the __slots__
declaration does
not have the expected effect, it just causes confusion. Maybe this should be
added as a feature just like __weakref__
though.)
initproc PyTypeObject.tp_init
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 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.
This field is inherited by subtypes.
allocfunc PyTypeObject.tp_alloc
An optional pointer to an instance allocation function.
The function signature is
PyObject *tp_alloc(PyTypeObject *self, Py_ssize_t nitems)
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
.
Do not use this function to do any other instance initialization, not even to
allocate additional memory; that should be done by tp_new
.
This field is inherited by static subtypes, but not by dynamic subtypes
(subtypes created by a class statement); in the latter, this field is always set
to PyType_GenericAlloc()
, to force a standard heap allocation strategy.
That is also the recommended value for statically defined types.
newfunc PyTypeObject.tp_new
An optional pointer to an instance creation function.
If this function is NULL for a particular type, that type cannot be called to create new instances; presumably there is some other way to create instances, like a factory 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 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
.
destructor PyTypeObject.tp_free
An optional pointer to an instance deallocation function. Its signature is
freefunc
:
void tp_free(void *)
An initializer that is compatible with this signature is PyObject_Free()
.
This field is inherited by static subtypes, but not by dynamic subtypes
(subtypes created by a class statement); in the latter, this field is set to a
deallocator suitable to match PyType_GenericAlloc()
and the value of the
Py_TPFLAGS_HAVE_GC
flag bit.
inquiry PyTypeObject.tp_is_gc
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
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.)
This field is inherited by subtypes.
PyObject* PyTypeObject.tp_bases
Tuple of base types.
This is set for types created by a class statement. It should be NULL for statically defined types.
This field is not inherited.
PyObject* PyTypeObject.tp_mro
Tuple containing the expanded set of base types, starting with the type itself
and ending with object
, in Method Resolution Order.
This field is not inherited; it is calculated fresh by PyType_Ready()
.
destructor PyTypeObject.tp_finalize
An optional pointer to an instance finalization function. Its signature is
destructor
:
void tp_finalize(PyObject *)
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:
static void
local_finalize(PyObject *self)
{
PyObject *error_type, *error_value, *error_traceback;
/* Save the current exception, if any. */
PyErr_Fetch(&error_type, &error_value, &error_traceback);
/* ... */
/* Restore the saved exception. */
PyErr_Restore(error_type, error_value, error_traceback);
}
For this field to be taken into account (even through inheritance),
you must also set the Py_TPFLAGS_HAVE_FINALIZE
flags bit.
This field is inherited by subtypes.
New in version 3.4.
See also
"Safe object finalization" (PEP 442)
PyObject* PyTypeObject.tp_cache
Unused. Not inherited. Internal use only.
PyObject* PyTypeObject.tp_subclasses
List of weak references to subclasses. Not inherited. Internal use only.
PyObject* PyTypeObject.tp_weaklist
Weak reference list head, for weak references to this type object. Not inherited. Internal use only.
The remaining fields are only defined if the feature test macro
COUNT_ALLOCS
is defined, and are for internal use only. They are
documented here for completeness. None of these fields are inherited by
subtypes.
Py_ssize_t PyTypeObject.tp_allocs
Number of allocations.
Py_ssize_t PyTypeObject.tp_frees
Number of frees.
Py_ssize_t PyTypeObject.tp_maxalloc
Maximum simultaneously allocated objects.
PyTypeObject* PyTypeObject.tp_next
Pointer to the next type object with a non-zero tp_allocs
field.
Also, note that, 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.
PyNumberMethods
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.
PyMappingMethods
This structure holds pointers to the functions which an object uses to implement the mapping protocol. It has three members:
lenfunc PyMappingMethods.mp_length
This function is used by PyMapping_Length()
and
PyObject_Size()
, and has the same signature. This slot may be set to
NULL if the object has no defined length.
binaryfunc PyMappingMethods.mp_subscript
This function is used by PyObject_GetItem()
and has the same
signature. This slot must be filled for the PyMapping_Check()
function to return 1
, it can be NULL otherwise.
objobjargproc PyMappingMethods.mp_ass_subscript
This function is used by PyObject_SetItem()
and
PyObject_DelItem()
. 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.
PySequenceMethods
This structure holds pointers to the functions which an object uses to implement the sequence protocol.
lenfunc PySequenceMethods.sq_length
This function is used by PySequence_Size()
and PyObject_Size()
,
and has the same signature.
binaryfunc PySequenceMethods.sq_concat
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.
ssizeargfunc PySequenceMethods.sq_repeat
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.
ssizeargfunc PySequenceMethods.sq_item
This function is used by PySequence_GetItem()
and has the same
signature. 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.
ssizeobjargproc PySequenceMethods.sq_ass_item
This function is used by PySequence_SetItem()
and has the same
signature. This slot may be left to NULL if the object does not support
item assignment and deletion.
objobjproc PySequenceMethods.sq_contains
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.
binaryfunc PySequenceMethods.sq_inplace_concat
This function is used by PySequence_InPlaceConcat()
and has the same
signature. It should modify its first operand, and return it.
ssizeargfunc PySequenceMethods.sq_inplace_repeat
This function is used by PySequence_InPlaceRepeat()
and has the same
signature. It should modify its first operand, and return it.
PyBufferProcs
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.
getbufferproc PyBufferProcs.bf_getbuffer
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
PyExc_BufferError
, setview->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 incrementview->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.
releasebufferproc PyBufferProcs.bf_releasebuffer
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.
New in version 3.5.
PyAsyncMethods
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;
} PyAsyncMethods;
unaryfunc PyAsyncMethods.am_await
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.
unaryfunc PyAsyncMethods.am_aiter
The signature of this function is:
PyObject *am_aiter(PyObject *self)
Must return an awaitable object. See __anext__()
for details.
This slot may be set to NULL if an object does not implement asynchronous iteration protocol.
unaryfunc PyAsyncMethods.am_anext
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.