Type Objects¶
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, coercion, intargfunc, intintargfunc, intobjargproc, intintobjargproc, objobjargproc, destructor, freefunc, printfunc, getattrfunc, getattrofunc, setattrfunc, setattrofunc, cmpfunc, 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
char *tp_name; /* For printing, in format "<module>.<name>" */
int tp_basicsize, tp_itemsize; /* For allocation */
/* Methods to implement standard operations */
destructor tp_dealloc;
printfunc tp_print;
getattrfunc tp_getattr;
setattrfunc tp_setattr;
cmpfunc tp_compare;
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 */
long tp_flags;
char *tp_doc; /* Documentation string */
/* Assigned meaning in release 2.0 */
/* call function for all accessible objects */
traverseproc tp_traverse;
/* delete references to contained objects */
inquiry tp_clear;
/* Assigned meaning in release 2.1 */
/* rich comparisons */
richcmpfunc tp_richcompare;
/* weak reference enabler */
long tp_weaklistoffset;
/* Added in release 2.2 */
/* 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;
long 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;
} 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 thePyObject_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 variablePYTHONDUMPREFS
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 thePyObject_HEAD_INIT
macro. Note that for statically allocated type objects, the type’s instances (objects whoseob_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.
Changed in version 2.5: This field used to be an
int
type. This might require changes in your code for properly supporting 64-bit systems.
-
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 thePyObject_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 ifob_type
is NULL, and if so, initializes it: in Python 2.2, it is set to&PyType_Type
; in Python 2.2.1 and later it is initialized to theob_type
field of the base class.PyType_Ready()
will not change this field if it is non-zero.In Python 2.2, this field is not inherited by subtypes. In 2.2.1, and in 2.3 and beyond, it 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.
-
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 moduleM
in subpackageQ
in packageP
should have thetp_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-zerotp_itemsize
field. For a type with fixed-length instances, all instances have the same size, given intp_basicsize
.For a type with variable-length instances, the instances must have an
ob_size
field, and the instance size istp_basicsize
plus N timestp_itemsize
, where N is the “length” of the object. The value of N is typically stored in the instance’sob_size
field. There are exceptions: for example, long ints use a negativeob_size
to indicate a negative number, and N isabs(ob_size)
there. Also, the presence of anob_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 meaningfulob_size
field).The basic size includes the fields in the instance declared by the macro
PyObject_HEAD
orPyObject_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 thetp_basicsize
is to use thesizeof
operator on the struct used to declare the instance layout. The basic size does not include the GC header size (this is new in Python 2.2; in 2.1 and 2.0, the GC header size was included intp_basicsize
).These fields are inherited separately by subtypes. If the base type has a non-zero
tp_itemsize
, it is generally not safe to settp_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 ofdouble
.tp_itemsize
issizeof(double)
. It is the programmer’s responsibility thattp_basicsize
is a multiple ofsizeof(double)
(assuming this is the alignment requirement fordouble
).
-
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
andEllipsis
).The destructor function is called by the
Py_DECREF()
andPy_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’stp_free
function. If the type is not subtypable (doesn’t have thePy_TPFLAGS_BASETYPE
flag bit set), it is permissible to call the object deallocator directly instead of viatp_free
. The object deallocator should be the one used to allocate the instance; this is normallyPyObject_Del()
if the instance was allocated usingPyObject_New()
orPyObject_VarNew()
, orPyObject_GC_Del()
if the instance was allocated usingPyObject_GC_New()
orPyObject_GC_NewVar()
.This field is inherited by subtypes.
-
printfunc
PyTypeObject.tp_print
¶ An optional pointer to the instance print function.
The print function is only called when the instance is printed to a real file; when it is printed to a pseudo-file (like a
StringIO
instance), the instance’stp_repr
ortp_str
function is called to convert it to a string. These are also called when the type’stp_print
field is NULL. A type should never implementtp_print
in a way that produces different output thantp_repr
ortp_str
would.The print function is called with the same signature as
PyObject_Print()
:int tp_print(PyObject *self, FILE *file, int flags)
. The self argument is the instance to be printed. The file argument is the stdio file to which it is to be printed. The flags argument is composed of flag bits. The only flag bit currently defined isPy_PRINT_RAW
. When thePy_PRINT_RAW
flag bit is set, the instance should be printed the same way astp_str
would format it; when thePy_PRINT_RAW
flag bit is clear, the instance should be printed the same was astp_repr
would format it. It should return-1
and set an exception condition when an error occurred during the comparison.It is possible that the
tp_print
field will be deprecated. In any case, it is recommended not to definetp_print
, but instead to rely ontp_repr
andtp_str
for printing.This field is inherited by subtypes.
-
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 isPyObject * tp_getattr(PyObject *o, char *attr_name);
This field is inherited by subtypes together with
tp_getattro
: a subtype inherits bothtp_getattr
andtp_getattro
from its base type when the subtype’stp_getattr
andtp_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 isPyObject * 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 bothtp_setattr
andtp_setattro
from its base type when the subtype’stp_setattr
andtp_setattro
are both NULL.
-
cmpfunc
PyTypeObject.tp_compare
¶ An optional pointer to the three-way comparison function.
The signature is the same as for
PyObject_Compare()
. The function should return1
if self greater than other,0
if self is equal to other, and-1
if self less than other. It should return-1
and set an exception condition when an error occurred during the comparison.This field is inherited by subtypes together with
tp_richcompare
andtp_hash
: a subtypes inherits all three oftp_compare
,tp_richcompare
, andtp_hash
when the subtype’stp_compare
,tp_richcompare
, andtp_hash
are all NULL.
-
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 toeval()
, 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 C long. 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, causingisinstance(o, collections.Hashable)
to correctly returnFalse
. Note that the converse is also true - setting__hash__ = None
on a class at the Python level will result in thetp_hash
slot being set toPyObject_HashNotImplemented()
.When this field is not set, two possibilities exist: if the
tp_compare
andtp_richcompare
fields are both NULL, a default hash value based on the object’s address is returned; otherwise, aTypeError
is raised.This field is inherited by subtypes together with
tp_richcompare
andtp_compare
: a subtypes inherits all three oftp_compare
,tp_richcompare
, andtp_hash
, when the subtype’stp_compare
,tp_richcompare
andtp_hash
are all 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 thatstr
is a type now, andstr()
calls the constructor for that type. This constructor callsPyObject_Str()
to do the actual work, andPyObject_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 by the print statement.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 toPyObject_GenericGetAttr()
, which implements the normal way of looking for object attributes.This field is inherited by subtypes together with
tp_getattr
: a subtype inherits bothtp_getattr
andtp_getattro
from its base type when the subtype’stp_getattr
andtp_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 toPyObject_GenericSetAttr()
, which implements the normal way of setting object attributes.This field is inherited by subtypes together with
tp_setattr
: a subtype inherits bothtp_setattr
andtp_setattro
from its base type when the subtype’stp_setattr
andtp_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.
-
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
, andtp_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 thetp_traverse
andtp_clear
fields, i.e. if thePy_TPFLAGS_HAVE_GC
flag bit is clear in the subtype and thetp_traverse
andtp_clear
fields in the subtype exist (as indicated by thePy_TPFLAGS_HAVE_RICHCOMPARE
flag bit) and have NULL values.The following bit masks are currently defined; these can be ORed together using the
|
operator to form the value of thetp_flags
field. The macroPyType_HasFeature()
takes a type and a flags value, tp and f, and checks whethertp->tp_flags & f
is non-zero.-
Py_TPFLAGS_HAVE_GETCHARBUFFER
¶ If this bit is set, the
PyBufferProcs
struct referenced bytp_as_buffer
has thebf_getcharbuffer
field.
-
Py_TPFLAGS_HAVE_SEQUENCE_IN
¶ If this bit is set, the
PySequenceMethods
struct referenced bytp_as_sequence
has thesq_contains
field.
-
Py_TPFLAGS_GC
¶ This bit is obsolete. The bit it used to name is no longer in use. The symbol is now defined as zero.
-
Py_TPFLAGS_HAVE_INPLACEOPS
¶ If this bit is set, the
PySequenceMethods
struct referenced bytp_as_sequence
and thePyNumberMethods
structure referenced bytp_as_number
contain the fields for in-place operators. In particular, this means that thePyNumberMethods
structure has the fieldsnb_inplace_add
,nb_inplace_subtract
,nb_inplace_multiply
,nb_inplace_divide
,nb_inplace_remainder
,nb_inplace_power
,nb_inplace_lshift
,nb_inplace_rshift
,nb_inplace_and
,nb_inplace_xor
, andnb_inplace_or
; and thePySequenceMethods
struct has the fieldssq_inplace_concat
andsq_inplace_repeat
.
-
Py_TPFLAGS_CHECKTYPES
¶ If this bit is set, the binary and ternary operations in the
PyNumberMethods
structure referenced bytp_as_number
accept arguments of arbitrary object types, and do their own type conversions if needed. If this bit is clear, those operations require that all arguments have the current type as their type, and the caller is supposed to perform a coercion operation first. This applies tonb_add
,nb_subtract
,nb_multiply
,nb_divide
,nb_remainder
,nb_divmod
,nb_power
,nb_lshift
,nb_rshift
,nb_and
,nb_xor
, andnb_or
.
-
Py_TPFLAGS_HAVE_RICHCOMPARE
¶ If this bit is set, the type object has the
tp_richcompare
field, as well as thetp_traverse
and thetp_clear
fields.
-
Py_TPFLAGS_HAVE_WEAKREFS
¶ If this bit is set, the
tp_weaklistoffset
field is defined. Instances of a type are weakly referenceable if the type’stp_weaklistoffset
field has a value greater than zero.
-
Py_TPFLAGS_HAVE_ITER
¶ If this bit is set, the type object has the
tp_iter
andtp_iternext
fields.
-
Py_TPFLAGS_HAVE_CLASS
¶ If this bit is set, the type object has several new fields defined starting in Python 2.2:
tp_methods
,tp_members
,tp_getset
,tp_base
,tp_dict
,tp_descr_get
,tp_descr_set
,tp_dictoffset
,tp_init
,tp_alloc
,tp_new
,tp_free
,tp_is_gc
,tp_bases
,tp_mro
,tp_cache
,tp_subclasses
, andtp_weaklist
.
-
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 usingPyObject_GC_Del()
. More information in section Supporting Cyclic Garbage Collection. This bit also implies that the GC-related fieldstp_traverse
andtp_clear
are present in the type object; but those fields also exist whenPy_TPFLAGS_HAVE_GC
is clear butPy_TPFLAGS_HAVE_RICHCOMPARE
is set.
-
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_GETCHARBUFFER
,Py_TPFLAGS_HAVE_SEQUENCE_IN
,Py_TPFLAGS_HAVE_INPLACEOPS
,Py_TPFLAGS_HAVE_RICHCOMPARE
,Py_TPFLAGS_HAVE_WEAKREFS
,Py_TPFLAGS_HAVE_ITER
, andPy_TPFLAGS_HAVE_CLASS
.
-
-
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.
The following three fields only exist if the
Py_TPFLAGS_HAVE_RICHCOMPARE
flag bit is set.
-
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 atp_traverse
function simply callsPy_VISIT()
on each of the instance’s members that are Python objects. For example, this is functionlocal_traverse()
from thethread
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 aself->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’sget_referents()
function will include it.Note that
Py_VISIT()
requires the visit and arg parameters tolocal_traverse()
to have these specific names; don’t name them just anything.This field is inherited by subtypes together with
tp_clear
and thePy_TPFLAGS_HAVE_GC
flag bit: the flag bit,tp_traverse
, andtp_clear
are all inherited from the base type if they are all zero in the subtype and the subtype has thePy_TPFLAGS_HAVE_RICHCOMPARE
flag bit set.
-
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, alltp_clear
functions in the system must combine to break all reference cycles. This is subtle, and if in any doubt supply atp_clear
function. For example, the tuple type does not implement atp_clear
function, because it’s possible to prove that no reference cycle can be composed entirely of tuples. Therefore thetp_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 implementingtp_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. ThePy_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’stp_dealloc
function to invoketp_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 thePy_TPFLAGS_HAVE_GC
flag bit: the flag bit,tp_traverse
, andtp_clear
are all inherited from the base type if they are all zero in the subtype and the subtype has thePy_TPFLAGS_HAVE_RICHCOMPARE
flag bit set.
-
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 function should return the result of the comparison (usually
Py_True
orPy_False
). If the comparison is undefined, it must returnPy_NotImplemented
, if another error occurred it must returnNULL
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 raiseTypeError
in the rich comparison function.This field is inherited by subtypes together with
tp_compare
andtp_hash
: a subtype inherits all three oftp_compare
,tp_richcompare
, andtp_hash
, when the subtype’stp_compare
,tp_richcompare
, andtp_hash
are all NULL.The following constants are defined to be used as the third argument for
tp_richcompare
and forPyObject_RichCompare()
:Constant
Comparison
Py_LT
<
Py_LE
<=
Py_EQ
==
Py_NE
!=
Py_GT
>
Py_GE
>=
The next field only exists if the Py_TPFLAGS_HAVE_WEAKREFS
flag bit is
set.
-
long
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 thePyWeakref_*()
functions. The instance structure needs to include a field of typePyObject*
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 thetp_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’stp_weaklistoffset
.When a type’s
__slots__
declaration does not contain a slot named__weakref__
, the type inherits itstp_weaklistoffset
from its base type.
The next two fields only exist if the Py_TPFLAGS_HAVE_ITER
flag bit is
set.
-
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, and classic instances always have this function, even if they don’t define an
__iter__()
method).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 normally signals that the instances of this type are iterators (although classic instances always have this function, even if they don’t define anext()
method).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.
The next fields, up to and including tp_weaklist
, only exist if the
Py_TPFLAGS_HAVE_CLASS
flag bit is set.
-
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 typeobject
).
-
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).
-
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.
-
long
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
ortuple
. Note that thetp_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
andtp_dictoffset
are taken from the type object, andob_size
is taken from the instance. The absolute value is taken because long ints use the sign ofob_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 thetp_dictoffset
is set to that slot’s offset.When a type defined by a class statement has a
__slots__
declaration, the type inherits itstp_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’stp_new
function has returned an instance of the type. If thetp_new
function returns an instance of some other type that is not a subtype of the original type, notp_init
function is called; iftp_new
returns an instance of a subtype of the original type, the subtype’stp_init
is called. (VERSION NOTE: described here is what is implemented in Python 2.2.1 and later. In Python 2.2, thetp_init
of the type of the object returned bytp_new
was always called, if not NULL.)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 to1
andob_type
set to the type argument. If the type’stp_itemsize
is non-zero, the object’sob_size
field should be initialized to nitems and the length of the allocated memory block should betp_basicsize + nitems*tp_itemsize
, rounded up to a multiple ofsizeof(void*)
; otherwise, nitems is not used and the length of the block should betp_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 callsubtype->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 thetp_init
handler. A good rule of thumb is that for immutable types, all initialization should take place intp_new
, while for mutable types, most initialization should be deferred totp_init
.This field is inherited by subtypes, except it is not inherited by static types whose
tp_base
is NULL or&PyBaseObject_Type
. The latter exception is a precaution so that old extension types don’t become callable simply by being linked with Python 2.2.
-
destructor
PyTypeObject.tp_free
¶ An optional pointer to an instance deallocation function.
The signature of this function has changed slightly: in Python 2.2 and 2.2.1, its signature is
destructor
:void tp_free(PyObject *)
In Python 2.3 and beyond, its signature is
freefunc
:void tp_free(void *)
The only initializer that is compatible with both versions is
_PyObject_Del
, whose definition has suitably adapted in Python 2.3.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 thePy_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 thePy_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 return1
for a collectible instance, and0
for a non-collectible instance. The signature isint 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. (VERSION NOTE: in Python 2.2, it was not inherited. It is inherited in 2.2.1 and later versions.)
-
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()
.
-
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. See the PYTHONSHOWALLOCCOUNT
environment variable.
-
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.
Number Object Structures¶
-
PyNumberMethods
¶ This structure holds pointers to the functions which an object uses to implement the number protocol. Almost every function below 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_divide; binaryfunc nb_remainder; binaryfunc nb_divmod; ternaryfunc nb_power; unaryfunc nb_negative; unaryfunc nb_positive; unaryfunc nb_absolute; inquiry nb_nonzero; /* Used by PyObject_IsTrue */ unaryfunc nb_invert; binaryfunc nb_lshift; binaryfunc nb_rshift; binaryfunc nb_and; binaryfunc nb_xor; binaryfunc nb_or; coercion nb_coerce; /* Used by the coerce() function */ unaryfunc nb_int; unaryfunc nb_long; unaryfunc nb_float; unaryfunc nb_oct; unaryfunc nb_hex; /* Added in release 2.0 */ binaryfunc nb_inplace_add; binaryfunc nb_inplace_subtract; binaryfunc nb_inplace_multiply; binaryfunc nb_inplace_divide; 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; /* Added in release 2.2 */ binaryfunc nb_floor_divide; binaryfunc nb_true_divide; binaryfunc nb_inplace_floor_divide; binaryfunc nb_inplace_true_divide; /* Added in release 2.5 */ unaryfunc nb_index; } PyNumberMethods;
Binary and ternary functions may receive different kinds of arguments, depending
on the flag bit Py_TPFLAGS_CHECKTYPES
:
If
Py_TPFLAGS_CHECKTYPES
is not set, the function arguments are guaranteed to be of the object’s type; the caller is responsible for calling the coercion method specified by thenb_coerce
member to convert the arguments:-
coercion
PyNumberMethods.nb_coerce
¶ This function is used by
PyNumber_CoerceEx()
and has the same signature. The first argument is always a pointer to an object of the defined type. If the conversion to a common “larger” type is possible, the function replaces the pointers with new references to the converted objects and returns0
. If the conversion is not possible, the function returns1
. If an error condition is set, it will return-1
.
-
coercion
If the
Py_TPFLAGS_CHECKTYPES
flag is set, 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). This is the recommended way; with Python 3 coercion will disappear completely.
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.
Mapping Object Structures¶
-
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()
andPyObject_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 thePyMapping_Check()
function to return1
, it can be NULL otherwise.
-
objobjargproc
PyMappingMethods.mp_ass_subscript
¶ This function is used by
PyObject_SetItem()
andPyObject_DelItem()
. It has the same signature asPyObject_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.
Sequence Object Structures¶
-
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()
andPyObject_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 thenb_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 thenb_multiply
slot.
-
ssizeargfunc
PySequenceMethods.sq_item
¶ This function is used by
PySequence_GetItem()
and has the same signature. This slot must be filled for thePySequence_Check()
function to return1
, 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 tosq_item
. Ifsq_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 casePySequence_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.
Buffer Object Structures¶
The buffer interface exports a model where an object can expose its internal data as a set of chunks of data, where each chunk is specified as a pointer/length pair. These chunks are called segments and are presumed to be non-contiguous in memory.
If an object does not export the buffer interface, then its tp_as_buffer
member in the PyTypeObject
structure should be NULL. Otherwise, the
tp_as_buffer
will point to a PyBufferProcs
structure.
Note
It is very important that your PyTypeObject
structure uses
Py_TPFLAGS_DEFAULT
for the value of the tp_flags
member rather
than 0
. This tells the Python runtime that your PyBufferProcs
structure contains the bf_getcharbuffer
slot. Older versions of Python
did not have this member, so a new Python interpreter using an old extension
needs to be able to test for its presence before using it.
-
PyBufferProcs
¶ Structure used to hold the function pointers which define an implementation of the buffer protocol.
The first slot is
bf_getreadbuffer
, of typereadbufferproc
. If this slot is NULL, then the object does not support reading from the internal data. This is non-sensical, so implementors should fill this in, but callers should test that the slot contains a non-NULL value.The next slot is
bf_getwritebuffer
having typewritebufferproc
. This slot may be NULL if the object does not allow writing into its returned buffers.The third slot is
bf_getsegcount
, with typesegcountproc
. This slot must not be NULL and is used to inform the caller how many segments the object contains. Simple objects such asPyString_Type
andPyBuffer_Type
objects contain a single segment.The last slot is
bf_getcharbuffer
, of typecharbufferproc
. This slot will only be present if thePy_TPFLAGS_HAVE_GETCHARBUFFER
flag is present in thetp_flags
field of the object’sPyTypeObject
. Before using this slot, the caller should test whether it is present by using thePyType_HasFeature()
function. If the flag is present,bf_getcharbuffer
may be NULL, indicating that the object’s contents cannot be used as 8-bit characters. The slot function may also raise an error if the object’s contents cannot be interpreted as 8-bit characters. For example, if the object is an array which is configured to hold floating point values, an exception may be raised if a caller attempts to usebf_getcharbuffer
to fetch a sequence of 8-bit characters. This notion of exporting the internal buffers as “text” is used to distinguish between objects that are binary in nature, and those which have character-based content.Note
The current policy seems to state that these characters may be multi-byte characters. This implies that a buffer size of N does not mean there are N characters present.
-
Py_TPFLAGS_HAVE_GETCHARBUFFER
Flag bit set in the type structure to indicate that the
bf_getcharbuffer
slot is known. This being set does not indicate that the object supports the buffer interface or that thebf_getcharbuffer
slot is non-NULL.
-
Py_ssize_t
(*readbufferproc)
(PyObject *self, Py_ssize_t segment, void **ptrptr)¶ Return a pointer to a readable segment of the buffer in
*ptrptr
. This function is allowed to raise an exception, in which case it must return-1
. The segment which is specified must be zero or positive, and strictly less than the number of segments returned by thebf_getsegcount
slot function. On success, it returns the length of the segment, and sets*ptrptr
to a pointer to that memory.
-
Py_ssize_t
(*writebufferproc)
(PyObject *self, Py_ssize_t segment, void **ptrptr)¶ Return a pointer to a writable memory buffer in
*ptrptr
, and the length of that segment as the function return value. The memory buffer must correspond to buffer segment segment. Must return-1
and set an exception on error.TypeError
should be raised if the object only supports read-only buffers, andSystemError
should be raised when segment specifies a segment that doesn’t exist.