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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.