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
       const char *tp_name; /* For printing, in format "<module>.<name>" */
       Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */

       /* Methods to implement standard operations */

       destructor tp_dealloc;
       printfunc tp_print;
       getattrfunc tp_getattr;
       setattrfunc tp_setattr;
       PyAsyncMethods *tp_as_async; /* formerly known as tp_compare (Python 2)
                                       or tp_reserved (Python 3) */
       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 */
       unsigned long tp_flags;

       const 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 */
       Py_ssize_t 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;
       Py_ssize_t 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;
       destructor tp_del;

       /* Type attribute cache version tag. Added in version 2.6 */
       unsigned int tp_version_tag;

       destructor tp_finalize;

   } 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.

const 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.
   Additionally, it will not be listed in module documentations
   created with pydoc.

   This field is not inherited by subtypes.

Py_ssize_t PyTypeObject.tp_basicsize
Py_ssize_t PyTypeObject.tp_itemsize

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

   There are two kinds of types: types with fixed-length instances
   have a zero "tp_itemsize" field, types with variable-length
   instances have a non-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_NewVar()".

   This field is inherited by subtypes.

printfunc PyTypeObject.tp_print

   Reserved slot, formerly used for print formatting in Python 2.x.

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

      PyObject * tp_getattr(PyObject *o, char *attr_name);

   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 function for setting and deleting
   attributes.

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

      PyObject * tp_setattr(PyObject *o, char *attr_name, PyObject *v);

   The *v* argument is set to *NULL* to delete the attribute. This
   field is inherited by subtypes together with "tp_setattro": a
   subtype inherits both "tp_setattr" and "tp_setattro" from its base
   type when the subtype's "tp_setattr" and "tp_setattro" are both
   *NULL*.

PyAsyncMethods* tp_as_async

   Pointer to an additional structure that contains fields relevant
   only to objects which implement *awaitable* and *asynchronous
   iterator* protocols at the C-level.  See Async Object Structures
   for details.

   3.5 版新加入: Formerly known as "tp_compare" and "tp_reserved".

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 value of the type Py_hash_t.  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 function for setting and deleting
   attributes.

   The signature is the same as for "PyObject_SetAttr()", but setting
   *v* to *NULL* to delete an attribute must be supported.  It is
   usually convenient to set this field 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.

unsigned 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".

   Py_TPFLAGS_LONG_SUBCLASS

   Py_TPFLAGS_LIST_SUBCLASS

   Py_TPFLAGS_TUPLE_SUBCLASS

   Py_TPFLAGS_BYTES_SUBCLASS

   Py_TPFLAGS_UNICODE_SUBCLASS

   Py_TPFLAGS_DICT_SUBCLASS

   Py_TPFLAGS_BASE_EXC_SUBCLASS

   Py_TPFLAGS_TYPE_SUBCLASS

      These flags are used by functions such as "PyLong_Check()" to
      quickly determine if a type is a subclass of a built-in type;
      such specific checks are faster than a generic check, like
      "PyObject_IsInstance()". Custom types that inherit from built-
      ins should have their "tp_flags" set appropriately, or the code
      that interacts with such types will behave differently depending
      on what kind of check is used.

   Py_TPFLAGS_HAVE_FINALIZE

      This bit is set when the "tp_finalize" slot is present in the
      type structure.

      3.4 版新加入.

const 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 first parameter is guaranteed to be an instance of
   the type that is defined by "PyTypeObject".

   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.

   備註: 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"          | ">="         |
   +------------------+--------------+

Py_ssize_t 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).

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).

   警告: It is not safe to use "PyDict_SetItem()" on or otherwise
     modify "tp_dict" with the dictionary C-API.

descrgetfunc PyTypeObject.tp_descr_get

   An optional pointer to a "descriptor get" function.

   The function signature is

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

   This field is inherited by subtypes.

descrsetfunc PyTypeObject.tp_descr_set

   An optional pointer to a function for setting and deleting a
   descriptor's value.

   The function signature is

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

   The *value* argument is set to *NULL* to delete the value. This
   field is inherited by subtypes.

Py_ssize_t 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()".

destructor PyTypeObject.tp_finalize

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

      void tp_finalize(PyObject *)

   If "tp_finalize" is set, the interpreter calls it once when
   finalizing an instance.  It is called either from the garbage
   collector (if the instance is part of an isolated reference cycle)
   or just before the object is deallocated.  Either way, it is
   guaranteed to be called before attempting to break reference
   cycles, ensuring that it finds the object in a sane state.

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

      static void
      local_finalize(PyObject *self)
      {
          PyObject *error_type, *error_value, *error_traceback;

          /* Save the current exception, if any. */
          PyErr_Fetch(&error_type, &error_value, &error_traceback);

          /* ... */

          /* Restore the saved exception. */
          PyErr_Restore(error_type, error_value, error_traceback);
      }

   For this field to be taken into account (even through inheritance),
   you must also set the "Py_TPFLAGS_HAVE_FINALIZE" flags bit.

   This field is inherited by subtypes.

   3.4 版新加入.

   也參考: "Safe object finalization" (**PEP 442**)

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_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;

           binaryfunc nb_matrix_multiply;
           binaryfunc nb_inplace_matrix_multiply;
      } PyNumberMethods;

   備註: 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.

   備註: 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_Size()" 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
   "PySequence_GetSlice()", and has the same signature as
   "PyObject_GetItem()".  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()",
   "PyObject_DelItem()", "PyObject_SetSlice()" and
   "PyObject_DelSlice()".  It has the same signature as
   "PyObject_SetItem()", but *v* can also be set to *NULL* to delete
   an item.  If this slot is *NULL*, the object does not support item
   assignment and deletion.


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

PySequenceMethods

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

lenfunc PySequenceMethods.sq_length

   This function is used by "PySequence_Size()" and "PyObject_Size()",
   and has the same signature.  It is also used for handling negative
   indices via the "sq_item" and the "sq_ass_item" slots.

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 "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 "nb_multiply" slot.

ssizeargfunc PySequenceMethods.sq_item

   This function is used by "PySequence_GetItem()" and has the same
   signature.  It is also used by "PyObject_GetItem()", after trying
   the subscription via the "mp_subscript" slot. 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.  It is also used by "PyObject_SetItem()" and
   "PyObject_DelItem()", after trying the item assignment and deletion
   via the "mp_ass_subscript" slot. This slot may be left to *NULL* if
   the object does not support item assignment and deletion.

objobjproc PySequenceMethods.sq_contains

   This function may be used by "PySequence_Contains()" and has the
   same signature.  This slot may be left to *NULL*, in this 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.
   This slot may be left to *NULL*, in this case
   "PySequence_InPlaceConcat()" will fall back to
   "PySequence_Concat()".  It is also used by the augmented assignment
   "+=", after trying numeric inplace addition via the
   "nb_inplace_add" slot.

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.
   This slot may be left to *NULL*, in this case
   "PySequence_InPlaceRepeat()" will fall back to
   "PySequence_Repeat()".  It is also used by the augmented assignment
   "*=", after trying numeric inplace multiplication via the
   "nb_inplace_multiply" slot.


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

PyBufferProcs

   This structure holds pointers to the functions required by the
   Buffer protocol. The protocol defines how an exporter object can
   expose its internal data to consumer objects.

getbufferproc PyBufferProcs.bf_getbuffer

   The signature of this function is:

      int (PyObject *exporter, Py_buffer *view, int flags);

   Handle a request to *exporter* to fill in *view* as specified by
   *flags*. Except for point (3), an implementation of this function
   MUST take these steps:

   1. Check if the request can be met. If not, raise
      "PyExc_BufferError", set "view->obj" to *NULL* and return "-1".

   2. Fill in the requested fields.

   3. Increment an internal counter for the number of exports.

   4. Set "view->obj" to *exporter* and increment "view->obj".

   5. Return "0".

   If *exporter* is part of a chain or tree of buffer providers, two
   main schemes can be used:

   * Re-export: Each member of the tree acts as the exporting object
     and sets "view->obj" to a new reference to itself.

   * Redirect: The buffer request is redirected to the root object
     of the tree. Here, "view->obj" will be a new reference to the
     root object.

   The individual fields of *view* are described in section Buffer
   structure, the rules how an exporter must react to specific
   requests are in section Buffer request types.

   All memory pointed to in the "Py_buffer" structure belongs to the
   exporter and must remain valid until there are no consumers left.
   "format", "shape", "strides", "suboffsets" and "internal" are read-
   only for the consumer.

   "PyBuffer_FillInfo()" provides an easy way of exposing a simple
   bytes buffer while dealing correctly with all request types.

   "PyObject_GetBuffer()" is the interface for the consumer that wraps
   this function.

releasebufferproc PyBufferProcs.bf_releasebuffer

   The signature of this function is:

      void (PyObject *exporter, Py_buffer *view);

   Handle a request to release the resources of the buffer. If no
   resources need to be released, "PyBufferProcs.bf_releasebuffer" may
   be *NULL*. Otherwise, a standard implementation of this function
   will take these optional steps:

   1. Decrement an internal counter for the number of exports.

   2. If the counter is "0", free all memory associated with
      *view*.

   The exporter MUST use the "internal" field to keep track of buffer-
   specific resources. This field is guaranteed to remain constant,
   while a consumer MAY pass a copy of the original buffer as the
   *view* argument.

   This function MUST NOT decrement "view->obj", since that is done
   automatically in "PyBuffer_Release()" (this scheme is useful for
   breaking reference cycles).

   "PyBuffer_Release()" is the interface for the consumer that wraps
   this function.


Async Object Structures
***********************

3.5 版新加入.

PyAsyncMethods

   This structure holds pointers to the functions required to
   implement *awaitable* and *asynchronous iterator* objects.

   Here is the structure definition:

      typedef struct {
          unaryfunc am_await;
          unaryfunc am_aiter;
          unaryfunc am_anext;
      } PyAsyncMethods;

unaryfunc PyAsyncMethods.am_await

   The signature of this function is:

      PyObject *am_await(PyObject *self)

   The returned object must be an iterator, i.e. "PyIter_Check()" must
   return "1" for it.

   This slot may be set to *NULL* if an object is not an *awaitable*.

unaryfunc PyAsyncMethods.am_aiter

   The signature of this function is:

      PyObject *am_aiter(PyObject *self)

   Must return an *awaitable* object.  See "__anext__()" for details.

   This slot may be set to *NULL* if an object does not implement
   asynchronous iteration protocol.

unaryfunc PyAsyncMethods.am_anext

   The signature of this function is:

      PyObject *am_anext(PyObject *self)

   Must return an *awaitable* object.  See "__anext__()" for details.
   This slot may be set to *NULL*.
