
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, intargfunc,
intintargfunc, intobjargproc, intintobjargproc, objobjargproc,
destructor, freefunc, printfunc, getattrfunc, getattrofunc,
setattrfunc, setattrofunc, reprfunc, hashfunc

The structure definition for ``PyTypeObject`` can be found in
``Include/object.h``.  For convenience of reference, this repeats the
definition found there:

   typedef struct _typeobject {
       PyObject_VAR_HEAD
       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;
       void *tp_reserved;
       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 */

       /* call function for all accessible objects */
       traverseproc tp_traverse;

       /* delete references to contained objects */
       inquiry tp_clear;

       /* rich comparisons */
       richcmpfunc tp_richcompare;

       /* weak reference enabler */
       long tp_weaklistoffset;

       /* Iterators */
       getiterfunc tp_iter;
       iternextfunc tp_iternext;

       /* Attribute descriptor and subclassing stuff */
       struct PyMethodDef *tp_methods;
       struct PyMemberDef *tp_members;
       struct PyGetSetDef *tp_getset;
       struct _typeobject *tp_base;
       PyObject *tp_dict;
       descrgetfunc tp_descr_get;
       descrsetfunc tp_descr_set;
       long tp_dictoffset;
       initproc tp_init;
       allocfunc tp_alloc;
       newfunc tp_new;
       freefunc tp_free; /* Low-level free-memory routine */
       inquiry tp_is_gc; /* For PyObject_IS_GC */
       PyObject *tp_bases;
       PyObject *tp_mro; /* method resolution order */
       PyObject *tp_cache;
       PyObject *tp_subclasses;
       PyObject *tp_weaklist;

   } PyTypeObject;

The type object structure extends the ``PyVarObject`` structure. The
``ob_size`` field is used for dynamic types (created by
``type_new()``, usually called from a class statement). Note that
``PyType_Type`` (the metatype) initializes ``tp_itemsize``, which
means that its instances (i.e. type objects) *must* have the
``ob_size`` field.

PyObject* PyObject._ob_next
PyObject* PyObject._ob_prev

   These fields are only present when the macro ``Py_TRACE_REFS`` is
   defined. Their initialization to *NULL* is taken care of by the
   ``PyObject_HEAD_INIT`` macro.  For statically allocated objects,
   these fields always remain *NULL*. For dynamically allocated
   objects, these two fields are used to link the object into a
   doubly-linked list of *all* live objects on the heap.  This could
   be used for various debugging purposes; currently the only use is
   to print the objects that are still alive at the end of a run when
   the environment variable **PYTHONDUMPREFS** is set.

   These fields are not inherited by subtypes.

Py_ssize_t PyObject.ob_refcnt

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

   This field is not inherited by subtypes.

PyTypeObject* PyObject.ob_type

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

      Foo_Type.ob_type = &PyType_Type;

   This should be done before any instances of the type are created.
   ``PyType_Ready`` checks if ``ob_type`` is *NULL*, and if so,
   initializes it to the ``ob_type`` field of the base class.
   ``PyType_Ready`` will not change this field if it is non-zero.

   This field is inherited by subtypes.

Py_ssize_t PyVarObject.ob_size

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

   This field is not inherited by subtypes.

char* PyTypeObject.tp_name

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

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

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

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

   This field is not inherited by subtypes.

Py_ssize_t PyTypeObject.tp_basicsize
Py_ssize_t PyTypeObject.tp_itemsize

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

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

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

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

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

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

destructor PyTypeObject.tp_dealloc

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

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

   This field is inherited by subtypes.

printfunc PyTypeObject.tp_print

   An optional pointer to the instance print function.

   The print function is only called when the instance is printed to a
   *real* file; when it is printed to a pseudo-file (like a
   ``StringIO`` instance), the instance'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 PyTypeObject.tp_getattr

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

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

void* PyTypeObject.tp_reserved

   Reserved slot, formerly known as tp_compare.

reprfunc PyTypeObject.tp_repr

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

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

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

   This field is inherited by subtypes.

PyNumberMethods* tp_as_number

   Pointer to an additional structure that contains fields relevant
   only to objects which implement the number protocol.  These fields
   are documented in *Number Object Structures*.

   The ``tp_as_number`` field is not inherited, but the contained
   fields are inherited individually.

PySequenceMethods* tp_as_sequence

   Pointer to an additional structure that contains fields relevant
   only to objects which implement the sequence protocol.  These
   fields are documented in *Sequence Object Structures*.

   The ``tp_as_sequence`` field is not inherited, but the contained
   fields are inherited individually.

PyMappingMethods* tp_as_mapping

   Pointer to an additional structure that contains fields relevant
   only to objects which implement the mapping protocol.  These fields
   are documented in *Mapping Object Structures*.

   The ``tp_as_mapping`` field is not inherited, but the contained
   fields are inherited individually.

hashfunc PyTypeObject.tp_hash

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

   The signature is the same as for ``PyObject_Hash``; it must return
   a C long.  The value ``-1`` should not be returned as a normal
   return value; when an error occurs during the computation of the
   hash value, the function should set an exception and return ``-1``.

   This field can be set explicitly to ``PyObject_HashNotImplemented``
   to block inheritance of the hash method from a parent type. This is
   interpreted as the equivalent of ``__hash__ = None`` at the Python
   level, causing ``isinstance(o, collections.Hashable)`` to correctly
   return ``False``. Note that the converse is also true - setting
   ``__hash__ = None`` on a class at the Python level will result in
   the ``tp_hash`` slot being set to ``PyObject_HashNotImplemented``.

   When this field is not set, an attempt to take the hash of the
   object raises ``TypeError``.

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

ternaryfunc PyTypeObject.tp_call

   An optional pointer to a function that implements calling the
   object.  This should be *NULL* if the object is not callable.  The
   signature is the same as for ``PyObject_Call``.

   This field is inherited by subtypes.

reprfunc PyTypeObject.tp_str

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

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

   When this field is not set, ``PyObject_Repr`` is called to return a
   string representation.

   This field is inherited by subtypes.

getattrofunc PyTypeObject.tp_getattro

   An optional pointer to the get-attribute function.

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

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

setattrofunc PyTypeObject.tp_setattro

   An optional pointer to the 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* PyTypeObject.tp_as_buffer

   Pointer to an additional structure that contains fields relevant
   only to objects which implement the buffer interface.  These fields
   are documented in *Buffer Object Structures*.

   The ``tp_as_buffer`` field is not inherited, but the contained
   fields are inherited individually.

long PyTypeObject.tp_flags

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

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

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

   Py_TPFLAGS_HEAPTYPE

      This bit is set when the type object itself is allocated on the
      heap.  In this case, the ``ob_type`` field of its instances is
      considered a reference to the type, and the type object is
      INCREF'ed when a new instance is created, and DECREF'ed when an
      instance is destroyed (this does not apply to instances of
      subtypes; only the type referenced by the instance's ob_type
      gets INCREF'ed or DECREF'ed).

   Py_TPFLAGS_BASETYPE

      This bit is set when the type can be used as the base type of
      another type.  If this bit is clear, the type cannot be subtyped
      (similar to a "final" class in Java).

   Py_TPFLAGS_READY

      This bit is set when the type object has been fully initialized
      by ``PyType_Ready``.

   Py_TPFLAGS_READYING

      This bit is set while ``PyType_Ready`` is in the process of
      initializing the type object.

   Py_TPFLAGS_HAVE_GC

      This bit is set when the object supports garbage collection.  If
      this bit is set, instances must be created using
      ``PyObject_GC_New`` and destroyed using ``PyObject_GC_Del``.
      More information in section *Supporting Cyclic Garbage
      Collection*.  This bit also implies that the GC-related fields
      ``tp_traverse`` and ``tp_clear`` are present in the type object.

   Py_TPFLAGS_DEFAULT

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

char* PyTypeObject.tp_doc

   An optional pointer to a NUL-terminated C string giving the
   docstring for this type object.  This is exposed as the ``__doc__``
   attribute on the type and instances of the type.

   This field is *not* inherited by subtypes.

traverseproc PyTypeObject.tp_traverse

   An optional pointer to a traversal function for the garbage
   collector.  This is only used if the ``Py_TPFLAGS_HAVE_GC`` flag
   bit is set.  More information about Python's garbage collection
   scheme can be found in section *Supporting Cyclic Garbage
   Collection*.

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

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

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

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

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

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

inquiry PyTypeObject.tp_clear

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

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

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

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

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

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

   More information about Python's garbage collection scheme can be
   found in section *Supporting Cyclic Garbage Collection*.

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

richcmpfunc PyTypeObject.tp_richcompare

   An optional pointer to the rich comparison function, whose
   signature is ``PyObject *tp_richcompare(PyObject *a, PyObject *b,
   int op)``.

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

   Note: If you want to implement a type for which only a limited set of
     comparisons makes sense (e.g. ``==`` and ``!=``, but not ``<``
     and friends), directly raise ``TypeError`` in the rich comparison
     function.

   This field is inherited by subtypes together with ``tp_hash``: a
   subtype inherits ``tp_richcompare`` and ``tp_hash`` when the
   subtype's ``tp_richcompare`` and ``tp_hash`` are both *NULL*.

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

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

long PyTypeObject.tp_weaklistoffset

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

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

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

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

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

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

getiterfunc PyTypeObject.tp_iter

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

   This function has the same signature as ``PyObject_GetIter``.

   This field is inherited by subtypes.

iternextfunc PyTypeObject.tp_iternext

   An optional pointer to a function that returns the next item in an
   iterator. When the iterator is exhausted, it must return *NULL*; a
   ``StopIteration`` exception may or may not be set.  When another
   error occurs, it must return *NULL* too.  Its presence signals that
   the instances of this type are iterators.

   Iterator types should also define the ``tp_iter`` function, and
   that function should return the iterator instance itself (not a new
   iterator instance).

   This function has the same signature as ``PyIter_Next``.

   This field is inherited by subtypes.

struct PyMethodDef* PyTypeObject.tp_methods

   An optional pointer to a static *NULL*-terminated array of
   ``PyMethodDef`` structures, declaring regular methods of this type.

   For each entry in the array, an entry is added to the type's
   dictionary (see ``tp_dict`` below) containing a method descriptor.

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

struct PyMemberDef* PyTypeObject.tp_members

   An optional pointer to a static *NULL*-terminated array of
   ``PyMemberDef`` structures, declaring regular data members (fields
   or slots) of instances of this type.

   For each entry in the array, an entry is added to the type's
   dictionary (see ``tp_dict`` below) containing a member descriptor.

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

struct PyGetSetDef* PyTypeObject.tp_getset

   An optional pointer to a static *NULL*-terminated array of
   ``PyGetSetDef`` structures, declaring computed attributes of
   instances of this type.

   For each entry in the array, an entry is added to the type's
   dictionary (see ``tp_dict`` below) containing a getset descriptor.

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

   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* PyTypeObject.tp_base

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

   This field is not inherited by subtypes (obviously), but it
   defaults to ``&PyBaseObject_Type`` (which to Python programmers is
   known as the type ``object``).

PyObject* PyTypeObject.tp_dict

   The type's dictionary is stored here by ``PyType_Ready``.

   This field should normally be initialized to *NULL* before
   PyType_Ready is called; it may also be initialized to a dictionary
   containing initial attributes for the type.  Once ``PyType_Ready``
   has initialized the type, extra attributes for the type may be
   added to this dictionary only if they don't correspond to
   overloaded operations (like ``__add__()``).

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

descrgetfunc PyTypeObject.tp_descr_get

   An optional pointer to a "descriptor get" function.

   The function signature is

      PyObject * tp_descr_get(PyObject *self, PyObject *obj, PyObject *type);

   XXX explain.

   This field is inherited by subtypes.

descrsetfunc PyTypeObject.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 explain.

long PyTypeObject.tp_dictoffset

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

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

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

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

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

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

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

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

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

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

initproc PyTypeObject.tp_init

   An optional pointer to an instance initialization function.

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

   The function signature is

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

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

   The ``tp_init`` function, if not *NULL*, is called when an instance
   is created normally by calling its type, after the type's
   ``tp_new`` function has returned an instance of the type.  If the
   ``tp_new`` function returns an instance of some other type that is
   not a subtype of the original type, no ``tp_init`` function is
   called; if ``tp_new`` returns an instance of a subtype of the
   original type, the subtype's ``tp_init`` is called.

   This field is inherited by subtypes.

allocfunc PyTypeObject.tp_alloc

   An optional pointer to an instance allocation function.

   The function signature is

      PyObject *tp_alloc(PyTypeObject *self, Py_ssize_t nitems)

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

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

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

newfunc PyTypeObject.tp_new

   An optional pointer to an instance creation function.

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

   The function signature is

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

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

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

   This field is inherited by subtypes, except it is not inherited by
   static types whose ``tp_base`` is *NULL* or ``&PyBaseObject_Type``.

destructor PyTypeObject.tp_free

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

      void tp_free(void *)

   An initializer that is compatible with this signature is
   ``PyObject_Free``.

   This field is inherited by static subtypes, but not by dynamic
   subtypes (subtypes created by a class statement); in the latter,
   this field is set to a deallocator suitable to match
   ``PyType_GenericAlloc`` and the value of the ``Py_TPFLAGS_HAVE_GC``
   flag bit.

inquiry PyTypeObject.tp_is_gc

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

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

      int tp_is_gc(PyObject *self)

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

   This field is inherited by subtypes.

PyObject* PyTypeObject.tp_bases

   Tuple of base types.

   This is set for types created by a class statement.  It should be
   *NULL* for statically defined types.

   This field is not inherited.

PyObject* PyTypeObject.tp_mro

   Tuple containing the expanded set of base types, starting with the
   type itself and ending with ``object``, in Method Resolution Order.

   This field is not inherited; it is calculated fresh by
   ``PyType_Ready``.

PyObject* PyTypeObject.tp_cache

   Unused.  Not inherited.  Internal use only.

PyObject* PyTypeObject.tp_subclasses

   List of weak references to subclasses.  Not inherited.  Internal
   use only.

PyObject* PyTypeObject.tp_weaklist

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

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

Py_ssize_t PyTypeObject.tp_allocs

   Number of allocations.

Py_ssize_t PyTypeObject.tp_frees

   Number of frees.

Py_ssize_t PyTypeObject.tp_maxalloc

   Maximum simultaneously allocated objects.

PyTypeObject* PyTypeObject.tp_next

   Pointer to the next type object with a non-zero ``tp_allocs``
   field.

Also, note that, in a garbage collected Python, tp_dealloc may be
called from any Python thread, not just the thread which created the
object (if the object becomes part of a refcount cycle, that cycle
might be collected by a garbage collection on any thread).  This is
not a problem for Python API calls, since the thread on which
tp_dealloc is called will own the Global Interpreter Lock (GIL).
However, if the object being destroyed in turn destroys objects from
some other C or C++ library, care should be taken to ensure that
destroying those objects on the thread which called tp_dealloc will
not violate any assumptions of the library.


Number Object Structures
************************

PyNumberMethods

   This structure holds pointers to the functions which an object uses
   to implement the number protocol.  Each function is used by the
   function of similar name documented in the *Number Protocol*
   section.

   Here is the structure definition:

      typedef struct {
           binaryfunc nb_add;
           binaryfunc nb_subtract;
           binaryfunc nb_multiply;
           binaryfunc nb_remainder;
           binaryfunc nb_divmod;
           ternaryfunc nb_power;
           unaryfunc nb_negative;
           unaryfunc nb_positive;
           unaryfunc nb_absolute;
           inquiry nb_bool;
           unaryfunc nb_invert;
           binaryfunc nb_lshift;
           binaryfunc nb_rshift;
           binaryfunc nb_and;
           binaryfunc nb_xor;
           binaryfunc nb_or;
           unaryfunc nb_int;
           void *nb_reserved;
           unaryfunc nb_float;

           binaryfunc nb_inplace_add;
           binaryfunc nb_inplace_subtract;
           binaryfunc nb_inplace_multiply;
           binaryfunc nb_inplace_divide;
           binaryfunc nb_inplace_remainder;
           ternaryfunc nb_inplace_power;
           binaryfunc nb_inplace_lshift;
           binaryfunc nb_inplace_rshift;
           binaryfunc nb_inplace_and;
           binaryfunc nb_inplace_xor;
           binaryfunc nb_inplace_or;

           binaryfunc nb_floor_divide;
           binaryfunc nb_true_divide;
           binaryfunc nb_inplace_floor_divide;
           binaryfunc nb_inplace_true_divide;

           unaryfunc nb_index;
      } PyNumberMethods;

   Note: Binary and ternary functions must check the type of all their
     operands, and implement the necessary conversions (at least one
     of the operands is an instance of the defined type).  If the
     operation is not defined for the given operands, binary and
     ternary functions must return ``Py_NotImplemented``, if another
     error occurred they must return ``NULL`` and set an exception.

   Note: The ``nb_reserved`` field should always be ``NULL``.  It was
     previously called ``nb_long``, and was renamed in Python 3.0.1.


Mapping Object Structures
*************************

PyMappingMethods

   This structure holds pointers to the functions which an object uses
   to implement the mapping protocol.  It has three members:

lenfunc PyMappingMethods.mp_length

   This function is used by ``PyMapping_Length`` and
   ``PyObject_Size``, and has the same signature.  This slot may be
   set to *NULL* if the object has no defined length.

binaryfunc PyMappingMethods.mp_subscript

   This function is used by ``PyObject_GetItem`` and has the same
   signature.  This slot must be filled for the ``PyMapping_Check``
   function to return ``1``, it can be *NULL* otherwise.

objobjargproc PyMappingMethods.mp_ass_subscript

   This function is used by ``PyObject_SetItem`` and has the same
   signature.  If this slot is *NULL*, the object does not support
   item assignment.


Sequence Object Structures
**************************

PySequenceMethods

   This structure holds pointers to the functions which an object uses
   to implement the sequence protocol.

lenfunc PySequenceMethods.sq_length

   This function is used by ``PySequence_Size`` and ``PyObject_Size``,
   and has the same signature.

binaryfunc PySequenceMethods.sq_concat

   This function is used by ``PySequence_Concat`` and has the same
   signature.  It is also used by the ``+`` operator, after trying the
   numeric addition via the ``tp_as_number.nb_add`` slot.

ssizeargfunc PySequenceMethods.sq_repeat

   This function is used by ``PySequence_Repeat`` and has the same
   signature.  It is also used by the ``*`` operator, after trying
   numeric multiplication via the ``tp_as_number.nb_mul`` slot.

ssizeargfunc PySequenceMethods.sq_item

   This function is used by ``PySequence_GetItem`` and has the same
   signature.  This slot must be filled for the ``PySequence_Check``
   function to return ``1``, it can be *NULL* otherwise.

   Negative indexes are handled as follows: if the ``sq_length`` slot
   is filled, it is called and the sequence length is used to compute
   a positive index which is passed to ``sq_item``.  If ``sq_length``
   is *NULL*, the index is passed as is to the function.

ssizeobjargproc PySequenceMethods.sq_ass_item

   This function is used by ``PySequence_SetItem`` and has the same
   signature.  This slot may be left to *NULL* if the object does not
   support item assignment.

objobjproc PySequenceMethods.sq_contains

   This function may be used by ``PySequence_Contains`` and has the
   same signature.  This slot may be left to *NULL*, in this case
   ``PySequence_Contains`` simply traverses the sequence until it
   finds a match.

binaryfunc PySequenceMethods.sq_inplace_concat

   This function is used by ``PySequence_InPlaceConcat`` and has the
   same signature.  It should modify its first operand, and return it.

ssizeargfunc PySequenceMethods.sq_inplace_repeat

   This function is used by ``PySequence_InPlaceRepeat`` and has the
   same signature.  It should modify its first operand, and return it.


Buffer Object Structures
************************

The buffer interface exports a model where an object can expose its
internal data.

If an object does not export the buffer interface, then its
``tp_as_buffer`` member in the ``PyTypeObject`` structure should be
*NULL*.  Otherwise, the ``tp_as_buffer`` will point to a
``PyBufferProcs`` structure.

PyBufferProcs

   Structure used to hold the function pointers which define an
   implementation of the buffer protocol.

   getbufferproc bf_getbuffer

      This should fill a ``Py_buffer`` with the necessary data for
      exporting the type.  The signature of ``getbufferproc`` is ``int
      (PyObject *obj, PyObject *view, int flags)``.  *obj* is the
      object to export, *view* is the ``Py_buffer`` struct to fill,
      and *flags* gives the conditions the caller wants the memory
      under.  (See ``PyObject_GetBuffer`` for all flags.)
      ``bf_getbuffer`` is responsible for filling *view* with the
      appropriate information. (``PyBuffer_FillView`` can be used in
      simple cases.)  See ``Py_buffer``s docs for what needs to be
      filled in.

   releasebufferproc bf_releasebuffer

      This should release the resources of the buffer.  The signature
      of ``releasebufferproc`` is ``void (PyObject *obj, Py_buffer
      *view)``. If the ``bf_releasebuffer`` function is not provided
      (i.e. it is *NULL*), then it does not ever need to be called.

      The exporter of the buffer interface must make sure that any
      memory pointed to in the ``Py_buffer`` structure remains valid
      until releasebuffer is called.  Exporters will need to define a
      ``bf_releasebuffer`` function if they can re-allocate their
      memory, strides, shape, suboffsets, or format variables which
      they might share through the struct bufferinfo.

      See ``PyBuffer_Release``.
