10.3 Type Objects

Python 2.5

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10.3 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* _ob_next
PyObject* _ob_prev: These fields are only present when the macro Py_TRACE_REFS is defined. Their initialization to NULL is taken care of by the PyObject_HEAD_INIT macro. For statically allocated objects, these fields always remain NULL. For dynamically allocated objects, these two fields are used to link the object into a doubly-linked list of all live objects on the heap. This could be used for various debugging purposes; currently the only use is to print the objects that are still alive at the end of a run when the environment variable PYTHONDUMPREFS is set.
These fields are not inherited by subtypes.

Py_ssize_t ob_refcnt: This is the type object's reference count, initialized to 1 by the PyObject_HEAD_INIT macro. Note that for statically allocated type objects, the type's instances (objects whose ob_type points back to the type) do not count as references. But for dynamically allocated type objects, the instances do count as references.
This field is not inherited by subtypes.

PyTypeObject* ob_type

This is the type's type, in other words its metatype. It is initialized by the argument to the PyObject_HEAD_INIT macro, and its value should normally be &PyType_Type. However, for dynamically loadable extension modules that must be usable on Windows (at least), the compiler complains that this is not a valid initializer. Therefore, the convention is to pass NULL to the PyObject_HEAD_INIT macro and to initialize this field explicitly at the start of the module's initialization function, before doing anything else. This is typically done like this:

Foo_Type.ob_type = &PyType_Type;

This should be done before any instances of the type are created. PyType_Ready() checks if ob_type is NULL, and if so, initializes it: in Python 2.2, it is set to &PyType_Type; in Python 2.2.1 and later it is initialized to the ob_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 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* tp_name

Pointer to a NUL-terminated string containing the name of the type. For types that are accessible as module globals, the string should be the full module name, followed by a dot, followed by the type name; for built-in types, it should be just the type name. If the module is a submodule of a package, the full package name is part of the full module name. For example, a type named T defined in module M in subpackage Q in package P should have the tp_name initializer "P.Q.M.T".

For dynamically allocated type objects, this should just be the type name, and the module name explicitly stored in the type dict as the value for key '__module__'.

For statically allocated type objects, the tp_name field should contain a dot. Everything before the last dot is made accessible as the __module__ attribute, and everything after the last dot is made accessible as the __name__ attribute.

If no dot is present, the entire tp_name field is made accessible as the __name__ attribute, and the __module__ attribute is undefined (unless explicitly set in the dictionary, as explained above). This means your type will be impossible to pickle.

This field is not inherited by subtypes.

Py_ssize_t tp_basicsize

Py_ssize_t tp_itemsize

These fields allow calculating the size in bytes of instances of the type.

There are two kinds of types: types with fixed-length instances have a zero tp_itemsize field, types with variable-length instances have a non-zero tp_itemsize field. For a type with fixed-length instances, all instances have the same size, given in tp_basicsize.

For a type with variable-length instances, the instances must have an ob_size field, and the instance size is tp_basicsize plus N times tp_itemsize, where N is the ``length'' of the object. The value of N is typically stored in the instance's ob_size field. There are exceptions: for example, long ints use a negative ob_size to indicate a negative number, and N is abs(ob_size) there. Also, the presence of an ob_size field in the instance layout doesn't mean that the instance structure is variable-length (for example, the structure for the list type has fixed-length instances, yet those instances have a meaningful ob_size field).

The basic size includes the fields in the instance declared by the macro PyObject_HEAD or PyObject_VAR_HEAD (whichever is used to declare the instance struct) and this in turn includes the _ob_prev and _ob_next fields if they are present. This means that the only correct way to get an initializer for the tp_basicsize is to use the sizeof operator on the struct used to declare the instance layout. The basic size does not include the GC header size (this is new in Python 2.2; in 2.1 and 2.0, the GC header size was included in tp_basicsize).

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

A note about alignment: if the variable items require a particular alignment, this should be taken care of by the value of tp_basicsize. Example: suppose a type implements an array of double. tp_itemsize is sizeof(double). It is the programmer's responsibility that tp_basicsize is a multiple of sizeof(double) (assuming this is the alignment requirement for double).

destructor tp_dealloc

A pointer to the instance destructor function. This function must be defined unless the type guarantees that its instances will never be deallocated (as is the case for the singletons None and Ellipsis).

The destructor function is called by the Py_DECREF() and Py_XDECREF() macros when the new reference count is zero. At this point, the instance is still in existence, but there are no references to it. The destructor function should free all references which the instance owns, free all memory buffers owned by the instance (using the freeing function corresponding to the allocation function used to allocate the buffer), and finally (as its last action) call the type's tp_free function. If the type is not subtypable (doesn't have the Py_TPFLAGS_BASETYPE flag bit set), it is permissible to call the object deallocator directly instead of via tp_free. The object deallocator should be the one used to allocate the instance; this is normally PyObject_Del() if the instance was allocated using PyObject_New() or PyObject_VarNew(), or PyObject_GC_Del() if the instance was allocated using PyObject_GC_New() or PyObject_GC_VarNew().

This field is inherited by subtypes.

printfunc 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's tp_repr or tp_str function is called to convert it to a string. These are also called when the type's tp_print field is NULL. A type should never implement tp_print in a way that produces different output than tp_repr or tp_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 is Py_PRINT_RAW. When the Py_PRINT_RAW flag bit is set, the instance should be printed the same way as tp_str would format it; when the Py_PRINT_RAW flag bit is clear, the instance should be printed the same was as tp_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 define tp_print, but instead to rely on tp_repr and tp_str for printing.

This field is inherited by subtypes.

getattrfunc tp_getattr

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

This field is deprecated. When it is defined, it should point to a function that acts the same as the tp_getattro function, but taking a C string instead of a Python string object to give the attribute name. The signature is the same as for PyObject_GetAttrString().

This field is inherited by subtypes together with tp_getattro: a subtype inherits both tp_getattr and tp_getattro from its base type when the subtype's tp_getattr and tp_getattro are both NULL.

setattrfunc tp_setattr

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

This field is deprecated. When it is defined, it should point to a function that acts the same as the tp_setattro function, but taking a C string instead of a Python string object to give the attribute name. The signature is the same as for PyObject_SetAttrString().

This field is inherited by subtypes together with tp_setattro: a subtype inherits both tp_setattr and tp_setattro from its base type when the subtype's tp_setattr and tp_setattro are both NULL.

cmpfunc tp_compare

An optional pointer to the three-way comparison function.

The signature is the same as for PyObject_Compare(). The function should return 1 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 and tp_hash: a subtypes inherits all three of tp_compare, tp_richcompare, and tp_hash when the subtype's tp_compare, tp_richcompare, and tp_hash are all NULL.

reprfunc tp_repr

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

The signature is the same as for PyObject_Repr(); it must return a string or a Unicode object. Ideally, this function should return a string that, when passed to eval(), given a suitable environment, returns an object with the same value. If this is not feasible, it should return a string starting with "<" and ending with ">" from which both the type and the value of the object can be deduced.

When this field is not set, a string of the form "<%s object at %p>" is returned, where %s is replaced by the type name, and %p by the object's memory address.

This field is inherited by subtypes.

PyNumberMethods *tp_as_number;

XXX

PySequenceMethods *tp_as_sequence;

XXX

PyMappingMethods *tp_as_mapping;

XXX

hashfunc 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.

When this field is not set, two possibilities exist: if the tp_compare and tp_richcompare fields are both NULL, a default hash value based on the object's address is returned; otherwise, a TypeError is raised.

This field is inherited by subtypes together with tp_richcompare and tp_compare: a subtypes inherits all three of tp_compare, tp_richcompare, and tp_hash, when the subtype's tp_compare, tp_richcompare and tp_hash are all NULL.

ternaryfunc 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 tp_str

An optional pointer to a function that implements the built-in operation str(). (Note that str is a type now, and str() calls the constructor for that type. This constructor calls PyObject_Str() to do the actual work, and PyObject_Str() will call this handler.)

The signature is the same as for PyObject_Str(); it must return a string or a Unicode object. This function should return a ``friendly'' string representation of the object, as this is the representation that will be used 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 tp_getattro

An optional pointer to the get-attribute function.

The signature is the same as for PyObject_GetAttr(). It is usually convenient to set this field to PyObject_GenericGetAttr(), which implements the normal way of looking for object attributes.

This field is inherited by subtypes together with tp_getattr: a subtype inherits both tp_getattr and tp_getattro from its base type when the subtype's tp_getattr and tp_getattro are both NULL.

setattrofunc tp_setattro

An optional pointer to the set-attribute function.

The signature is the same as for PyObject_SetAttr(). It is usually convenient to set this field to PyObject_GenericSetAttr(), which implements the normal way of setting object attributes.

This field is inherited by subtypes together with tp_setattr: a subtype inherits both tp_setattr and tp_setattro from its base type when the subtype's tp_setattr and tp_setattro are both NULL.

PyBufferProcs* 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'' (section 10.7).
The tp_as_buffer field is not inherited, but the contained fields are inherited individually.

long tp_flags

This field is a bit mask of various flags. Some flags indicate variant semantics for certain situations; others are used to indicate that certain fields in the type object (or in the extension structures referenced via tp_as_number, tp_as_sequence, tp_as_mapping, and tp_as_buffer) that were historically not always present are valid; if such a flag bit is clear, the type fields it guards must not be accessed and must be considered to have a zero or NULL value instead.

Inheritance of this field is complicated. Most flag bits are inherited individually, i.e. if the base type has a flag bit set, the subtype inherits this flag bit. The flag bits that pertain to extension structures are strictly inherited if the extension structure is inherited, i.e. the base type's value of the flag bit is copied into the subtype together with a pointer to the extension structure. The Py_TPFLAGS_HAVE_GC flag bit is inherited together with the tp_traverse and tp_clear fields, i.e. if the Py_TPFLAGS_HAVE_GC flag bit is clear in the subtype and the tp_traverse and tp_clear fields in the subtype exist (as indicated by the Py_TPFLAGS_HAVE_RICHCOMPARE flag bit) and have NULL values.

The following bit masks are currently defined; these can be or-ed together using the | operator to form the value of the tp_flags field. The macro PyType_HasFeature() takes a type and a flags value, tp and f, and checks whether tp->tp_flags & f is non-zero.

Py_TPFLAGS_HAVE_GETCHARBUFFER: If this bit is set, the PyBufferProcs struct referenced by tp_as_buffer has the bf_getcharbuffer field.

Py_TPFLAGS_HAVE_SEQUENCE_IN: If this bit is set, the PySequenceMethods struct referenced by tp_as_sequence has the sq_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 by tp_as_sequence and the PyNumberMethods structure referenced by tp_as_number contain the fields for in-place operators. In particular, this means that the PyNumberMethods structure has the fields nb_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, and nb_inplace_or; and the PySequenceMethods struct has the fields sq_inplace_concat and sq_inplace_repeat.

Py_TPFLAGS_CHECKTYPES: If this bit is set, the binary and ternary operations in the PyNumberMethods structure referenced by tp_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 to nb_add, nb_subtract, nb_multiply, nb_divide, nb_remainder, nb_divmod, nb_power, nb_lshift, nb_rshift, nb_and, nb_xor, and nb_or.

Py_TPFLAGS_HAVE_RICHCOMPARE: If this bit is set, the type object has the tp_richcompare field, as well as the tp_traverse and the tp_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's tp_weaklistoffset field has a value greater than zero.

Py_TPFLAGS_HAVE_ITER: If this bit is set, the type object has the tp_iter and tp_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, and tp_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 using PyObject_GC_Del(). More information in section XXX about garbage collection. This bit also implies that the GC-related fields tp_traverse and tp_clear are present in the type object; but those fields also exist when Py_TPFLAGS_HAVE_GC is clear but Py_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, and Py_TPFLAGS_HAVE_CLASS.

char* 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 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 10.9.

The tp_traverse pointer is used by the garbage collector to detect reference cycles. A typical implementation of a tp_traverse function simply calls Py_VISIT() on each of the instance's members that are Python objects. For exampe, this is function local_traverse from the thread extension module:

  static int
  local_traverse(localobject *self, visitproc visit, void *arg)
  {
      Py_VISIT(self->args);
      Py_VISIT(self->kw);
      Py_VISIT(self->dict);
      return 0;
  }

Note that Py_VISIT() is called only on those members that can participate in reference cycles. Although there is also a "self->key" member, it can only be NULL or a Python string and therefore cannot be part of a reference cycle.

On the other hand, even if you know a member can never be part of a cycle, as a debugging aid you may want to visit it anyway just so the gc module's get_referents() function will include it.

Note that Py_VISIT() requires the visit and arg parameters to local_traverse to have these specific names; don't name them just anything.

This field is inherited by subtypes together with tp_clear and the Py_TPFLAGS_HAVE_GC flag bit: the flag bit, tp_traverse, and tp_clear are all inherited from the base type if they are all zero in the subtype and the subtype has the Py_TPFLAGS_HAVE_RICHCOMPARE flag bit set.

inquiry tp_clear

An optional pointer to a clear function for the garbage collector. This is only used if the Py_TPFLAGS_HAVE_GC flag bit is set.

The tp_clear member function is used to break reference cycles in cyclic garbage detected by the garbage collector. Taken together, all tp_clear functions in the system must combine to break all reference cycles. This is subtle, and if in any doubt supply a tp_clear function. For example, the tuple type does not implement a tp_clear function, because it's possible to prove that no reference cycle can be composed entirely of tuples. Therefore the tp_clear functions of other types must be sufficient to break any cycle containing a tuple. This isn't immediately obvious, and there's rarely a good reason to avoid implementing tp_clear.

Implementations of tp_clear should drop the instance's references to those of its members that may be Python objects, and set its pointers to those members to NULL, as in the following example:

  static int
  local_clear(localobject *self)
  {
      Py_CLEAR(self->key);
      Py_CLEAR(self->args);
      Py_CLEAR(self->kw);
      Py_CLEAR(self->dict);
      return 0;
  }

The Py_CLEAR() macro should be used, because clearing references is delicate: the reference to the contained object must not be decremented until after the pointer to the contained object is set to NULL. This is because decrementing the reference count may cause the contained object to become trash, triggering a chain of reclamation activity that may include invoking arbitrary Python code (due to finalizers, or weakref callbacks, associated with the contained object). If it's possible for such code to reference self again, it's important that the pointer to the contained object be NULL at that time, so that self knows the contained object can no longer be used. The Py_CLEAR() macro performs the operations in a safe order.

Because the goal of tp_clear functions is to break reference cycles, it's not necessary to clear contained objects like Python strings or Python integers, which can't participate in reference cycles. On the other hand, it may be convenient to clear all contained Python objects, and write the type's tp_dealloc function to invoke tp_clear.

More information about Python's garbage collection scheme can be found in section 10.9.

This field is inherited by subtypes together with tp_traverse and the Py_TPFLAGS_HAVE_GC flag bit: the flag bit, tp_traverse, and tp_clear are all inherited from the base type if they are all zero in the subtype and the subtype has the Py_TPFLAGS_HAVE_RICHCOMPARE flag bit set.

richcmpfunc tp_richcompare

An optional pointer to the rich comparison function.

The signature is the same as for PyObject_RichCompare(). The function should return the result of the comparison (usually Py_True or Py_False). If the comparison is undefined, it must return Py_NotImplemented, if another error occurred it must return NULL and set an exception condition.

This field is inherited by subtypes together with tp_compare and tp_hash: a subtype inherits all three of tp_compare, tp_richcompare, and tp_hash, when the subtype's tp_compare, tp_richcompare, and tp_hash are all NULL.

The following constants are defined to be used as the third argument for tp_richcompare and for PyObject_RichCompare():

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

The next field only exists if the Py_TPFLAGS_HAVE_WEAKREFS flag bit is set.

long tp_weaklistoffset

If the instances of this type are weakly referenceable, this field is greater than zero and contains the offset in the instance structure of the weak reference list head (ignoring the GC header, if present); this offset is used by PyObject_ClearWeakRefs() and the PyWeakref_*() functions. The instance structure needs to include a field of type PyObject* which is initialized to NULL.

Do not confuse this field with tp_weaklist; that is the list head for weak references to the type object itself.

This field is inherited by subtypes, but see the rules listed below. A subtype may override this offset; this means that the subtype uses a different weak reference list head than the base type. Since the list head is always found via tp_weaklistoffset, this should not be a problem.

When a type defined by a class statement has no __slots__ declaration, and none of its base types are weakly referenceable, the type is made weakly referenceable by adding a weak reference list head slot to the instance layout and setting the tp_weaklistoffset of that slot's offset.

When a type's __slots__ declaration contains a slot named __weakref__, that slot becomes the weak reference list head for instances of the type, and the slot's offset is stored in the type's tp_weaklistoffset.

When a type's __slots__ declaration does not contain a slot named __weakref__, the type inherits its tp_weaklistoffset from its base type.

The next two fields only exist if the Py_TPFLAGS_HAVE_CLASS flag bit is set.

getiterfunc 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 tp_iternext

An optional pointer to a function that returns the next item in an iterator, or raises StopIteration when the iterator is exhausted. 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 a next() 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* 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* 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* 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).

Docs for PyGetSetDef (XXX belong elsewhere):

typedef PyObject *(*getter)(PyObject *, void *);
typedef int (*setter)(PyObject *, PyObject *, void *);

typedef struct PyGetSetDef {
    char *name;    /* attribute name */
    getter get;    /* C function to get the attribute */
    setter set;    /* C function to set the attribute */
    char *doc;     /* optional doc string */
    void *closure; /* optional additional data for getter and setter */
} PyGetSetDef;

PyTypeObject* tp_base: An optional pointer to a base type from which type properties are inherited. At this level, only single inheritance is supported; multiple inheritance require dynamically creating a type object by calling the metatype.
This field is not inherited by subtypes (obviously), but it defaults to &PyBaseObject_Type (which to Python programmers is known as the type object).

PyObject* 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 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);

XXX blah, blah.

This field is inherited by subtypes.

descrsetfunc tp_descr_set

An optional pointer to a "descriptor set" function.

The function signature is

int tp_descr_set(PyObject *self, PyObject *obj, PyObject *value);

This field is inherited by subtypes.

XXX blah, blah.

long tp_dictoffset

If the instances of this type have a dictionary containing instance variables, this field is non-zero and contains the offset in the instances of the type of the instance variable dictionary; this offset is used by PyObject_GenericGetAttr().

Do not confuse this field with tp_dict; that is the dictionary for attributes of the type object itself.

If the value of this field is greater than zero, it specifies the offset from the start of the instance structure. If the value is less than zero, it specifies the offset from the end of the instance structure. A negative offset is more expensive to use, and should only be used when the instance structure contains a variable-length part. This is used for example to add an instance variable dictionary to subtypes of str or tuple. Note that the tp_basicsize field should account for the dictionary added to the end in that case, even though the dictionary is not included in the basic object layout. On a system with a pointer size of 4 bytes, tp_dictoffset should be set to -4 to indicate that the dictionary is at the very end of the structure.

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

dictoffset = tp_basicsize + abs(ob_size)*tp_itemsize + tp_dictoffset
if dictoffset is not aligned on sizeof(void*):
    round up to sizeof(void*)

where tp_basicsize, tp_itemsize and tp_dictoffset are taken from the type object, and ob_size is taken from the instance. The absolute value is taken because long ints use the sign of ob_size to store the sign of the number. (There's never a need to do this calculation yourself; it is done for you by _PyObject_GetDictPtr().)

This field is inherited by subtypes, but see the rules listed below. A subtype may override this offset; this means that the subtype instances store the dictionary at a difference offset than the base type. Since the dictionary is always found via tp_dictoffset, this should not be a problem.

When a type defined by a class statement has no __slots__ declaration, and none of its base types has an instance variable dictionary, a dictionary slot is added to the instance layout and the tp_dictoffset is set to that slot's offset.

When a type defined by a class statement has a __slots__ declaration, the type inherits its tp_dictoffset from its base type.

(Adding a slot named __dict__ to the __slots__ declaration does not have the expected effect, it just causes confusion. Maybe this should be added as a feature just like __weakref__ though.)

initproc tp_init

An optional pointer to an instance initialization function.

This function corresponds to the __init__() method of classes. Like __init__(), it is possible to create an instance without calling __init__(), and it is possible to reinitialize an instance by calling its __init__() method again.

The function signature is

int tp_init(PyObject *self, PyObject *args, PyObject *kwds)

The self argument is the instance to be initialized; the args and kwds arguments represent positional and keyword arguments of the call to __init__().

The tp_init function, if not NULL, is called when an instance is created normally by calling its type, after the type's tp_new function has returned an instance of the type. If the tp_new function returns an instance of some other type that is not a subtype of the original type, no tp_init function is called; if tp_new returns an instance of a subtype of the original type, the subtype's tp_init is called. (VERSION NOTE: described here is what is implemented in Python 2.2.1 and later. In Python 2.2, the tp_init of the type of the object returned by tp_new was always called, if not NULL.)

This field is inherited by subtypes.

allocfunc tp_alloc

An optional pointer to an instance allocation function.

The function signature is

PyObject *tp_alloc(PyTypeObject *self, Py_ssize_t nitems)

The purpose of this function is to separate memory allocation from memory initialization. It should return a pointer to a block of memory of adequate length for the instance, suitably aligned, and initialized to zeros, but with ob_refcnt set to 1 and ob_type set to the type argument. If the type's tp_itemsize is non-zero, the object's ob_size field should be initialized to nitems and the length of the allocated memory block should be tp_basicsize + nitems*tp_itemsize, rounded up to a multiple of sizeof(void*); otherwise, nitems is not used and the length of the block should be tp_basicsize.

Do not use this function to do any other instance initialization, not even to allocate additional memory; that should be done by tp_new.

This field is inherited by static subtypes, but not by dynamic subtypes (subtypes created by a class statement); in the latter, this field is always set to PyType_GenericAlloc(), to force a standard heap allocation strategy. That is also the recommended value for statically defined types.

newfunc tp_new

An optional pointer to an instance creation function.

If this function is NULL for a particular type, that type cannot be called to create new instances; presumably there is some other way to create instances, like a factory function.

The function signature is

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

The subtype argument is the type of the object being created; the args and kwds arguments represent positional and keyword arguments of the call to the type. Note that subtype doesn't have to equal the type whose tp_new function is called; it may be a subtype of that type (but not an unrelated type).

The tp_new function should call subtype->tp_alloc(subtype, nitems) to allocate space for the object, and then do only as much further initialization as is absolutely necessary. Initialization that can safely be ignored or repeated should be placed in the tp_init handler. A good rule of thumb is that for immutable types, all initialization should take place in tp_new, while for mutable types, most initialization should be deferred to tp_init.

This field is inherited by subtypes, except it is not inherited by static types whose tp_base is NULL or &PyBaseObject_Type. The latter exception is a precaution so that old extension types don't become callable simply by being linked with Python 2.2.

destructor 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 the Py_TPFLAGS_HAVE_GC flag bit.

inquiry tp_is_gc

An optional pointer to a function called by the garbage collector.

The garbage collector needs to know whether a particular object is collectible or not. Normally, it is sufficient to look at the object's type's tp_flags field, and check the Py_TPFLAGS_HAVE_GC flag bit. But some types have a mixture of statically and dynamically allocated instances, and the statically allocated instances are not collectible. Such types should define this function; it should return 1 for a collectible instance, and 0 for a non-collectible instance. The signature is

int tp_is_gc(PyObject *self)

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

This field is inherited by subtypes. (VERSION NOTE: in Python 2.2, it was not inherited. It is inherited in 2.2.1 and later versions.)

PyObject* 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* 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* tp_cache: Unused. Not inherited. Internal use only.

PyObject* tp_subclasses: List of weak references to subclasses. Not inherited. Internal use only.

PyObject* tp_weaklist: Weak reference list head, for weak references to this type object. Not inherited. Internal use only.

The remaining fields are only defined if the feature test macro COUNT_ALLOCS is defined, and are for internal use only. They are documented here for completeness. None of these fields are inherited by subtypes.

Py_ssize_t tp_allocs: Number of allocations.

Py_ssize_t tp_frees: Number of frees.

Py_ssize_t tp_maxalloc: Maximum simultaneously allocated objects.

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.

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