Quick Reference¶

PyTypeObject Definition¶

The structure definition forPyTypeObject can be found inInclude/object.h. For convenience of reference, this repeats thedefinition found there:

typedefstruct_typeobject{PyObject_VAR_HEADconstchar*tp_name;/* For printing, in format "<module>.<name>" */Py_ssize_ttp_basicsize,tp_itemsize;/* For allocation *//* Methods to implement standard operations */destructortp_dealloc;Py_ssize_ttp_vectorcall_offset;getattrfunctp_getattr;setattrfunctp_setattr;PyAsyncMethods*tp_as_async;/* formerly known as tp_compare (Python 2)                                    or tp_reserved (Python 3) */reprfunctp_repr;/* Method suites for standard classes */PyNumberMethods*tp_as_number;PySequenceMethods*tp_as_sequence;PyMappingMethods*tp_as_mapping;/* More standard operations (here for binary compatibility) */hashfunctp_hash;ternaryfunctp_call;reprfunctp_str;getattrofunctp_getattro;setattrofunctp_setattro;/* Functions to access object as input/output buffer */PyBufferProcs*tp_as_buffer;/* Flags to define presence of optional/expanded features */unsignedlongtp_flags;constchar*tp_doc;/* Documentation string *//* call function for all accessible objects */traverseproctp_traverse;/* delete references to contained objects */inquirytp_clear;/* rich comparisons */richcmpfunctp_richcompare;/* weak reference enabler */Py_ssize_ttp_weaklistoffset;/* Iterators */getiterfunctp_iter;iternextfunctp_iternext;/* Attribute descriptor and subclassing stuff */structPyMethodDef*tp_methods;structPyMemberDef*tp_members;structPyGetSetDef*tp_getset;struct_typeobject*tp_base;PyObject*tp_dict;descrgetfunctp_descr_get;descrsetfunctp_descr_set;Py_ssize_ttp_dictoffset;initproctp_init;allocfunctp_alloc;newfunctp_new;freefunctp_free;/* Low-level free-memory routine */inquirytp_is_gc;/* For PyObject_IS_GC */PyObject*tp_bases;PyObject*tp_mro;/* method resolution order */PyObject*tp_cache;PyObject*tp_subclasses;PyObject*tp_weaklist;destructortp_del;/* Type attribute cache version tag. Added in version 2.6 */unsignedinttp_version_tag;destructortp_finalize;}PyTypeObject;

PyObject Slots¶

The type object structure extends thePyVarObject structure. Theob_size field is used for dynamic types (created bytype_new(),usually called from a class statement). Note thatPyType_Type (themetatype) initializestp_itemsize, which means that its instances (i.e.type objects)must have theob_size field.

PyObject*PyObject._ob_next¶

PyObject*PyObject._ob_prev¶

These fields are only present when the macroPy_TRACE_REFS is defined.Their initialization toNULL is taken care of by thePyObject_HEAD_INITmacro. For statically allocated objects, these fields always remainNULL.For dynamically allocated objects, these two fields are used to link the objectinto a doubly-linked list ofall live objects on the heap. This could be usedfor various debugging purposes; currently the only use is to print the objectsthat are still alive at the end of a run when the environment variablePYTHONDUMPREFS is set.

Inheritance:

These fields are not inherited by subtypes.

Py_ssize_tPyObject.ob_refcnt¶

This is the type object’s reference count, initialized to1 by thePyObject_HEAD_INIT macro. Note that for statically allocated type objects,the type’s instances (objects whoseob_type points back to the type) donot count as references. But for dynamically allocated type objects, theinstancesdo count as references.

Inheritance:

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 theargument to thePyObject_HEAD_INIT macro, and its value should normally be&PyType_Type. However, for dynamically loadable extension modules that mustbe usable on Windows (at least), the compiler complains that this is not a validinitializer. Therefore, the convention is to passNULL to thePyObject_HEAD_INIT macro and to initialize this field explicitly at thestart of the module’s initialization function, before doing anything else. Thisis 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 isNULL, and if so,initializes it to theob_type field of the base class.PyType_Ready() will not change this field if it is non-zero.

Inheritance:

This field is inherited by subtypes.

PyVarObject Slots¶

Py_ssize_tPyVarObject.ob_size¶

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

Inheritance:

This field is not inherited by subtypes.

PyTypeObject Slots¶

Each slot has a section describing inheritance. IfPyType_Ready()may set a value when the field is set toNULL then there will also bea “Default” section. (Note that many fields set onPyBaseObject_TypeandPyType_Type effectively act as defaults.)

const char*PyTypeObject.tp_name¶

Pointer to a NUL-terminated string containing the name of the type. For typesthat are accessible as module globals, the string should be the full modulename, followed by a dot, followed by the type name; for built-in types, itshould be just the type name. If the module is a submodule of a package, thefull package name is part of the full module name. For example, a type namedT defined in moduleM in subpackageQ in packagePshould have thetp_name initializer"P.Q.M.T".

For dynamically allocated type objects, this should just be the type name, andthe 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 entiretp_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 yourtype will be impossible to pickle. Additionally, it will not be listed inmodule documentations created with pydoc.

This field must not beNULL. It is the only required fieldinPyTypeObject() (other than potentiallytp_itemsize).

Inheritance:

This field is not inherited by subtypes.

Py_ssize_tPyTypeObject.tp_basicsize¶

Py_ssize_tPyTypeObject.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 zerotp_itemsize field, types with variable-length instances have a non-zerotp_itemsize field. For a type with fixed-length instances, allinstances have the same size, given intp_basicsize.

For a type with variable-length instances, the instances must have anob_size field, and the instance size istp_basicsize plus Ntimestp_itemsize, where N is the “length” of the object. The value ofN is typically stored in the instance’sob_size field. There areexceptions: for example, ints use a negativeob_size to indicate anegative number, and N isabs(ob_size) there. Also, the presence of anob_size field in the instance layout doesn’t mean that the instancestructure is variable-length (for example, the structure for the list type hasfixed-length instances, yet those instances have a meaningfulob_sizefield).

The basic size includes the fields in the instance declared by the macroPyObject_HEAD orPyObject_VAR_HEAD (whichever is used todeclare the instance struct) and this in turn includes the_ob_prev and_ob_next fields if they are present. This means that the only correctway 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.

A note about alignment: if the variable items require a particular alignment,this should be taken care of by the value oftp_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 thealignment requirement fordouble).

For any type with variable-length instances, this field must not beNULL.

Inheritance:

These fields are inherited separately by subtypes. If the base type has anon-zerotp_itemsize, it is generally not safe to settp_itemsize to a different non-zero value in a subtype (though thisdepends on the implementation of the base type).

destructorPyTypeObject.tp_dealloc¶

A pointer to the instance destructor function. This function must be definedunless the type guarantees that its instances will never be deallocated (as isthe case for the singletonsNone andEllipsis). The function signature is:

voidtp_dealloc(PyObject*self);

The destructor function is called by thePy_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. Thedestructor function should free all references which the instance owns, free allmemory buffers owned by the instance (using the freeing function correspondingto the allocation function used to allocate the buffer), and call the type’stp_free function. If the type is not subtypable(doesn’t have thePy_TPFLAGS_BASETYPE flag bit set), it ispermissible to call the object deallocator directly instead of viatp_free. The object deallocator should be the one used to allocate theinstance; this is normallyPyObject_Del() if the instance was allocatedusingPyObject_New() orPyObject_VarNew(), orPyObject_GC_Del() if the instance was allocated usingPyObject_GC_New() orPyObject_GC_NewVar().

Finally, if the type is heap allocated (Py_TPFLAGS_HEAPTYPE), thedeallocator should decrement the reference count for its type object aftercalling the type deallocator. In order to avoid dangling pointers, therecommended way to achieve this is:

staticvoidfoo_dealloc(foo_object*self){PyTypeObject*tp=Py_TYPE(self);// free references and buffers heretp->tp_free(self);Py_DECREF(tp);}

Inheritance:

This field is inherited by subtypes.

Py_ssize_tPyTypeObject.tp_vectorcall_offset¶

An optional offset to a per-instance function that implements callingthe object using thevectorcall protocol, a more efficient alternativeof the simplertp_call.

This field is only used if the flag_Py_TPFLAGS_HAVE_VECTORCALLis set. If so, this must be a positive integer containing the offset in theinstance of avectorcallfunc pointer.The signature is the same as for_PyObject_Vectorcall():

PyObject*vectorcallfunc(PyObject*callable,PyObject*const*args,size_tnargsf,PyObject*kwnames)

Thevectorcallfunc pointer may be zero, in which case the instance behavesas if_Py_TPFLAGS_HAVE_VECTORCALL was not set: calling the instancefalls back totp_call.

Any class that sets_Py_TPFLAGS_HAVE_VECTORCALL must also settp_call and make sure its behaviour is consistentwith thevectorcallfunc function.This can be done by settingtp_call toPyVectorcall_Call:

PyObject *PyVectorcall_Call(PyObject *callable,PyObject *tuple,PyObject *dict)¶

Callcallable’svectorcallfunc with positional and keywordarguments given in a tuple and dict, respectively.

This function is intended to be used in thetp_call slot.It does not fall back totp_call and it currently does not check the_Py_TPFLAGS_HAVE_VECTORCALL flag.To call an object, use one of thePyObject_Callfunctions instead.

Note

It is not recommended forheap types to implementthe vectorcall protocol.When a user sets__call__ in Python code, onlytp_call is updated,possibly making it inconsistent with the vectorcall function.

Note

The semantics of thetp_vectorcall_offset slot are provisional andexpected to be finalized in Python 3.9.If you use vectorcall, plan for updating your code for Python 3.9.

Changed in version 3.8:This slot was used for print formatting in Python 2.x.In Python 3.0 to 3.7, it was reserved and namedtp_print.

Inheritance:

This field is inherited by subtypes together withtp_call: a subtype inheritstp_vectorcall_offset from its base type whenthe subtype’stp_call isNULL.

Note thatheap types (including subclasses defined in Python) do notinherit the_Py_TPFLAGS_HAVE_VECTORCALL flag.

getattrfuncPyTypeObject.tp_getattr¶

An optional pointer to the get-attribute-string function.

This field is deprecated. When it is defined, it should point to a functionthat acts the same as thetp_getattro function, but taking a C stringinstead of a Python string object to give the attribute name.

Inheritance:

Group:tp_getattr,tp_getattro

This field is inherited by subtypes together withtp_getattro: a subtypeinherits bothtp_getattr andtp_getattro from its base type whenthe subtype’stp_getattr andtp_getattro are bothNULL.

setattrfuncPyTypeObject.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 functionthat acts the same as thetp_setattro function, but taking a C stringinstead of a Python string object to give the attribute name.

Inheritance:

Group:tp_setattr,tp_setattro

This field is inherited by subtypes together withtp_setattro: a subtypeinherits bothtp_setattr andtp_setattro from its base type whenthe subtype’stp_setattr andtp_setattro are bothNULL.

PyAsyncMethods*PyTypeObject.tp_as_async¶

Pointer to an additional structure that contains fields relevant only toobjects which implementawaitable andasynchronous iteratorprotocols at the C-level. SeeAsync Object Structures for details.

New in version 3.5:Formerly known astp_compare andtp_reserved.

Inheritance:

Thetp_as_async field is not inherited,but the contained fields are inherited individually.

reprfuncPyTypeObject.tp_repr¶

An optional pointer to a function that implements the built-in functionrepr().

The signature is the same as forPyObject_Repr():

PyObject*tp_repr(PyObject*self);

The function must return a string or a Unicode object. Ideally,this function should return a string that, when passed toeval(), given a suitable environment, returns an object with thesame 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 theobject can be deduced.

Inheritance:

This field is inherited by subtypes.

Default:

When this field is not set, a string of the form<%sobjectat%p> isreturned, where%s is replaced by the type name, and%p by the object’smemory address.

PyNumberMethods*PyTypeObject.tp_as_number¶

Pointer to an additional structure that contains fields relevant only toobjects which implement the number protocol. These fields are documented inNumber Object Structures.

Inheritance:

Thetp_as_number field is not inherited, but the contained fields areinherited individually.

PySequenceMethods*PyTypeObject.tp_as_sequence¶

Pointer to an additional structure that contains fields relevant only toobjects which implement the sequence protocol. These fields are documentedinSequence Object Structures.

Inheritance:

Thetp_as_sequence field is not inherited, but the contained fieldsare inherited individually.

PyMappingMethods*PyTypeObject.tp_as_mapping¶

Pointer to an additional structure that contains fields relevant only toobjects which implement the mapping protocol. These fields are documented inMapping Object Structures.

Inheritance:

Thetp_as_mapping field is not inherited, but the contained fieldsare inherited individually.

hashfuncPyTypeObject.tp_hash¶

An optional pointer to a function that implements the built-in functionhash().

The signature is the same as forPyObject_Hash():

Py_hash_ttp_hash(PyObject*);

The value-1 should not be returned as anormal return value; when an error occurs during the computation of the hashvalue, the function should set an exception and return-1.

When this field is not set (andtp_richcompare is not set),an attempt to take the hash of the object raisesTypeError.This is the same as setting it toPyObject_HashNotImplemented().

This field can be set explicitly toPyObject_HashNotImplemented() toblock inheritance of the hash method from a parent type. This is interpretedas the equivalent of__hash__=None at the Python level, causingisinstance(o,collections.Hashable) to correctly returnFalse. Notethat the converse is also true - setting__hash__=None on a class atthe Python level will result in thetp_hash slot being set toPyObject_HashNotImplemented().

Inheritance:

Group:tp_hash,tp_richcompare

This field is inherited by subtypes together withtp_richcompare: a subtype inherits both oftp_richcompare andtp_hash, when the subtype’stp_richcompare andtp_hash are bothNULL.

ternaryfuncPyTypeObject.tp_call¶

An optional pointer to a function that implements calling the object. Thisshould beNULL if the object is not callable. The signature is the same asforPyObject_Call():

PyObject*tp_call(PyObject*self,PyObject*args,PyObject*kwargs);

Inheritance:

This field is inherited by subtypes.

reprfuncPyTypeObject.tp_str¶

An optional pointer to a function that implements the built-in operationstr(). (Note thatstr is a type now, andstr() calls theconstructor for that type. This constructor callsPyObject_Str() to dothe actual work, andPyObject_Str() will call this handler.)

The signature is the same as forPyObject_Str():

PyObject*tp_str(PyObject*self);

The function must return a string or a Unicode object. It should be a “friendly” stringrepresentation of the object, as this is the representation that will be used,among other things, by theprint() function.

Inheritance:

This field is inherited by subtypes.

Default:

When this field is not set,PyObject_Repr() is called to return a stringrepresentation.

getattrofuncPyTypeObject.tp_getattro¶

An optional pointer to the get-attribute function.

The signature is the same as forPyObject_GetAttr():

PyObject*tp_getattro(PyObject*self,PyObject*attr);

It is usually convenient to set this field toPyObject_GenericGetAttr(),which implements the normal way of looking for object attributes.

Inheritance:

Group:tp_getattr,tp_getattro

This field is inherited by subtypes together withtp_getattr: a subtypeinherits bothtp_getattr andtp_getattro from its base type whenthe subtype’stp_getattr andtp_getattro are bothNULL.

Default:

PyBaseObject_Type usesPyObject_GenericGetAttr().

setattrofuncPyTypeObject.tp_setattro¶

An optional pointer to the function for setting and deleting attributes.

The signature is the same as forPyObject_SetAttr():

PyObject*tp_setattro(PyObject*self,PyObject*attr,PyObject*value);

In addition, settingvalue toNULL to delete an attribute must besupported. It is usually convenient to set this field toPyObject_GenericSetAttr(), which implements the normalway of setting object attributes.

Inheritance:

Group:tp_setattr,tp_setattro

This field is inherited by subtypes together withtp_setattr: a subtypeinherits bothtp_setattr andtp_setattro from its base type whenthe subtype’stp_setattr andtp_setattro are bothNULL.

Default:

PyBaseObject_Type usesPyObject_GenericSetAttr().

PyBufferProcs*PyTypeObject.tp_as_buffer¶

Pointer to an additional structure that contains fields relevant only to objectswhich implement the buffer interface. These fields are documented inBuffer Object Structures.

Inheritance:

Thetp_as_buffer field is not inherited,but the contained fields are inherited individually.

unsigned longPyTypeObject.tp_flags¶

This field is a bit mask of various flags. Some flags indicate variantsemantics for certain situations; others are used to indicate that certainfields in the type object (or in the extension structures referenced viatp_as_number,tp_as_sequence,tp_as_mapping, andtp_as_buffer) that were historically not always present are valid; ifsuch a flag bit is clear, the type fields it guards must not be accessed andmust be considered to have a zero orNULL value instead.

Inheritance:

Inheritance of this field is complicated. Most flag bits are inheritedindividually, i.e. if the base type has a flag bit set, the subtype inheritsthis flag bit. The flag bits that pertain to extension structures are strictlyinherited if the extension structure is inherited, i.e. the base type’s value ofthe flag bit is copied into the subtype together with a pointer to the extensionstructure. ThePy_TPFLAGS_HAVE_GC flag bit is inherited together withthetp_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 and haveNULL values.

Default:

PyBaseObject_Type usesPy_TPFLAGS_DEFAULT|Py_TPFLAGS_BASETYPE.

Bit Masks:

The following bit masks are currently defined; these can be ORed together usingthe| operator to form the value of thetp_flags field. The macroPyType_HasFeature() takes a type and a flags value,tp andf, andchecks whethertp->tp_flags&f is non-zero.

Py_TPFLAGS_HEAPTYPE¶

This bit is set when the type object itself is allocated on the heap, forexample, types created dynamically usingPyType_FromSpec(). In thiscase, theob_type field of its instances is considered a reference tothe type, and the type object is INCREF’ed when a new instance is created, andDECREF’ed when an instance is destroyed (this does not apply to instances ofsubtypes; only the type referenced by the instance’s ob_type gets INCREF’ed orDECREF’ed).

Inheritance:

???

Py_TPFLAGS_BASETYPE¶

This bit is set when the type can be used as the base type of another type. Ifthis bit is clear, the type cannot be subtyped (similar to a “final” class inJava).

Inheritance:

???

Py_TPFLAGS_READY¶

This bit is set when the type object has been fully initialized byPyType_Ready().

Inheritance:

???

Py_TPFLAGS_READYING¶

This bit is set whilePyType_Ready() is in the process of initializingthe type object.

Inheritance:

???

Py_TPFLAGS_HAVE_GC¶

This bit is set when the object supports garbage collection. If this bitis set, instances must be created usingPyObject_GC_New() anddestroyed usingPyObject_GC_Del(). More information in sectionSupporting Cyclic Garbage Collection. This bit also implies that theGC-related fieldstp_traverse andtp_clear are present inthe type object.

Inheritance:

Group:Py_TPFLAGS_HAVE_GC,tp_traverse,tp_clear

ThePy_TPFLAGS_HAVE_GC flag bit is inheritedtogether with thetp_traverse andtp_clearfields, i.e. if thePy_TPFLAGS_HAVE_GC flag bit isclear in the subtype and thetp_traverse andtp_clear fields in the subtype exist and haveNULLvalues.

Py_TPFLAGS_DEFAULT¶

This is a bitmask of all the bits that pertain to the existence of certainfields in the type object and its extension structures. Currently, it includesthe following bits:Py_TPFLAGS_HAVE_STACKLESS_EXTENSION,Py_TPFLAGS_HAVE_VERSION_TAG.

Inheritance:

???

Py_TPFLAGS_METHOD_DESCRIPTOR¶

This bit indicates that objects behave like unbound methods.

If this flag is set fortype(meth), then:

• meth.__get__(obj,cls)(*args,**kwds) (withobj not None)must be equivalent tometh(obj,*args,**kwds).

• meth.__get__(None,cls)(*args,**kwds)must be equivalent tometh(*args,**kwds).

This flag enables an optimization for typical method calls likeobj.meth(): it avoids creating a temporary “bound method” object forobj.meth.

New in version 3.8.

Inheritance:

This flag is never inherited by heap types.For extension types, it is inherited whenevertp_descr_get is inherited.

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 asPyLong_Check() to quickly determine if a type is a subclassof a built-in type; such specific checks are faster than a genericcheck, likePyObject_IsInstance(). Custom types that inheritfrom built-ins should have theirtp_flagsset appropriately, or the code that interacts with such typeswill behave differently depending on what kind of check is used.

Py_TPFLAGS_HAVE_FINALIZE¶

This bit is set when thetp_finalize slot is present in thetype structure.

New in version 3.4.

Deprecated since version 3.8:This flag isn’t necessary anymore, as the interpreter assumes thetp_finalize slot is always present in thetype structure.

_Py_TPFLAGS_HAVE_VECTORCALL¶

This bit is set when the class implements the vectorcall protocol.Seetp_vectorcall_offset for details.

Inheritance:

This bit is set onstatic subtypes iftp_flags is not overridden:a subtype inherits_Py_TPFLAGS_HAVE_VECTORCALL from its base typewhen the subtype’stp_call isNULLand the subtype’sPy_TPFLAGS_HEAPTYPE is not set.

Heap types do not inherit_Py_TPFLAGS_HAVE_VECTORCALL.

Note

This flag is provisional and expected to become public in Python 3.9,with a different name and, possibly, changed semantics.If you use vectorcall, plan for updating your code for Python 3.9.

New in version 3.8.

const char*PyTypeObject.tp_doc¶

An optional pointer to a NUL-terminated C string giving the docstring for thistype object. This is exposed as the__doc__ attribute on the type andinstances of the type.

Inheritance:

This field isnot inherited by subtypes.

traverseprocPyTypeObject.tp_traverse¶

An optional pointer to a traversal function for the garbage collector. This isonly used if thePy_TPFLAGS_HAVE_GC flag bit is set. The signature is:

inttp_traverse(PyObject*self,visitprocvisit,void*arg);

More information about Python’s garbage collection scheme can be foundin sectionSupporting Cyclic Garbage Collection.

Thetp_traverse pointer is used by the garbage collector to detectreference cycles. A typical implementation of atp_traverse functionsimply callsPy_VISIT() on each of the instance’s members that are Pythonobjects that the instance owns. For example, this is functionlocal_traverse() from the_thread extension module:

staticintlocal_traverse(localobject*self,visitprocvisit,void*arg){Py_VISIT(self->args);Py_VISIT(self->kw);Py_VISIT(self->dict);return0;}

Note thatPy_VISIT() is called only on those members that can participatein reference cycles. Although there is also aself->key member, it can onlybeNULL 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 adebugging aid you may want to visit it anyway just so thegc module’sget_referents() function will include it.

Warning

When implementingtp_traverse, only the membersthat the instanceowns (by having strong references to them) must bevisited. For instance, if an object supports weak references via thetp_weaklist slot, the pointer supportingthe linked list (whattp_weaklist points to) mustnot bevisited as the instance does not directly own the weak references to itself(the weakreference list is there to support the weak reference machinery,but the instance has no strong reference to the elements inside it, as theyare allowed to be removed even if the instance is still alive).

Note thatPy_VISIT() requires thevisit andarg parameters tolocal_traverse() to have these specific names; don’t name them justanything.

Inheritance:

Group:Py_TPFLAGS_HAVE_GC,tp_traverse,tp_clear

This field is inherited by subtypes together withtp_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 inthe subtype.

inquiryPyTypeObject.tp_clear¶

An optional pointer to a clear function for the garbage collector. This is onlyused if thePy_TPFLAGS_HAVE_GC flag bit is set. The signature is:

inttp_clear(PyObject*);

Thetp_clear member function is used to break reference cycles in cyclicgarbage detected by the garbage collector. Taken together, alltp_clearfunctions in the system must combine to break all reference cycles. This issubtle, and if in any doubt supply atp_clear function. For example,the tuple type does not implement atp_clear function, because it’spossible to prove that no reference cycle can be composed entirely of tuples.Therefore thetp_clear functions of other types must be sufficient tobreak any cycle containing a tuple. This isn’t immediately obvious, and there’srarely a good reason to avoid implementingtp_clear.

Implementations oftp_clear should drop the instance’s references tothose of its members that may be Python objects, and set its pointers to thosemembers toNULL, as in the following example:

staticintlocal_clear(localobject*self){Py_CLEAR(self->key);Py_CLEAR(self->args);Py_CLEAR(self->kw);Py_CLEAR(self->dict);return0;}

ThePy_CLEAR() macro should be used, because clearing references isdelicate: the reference to the contained object must not be decremented untilafter the pointer to the contained object is set toNULL. This is becausedecrementing the reference count may cause the contained object to become trash,triggering a chain of reclamation activity that may include invoking arbitraryPython code (due to finalizers, or weakref callbacks, associated with thecontained object). If it’s possible for such code to referenceself again,it’s important that the pointer to the contained object beNULL at that time,so thatself knows the contained object can no longer be used. ThePy_CLEAR() macro performs the operations in a safe order.

Because the goal oftp_clear functions is to break reference cycles,it’s not necessary to clear contained objects like Python strings or Pythonintegers, which can’t participate in reference cycles. On the other hand, it maybe 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 insectionSupporting Cyclic Garbage Collection.

Inheritance:

Group:Py_TPFLAGS_HAVE_GC,tp_traverse,tp_clear

This field is inherited by subtypes together withtp_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 inthe subtype.

richcmpfuncPyTypeObject.tp_richcompare¶

An optional pointer to the rich comparison function, whose signature is:

PyObject*tp_richcompare(PyObject*self,PyObject*other,intop);

The first parameter is guaranteed to be an instance of the typethat is defined byPyTypeObject.

The function should return the result of the comparison (usuallyPy_TrueorPy_False). If the comparison is undefined, it must returnPy_NotImplemented, if another error occurred it must returnNULL andset an exception condition.

The following constants are defined to be used as the third argument fortp_richcompare and forPyObject_RichCompare():

Constant	Comparison
Py_LT	<
Py_LE	<=
Py_EQ	==
Py_NE	!=
Py_GT	>
Py_GE	>=

The following macro is defined to ease writing rich comparison functions:

Py_RETURN_RICHCOMPARE(VAL_A, VAL_B, op)¶

ReturnPy_True orPy_False from the function, depending on theresult of a comparison.VAL_A and VAL_B must be orderable by C comparison operators (for example,they may be C ints or floats). The third argument specifies the requestedoperation, as forPyObject_RichCompare().

The return value’s reference count is properly incremented.

On error, sets an exception and returnsNULL from the function.

New in version 3.7.

Inheritance:

Group:tp_hash,tp_richcompare

This field is inherited by subtypes together withtp_hash:a subtype inheritstp_richcompare andtp_hash whenthe subtype’stp_richcompare andtp_hash are bothNULL.

Default:

PyBaseObject_Type provides atp_richcompareimplementation, which may be inherited. However, if onlytp_hash is defined, not even the inherited function is usedand instances of the type will not be able to participate in anycomparisons.

Py_ssize_tPyTypeObject.tp_weaklistoffset¶

If the instances of this type are weakly referenceable, this field is greaterthan zero and contains the offset in the instance structure of the weakreference list head (ignoring the GC header, if present); this offset is used byPyObject_ClearWeakRefs() and thePyWeakref_*() functions. Theinstance structure needs to include a field of typePyObject* which isinitialized toNULL.

Do not confuse this field withtp_weaklist; that is the list head forweak references to the type object itself.

Inheritance:

This field is inherited by subtypes, but see the rules listed below. A subtypemay override this offset; this means that the subtype uses a different weakreference list head than the base type. Since the list head is always found viatp_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 weaklyreferenceable by adding a weak reference list head slot to the instance layoutand 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 forinstances 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 itsbase type.

getiterfuncPyTypeObject.tp_iter¶

An optional pointer to a function that returns an iterator for the object. Itspresence normally signals that the instances of this type are iterable (althoughsequences may be iterable without this function).

This function has the same signature asPyObject_GetIter():

PyObject*tp_iter(PyObject*self);

Inheritance:

This field is inherited by subtypes.

iternextfuncPyTypeObject.tp_iternext¶

An optional pointer to a function that returns the next item in an iterator.The signature is:

PyObject*tp_iternext(PyObject*self);

When the iterator is exhausted, it must returnNULL; aStopIterationexception may or may not be set. When another error occurs, it must returnNULL too. Its presence signals that the instances of this type areiterators.

Iterator types should also define thetp_iter function, and thatfunction should return the iterator instance itself (not a new iteratorinstance).

This function has the same signature asPyIter_Next().

Inheritance:

This field is inherited by subtypes.

structPyMethodDef*PyTypeObject.tp_methods¶

An optional pointer to a staticNULL-terminated array ofPyMethodDefstructures, declaring regular methods of this type.

For each entry in the array, an entry is added to the type’s dictionary (seetp_dict below) containing a method descriptor.

Inheritance:

This field is not inherited by subtypes (methods are inherited through adifferent mechanism).

structPyMemberDef*PyTypeObject.tp_members¶

An optional pointer to a staticNULL-terminated array ofPyMemberDefstructures, declaring regular data members (fields or slots) of instances ofthis type.

For each entry in the array, an entry is added to the type’s dictionary (seetp_dict below) containing a member descriptor.

Inheritance:

This field is not inherited by subtypes (members are inherited through adifferent mechanism).

structPyGetSetDef*PyTypeObject.tp_getset¶

An optional pointer to a staticNULL-terminated array ofPyGetSetDefstructures, declaring computed attributes of instances of this type.

For each entry in the array, an entry is added to the type’s dictionary (seetp_dict below) containing a getset descriptor.

Inheritance:

This field is not inherited by subtypes (computed attributes are inheritedthrough a different mechanism).

PyTypeObject*PyTypeObject.tp_base¶

An optional pointer to a base type from which type properties are inherited. Atthis level, only single inheritance is supported; multiple inheritance requiredynamically creating a type object by calling the metatype.

Note

Slot initialization is subject to the rules of initializing globals.C99 requires the initializers to be “address constants”. Functiondesignators likePyType_GenericNew(), with implicit conversionto a pointer, are valid C99 address constants.

However, the unary ‘&’ operator applied to a non-static variablelikePyBaseObject_Type() is not required to produce an addressconstant. Compilers may support this (gcc does), MSVC does not.Both compilers are strictly standard conforming in this particularbehavior.

Consequently,tp_base should be set inthe extension module’s init function.

Inheritance:

This field is not inherited by subtypes (obviously).

Default:

This field defaults to&PyBaseObject_Type (which to Pythonprogrammers is known as the typeobject).

PyObject*PyTypeObject.tp_dict¶

The type’s dictionary is stored here byPyType_Ready().

This field should normally be initialized toNULL before PyType_Ready iscalled; it may also be initialized to a dictionary containing initial attributesfor the type. OncePyType_Ready() has initialized the type, extraattributes for the type may be added to this dictionary only if they don’tcorrespond to overloaded operations (like__add__()).

Inheritance:

This field is not inherited by subtypes (though the attributes defined in hereare inherited through a different mechanism).

Default:

If this field isNULL,PyType_Ready() will assign a newdictionary to it.

Warning

It is not safe to usePyDict_SetItem() on or otherwise modifytp_dict with the dictionary C-API.

descrgetfuncPyTypeObject.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);

Inheritance:

This field is inherited by subtypes.

descrsetfuncPyTypeObject.tp_descr_set¶

An optional pointer to a function for setting and deletinga descriptor’s value.

The function signature is:

inttp_descr_set(PyObject*self,PyObject*obj,PyObject*value);

Thevalue argument is set toNULL to delete the value.

Inheritance:

This field is inherited by subtypes.

Py_ssize_tPyTypeObject.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 ofthe instance variable dictionary; this offset is used byPyObject_GenericGetAttr().

Do not confuse this field withtp_dict; that is the dictionary forattributes of the type object itself.

If the value of this field is greater than zero, it specifies the offset fromthe start of the instance structure. If the value is less than zero, itspecifies the offset from theend of the instance structure. A negativeoffset is more expensive to use, and should only be used when the instancestructure contains a variable-length part. This is used for example to add aninstance variable dictionary to subtypes ofstr ortuple. Notethat thetp_basicsize field should account for the dictionary added tothe end in that case, even though the dictionary is not included in the basicobject layout. On a system with a pointer size of 4 bytes,tp_dictoffset should be set to-4 to indicate that the dictionary isat the very end of the structure.

The real dictionary offset in an instance can be computed from a negativetp_dictoffset as follows:

dictoffset=tp_basicsize+abs(ob_size)*tp_itemsize+tp_dictoffsetifdictoffsetisnotalignedonsizeof(void*):rounduptosizeof(void*)

wheretp_basicsize,tp_itemsize andtp_dictoffset aretaken from the type object, andob_size is taken from the instance. Theabsolute value is taken because ints use the sign ofob_size tostore the sign of the number. (There’s never a need to do this calculationyourself; it is done for you by_PyObject_GetDictPtr().)

Inheritance:

This field is inherited by subtypes, but see the rules listed below. A subtypemay override this offset; this means that the subtype instances store thedictionary at a difference offset than the base type. Since the dictionary isalways found viatp_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 dictionaryslot is added to the instance layout and thetp_dictoffset is set tothat 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 doesnot have the expected effect, it just causes confusion. Maybe this should beadded as a feature just like__weakref__ though.)

Default:

This slot has no default. For static types, if the field isNULL then no__dict__ gets created for instances.

initprocPyTypeObject.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:

inttp_init(PyObject*self,PyObject*args,PyObject*kwds);

The self argument is the instance to be initialized; theargs andkwdsarguments represent positional and keyword arguments of the call to__init__().

Thetp_init function, if notNULL, is called when an instance iscreated normally by calling its type, after the type’stp_new functionhas returned an instance of the type. If thetp_new function returns aninstance of some other type that is not a subtype of the original type, notp_init function is called; iftp_new returns an instance of asubtype of the original type, the subtype’stp_init is called.

Returns0 on success,-1 and sets an exception on error.

Inheritance:

This field is inherited by subtypes.

Default:

For static types this field does not have a default.

allocfuncPyTypeObject.tp_alloc¶

An optional pointer to an instance allocation function.

The function signature is:

PyObject*tp_alloc(PyTypeObject*self,Py_ssize_tnitems);

Inheritance:

This field is inherited by static subtypes, but not by dynamicsubtypes (subtypes created by a class statement).

Default:

For dynamic subtypes, this field is always set toPyType_GenericAlloc(), to force a standard heapallocation strategy.

For static subtypes,PyBaseObject_Type usesPyType_GenericAlloc(). That is the recommended valuefor all statically defined types.

newfuncPyTypeObject.tp_new¶

An optional pointer to an instance creation function.

The function signature is:

PyObject*tp_new(PyTypeObject*subtype,PyObject*args,PyObject*kwds);

The subtype argument is the type of the object being created; theargs andkwds arguments represent positional and keyword arguments of the call to thetype. Note that subtype doesn’t have to equal the type whosetp_newfunction is called; it may be a subtype of that type (but not an unrelatedtype).

Thetp_new function should callsubtype->tp_alloc(subtype,nitems)to allocate space for the object, and then do only as much furtherinitialization as is absolutely necessary. Initialization that can safely beignored or repeated should be placed in thetp_init handler. A goodrule of thumb is that for immutable types, all initialization should take placeintp_new, while for mutable types, most initialization should bedeferred totp_init.

Inheritance:

This field is inherited by subtypes, except it is not inherited by static typeswhosetp_base isNULL or&PyBaseObject_Type.

Default:

For static types this field has no default. This means if theslot is defined asNULL, the type cannot be called to create newinstances; presumably there is some other way to createinstances, like a factory function.

freefuncPyTypeObject.tp_free¶

An optional pointer to an instance deallocation function. Its signature is:

voidtp_free(void*self);

An initializer that is compatible with this signature isPyObject_Free().

Inheritance:

This field is inherited by static subtypes, but not by dynamicsubtypes (subtypes created by a class statement)

Default:

In dynamic subtypes, this field is set to a deallocator suitable tomatchPyType_GenericAlloc() and the value of thePy_TPFLAGS_HAVE_GC flag bit.

For static subtypes,PyBaseObject_Type uses PyObject_Del.

inquiryPyTypeObject.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 collectibleor not. Normally, it is sufficient to look at the object’s type’stp_flags field, and check thePy_TPFLAGS_HAVE_GC flag bit. Butsome types have a mixture of statically and dynamically allocated instances, andthe statically allocated instances are not collectible. Such types shoulddefine this function; it should return1 for a collectible instance, and0 for a non-collectible instance. The signature is:

inttp_is_gc(PyObject*self);

(The only example of this are types themselves. The metatype,PyType_Type, defines this function to distinguish between staticallyand dynamically allocated types.)

Inheritance:

This field is inherited by subtypes.

Default:

This slot has no default. If this field isNULL,Py_TPFLAGS_HAVE_GC is used as the functional equivalent.

PyObject*PyTypeObject.tp_bases¶

Tuple of base types.

This is set for types created by a class statement. It should beNULL forstatically defined types.

Inheritance:

This field is not inherited.

PyObject*PyTypeObject.tp_mro¶

Tuple containing the expanded set of base types, starting with the type itselfand ending withobject, in Method Resolution Order.

Inheritance:

This field is not inherited; it is calculated fresh byPyType_Ready().

PyObject*PyTypeObject.tp_cache¶

Unused. Internal use only.

Inheritance:

This field is not inherited.

PyObject*PyTypeObject.tp_subclasses¶

List of weak references to subclasses. Internal use only.

Inheritance:

This field is not inherited.

PyObject*PyTypeObject.tp_weaklist¶

Weak reference list head, for weak references to this type object. Notinherited. Internal use only.

Inheritance:

This field is not inherited.

destructorPyTypeObject.tp_del¶

This field is deprecated. Usetp_finalize instead.

unsigned intPyTypeObject.tp_version_tag¶

Used to index into the method cache. Internal use only.

Inheritance:

This field is not inherited.

destructorPyTypeObject.tp_finalize¶

An optional pointer to an instance finalization function. Its signature is:

voidtp_finalize(PyObject*self);

Iftp_finalize is set, the interpreter calls it once whenfinalizing an instance. It is called either from the garbagecollector (if the instance is part of an isolated reference cycle) orjust before the object is deallocated. Either way, it is guaranteedto be called before attempting to break reference cycles, ensuringthat it finds the object in a sane state.

tp_finalize should not mutate the current exception status;therefore, a recommended way to write a non-trivial finalizer is:

staticvoidlocal_finalize(PyObject*self){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 thePy_TPFLAGS_HAVE_FINALIZE flags bit.

Inheritance:

This field is inherited by subtypes.

New in version 3.4.

See also

“Safe object finalization” (PEP 442)

The remaining fields are only defined if the feature test macroCOUNT_ALLOCS is defined, and are for internal use only. They aredocumented here for completeness. None of these fields are inherited bysubtypes.

Py_ssize_tPyTypeObject.tp_allocs¶

Number of allocations.

Py_ssize_tPyTypeObject.tp_frees¶

Number of frees.

Py_ssize_tPyTypeObject.tp_maxalloc¶

Maximum simultaneously allocated objects.

PyTypeObject*PyTypeObject.tp_prev¶

Pointer to the previous type object with a non-zerotp_allocs field.

PyTypeObject*PyTypeObject.tp_next¶

Pointer to the next type object with a non-zerotp_allocs field.

Also, note that, in a garbage collected Python,tp_dealloc may be called fromany Python thread, not just the thread which created the object (if the objectbecomes part of a refcount cycle, that cycle might be collected by a garbagecollection on any thread). This is not a problem for Python API calls, sincethe 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 someother C or C++ library, care should be taken to ensure that destroying thoseobjects on the thread which called tp_dealloc will not violate any assumptionsof the library.

Heap Types¶

Traditionally, types defined in C code arestatic, that is,a staticPyTypeObject structure is defined directly in codeand initialized usingPyType_Ready().

This results in types that are limited relative to types defined in Python:

• Static types are limited to one base, i.e. they cannot use multipleinheritance.

• Static type objects (but not necessarily their instances) are immutable.It is not possible to add or modify the type object’s attributes from Python.

• Static type objects are shared acrosssub-interpreters, so they should notinclude any subinterpreter-specific state.

Also, sincePyTypeObject is not part of thestable ABI,any extension modules using static types must be compiled for a specificPython minor version.

An alternative to static types isheap-allocated types, orheap typesfor short, which correspond closely to classes created by Python’sclass statement.

This is done by filling aPyType_Spec structure and callingPyType_FromSpecWithBases().