
Common Object Structures
************************

There are a large number of structures which are used in the
definition of object types for Python.  This section describes these
structures and how they are used.

All Python objects ultimately share a small number of fields at the
beginning of the object's representation in memory.  These are
represented by the ``PyObject`` and ``PyVarObject`` types, which are
defined, in turn, by the expansions of some macros also used, whether
directly or indirectly, in the definition of all other Python objects.

PyObject

   All object types are extensions of this type.  This is a type which
   contains the information Python needs to treat a pointer to an
   object as an object.  In a normal "release" build, it contains only
   the object's reference count and a pointer to the corresponding
   type object.  It corresponds to the fields defined by the expansion
   of the ``PyObject_HEAD`` macro.

PyVarObject

   This is an extension of ``PyObject`` that adds the ``ob_size``
   field.  This is only used for objects that have some notion of
   *length*. This type does not often appear in the Python/C API.  It
   corresponds to the fields defined by the expansion of the
   ``PyObject_VAR_HEAD`` macro.

These macros are used in the definition of ``PyObject`` and
``PyVarObject``:

PyObject_HEAD

   This is a macro which expands to the declarations of the fields of
   the ``PyObject`` type; it is used when declaring new types which
   represent objects without a varying length.  The specific fields it
   expands to depend on the definition of ``Py_TRACE_REFS``.  By
   default, that macro is not defined, and ``PyObject_HEAD`` expands
   to:

      Py_ssize_t ob_refcnt;
      PyTypeObject *ob_type;

   When ``Py_TRACE_REFS`` is defined, it expands to:

      PyObject *_ob_next, *_ob_prev;
      Py_ssize_t ob_refcnt;
      PyTypeObject *ob_type;

PyObject_VAR_HEAD

   This is a macro which expands to the declarations of the fields of
   the ``PyVarObject`` type; it is used when declaring new types which
   represent objects with a length that varies from instance to
   instance. This macro always expands to:

      PyObject_HEAD
      Py_ssize_t ob_size;

   Note that ``PyObject_HEAD`` is part of the expansion, and that its
   own expansion varies depending on the definition of
   ``Py_TRACE_REFS``.

PyObject_HEAD_INIT(type)

   This is a macro which expands to initialization values for a new
   ``PyObject`` type.  This macro expands to:

      _PyObject_EXTRA_INIT
      1, type,

PyVarObject_HEAD_INIT(type, size)

   This is a macro which expands to initialization values for a new
   ``PyVarObject`` type, including the ``ob_size`` field. This macro
   expands to:

      _PyObject_EXTRA_INIT
      1, type, size,

PyCFunction

   Type of the functions used to implement most Python callables in C.
   Functions of this type take two ``PyObject*`` parameters and return
   one such value.  If the return value is *NULL*, an exception shall
   have been set.  If not *NULL*, the return value is interpreted as
   the return value of the function as exposed in Python.  The
   function must return a new reference.

PyCFunctionWithKeywords

   Type of the functions used to implement Python callables in C that
   take keyword arguments: they take three ``PyObject*`` parameters
   and return one such value.  See ``PyCFunction`` above for the
   meaning of the return value.

PyMethodDef

   Structure used to describe a method of an extension type.  This
   structure has four fields:

   +--------------------+---------------+---------------------------------+
   | Field              | C Type        | Meaning                         |
   +====================+===============+=================================+
   | ``ml_name``        | char *        | name of the method              |
   +--------------------+---------------+---------------------------------+
   | ``ml_meth``        | PyCFunction   | pointer to the C implementation |
   +--------------------+---------------+---------------------------------+
   | ``ml_flags``       | int           | flag bits indicating how the    |
   |                    |               | call should be constructed      |
   +--------------------+---------------+---------------------------------+
   | ``ml_doc``         | char *        | points to the contents of the   |
   |                    |               | docstring                       |
   +--------------------+---------------+---------------------------------+

The ``ml_meth`` is a C function pointer.  The functions may be of
different types, but they always return ``PyObject*``.  If the
function is not of the ``PyCFunction``, the compiler will require a
cast in the method table. Even though ``PyCFunction`` defines the
first parameter as ``PyObject*``, it is common that the method
implementation uses a the specific C type of the *self* object.

The ``ml_flags`` field is a bitfield which can include the following
flags. The individual flags indicate either a calling convention or a
binding convention.  Of the calling convention flags, only
``METH_VARARGS`` and ``METH_KEYWORDS`` can be combined (but note that
``METH_KEYWORDS`` alone is equivalent to ``METH_VARARGS |
METH_KEYWORDS``). Any of the calling convention flags can be combined
with a binding flag.

METH_VARARGS

   This is the typical calling convention, where the methods have the
   type ``PyCFunction``. The function expects two ``PyObject*``
   values. The first one is the *self* object for methods; for module
   functions, it is the module object.  The second parameter (often
   called *args*) is a tuple object representing all arguments. This
   parameter is typically processed using ``PyArg_ParseTuple()`` or
   ``PyArg_UnpackTuple()``.

METH_KEYWORDS

   Methods with these flags must be of type
   ``PyCFunctionWithKeywords``. The function expects three parameters:
   *self*, *args*, and a dictionary of all the keyword arguments.  The
   flag is typically combined with ``METH_VARARGS``, and the
   parameters are typically processed using
   ``PyArg_ParseTupleAndKeywords()``.

METH_NOARGS

   Methods without parameters don't need to check whether arguments
   are given if they are listed with the ``METH_NOARGS`` flag.  They
   need to be of type ``PyCFunction``.  The first parameter is
   typically named *self* and will hold a reference to the module or
   object instance.  In all cases the second parameter will be *NULL*.

METH_O

   Methods with a single object argument can be listed with the
   ``METH_O`` flag, instead of invoking ``PyArg_ParseTuple()`` with a
   ``"O"`` argument. They have the type ``PyCFunction``, with the
   *self* parameter, and a ``PyObject*`` parameter representing the
   single argument.

These two constants are not used to indicate the calling convention
but the binding when use with methods of classes.  These may not be
used for functions defined for modules.  At most one of these flags
may be set for any given method.

METH_CLASS

   The method will be passed the type object as the first parameter
   rather than an instance of the type.  This is used to create *class
   methods*, similar to what is created when using the
   ``classmethod()`` built-in function.

METH_STATIC

   The method will be passed *NULL* as the first parameter rather than
   an instance of the type.  This is used to create *static methods*,
   similar to what is created when using the ``staticmethod()`` built-
   in function.

One other constant controls whether a method is loaded in place of
another definition with the same method name.

METH_COEXIST

   The method will be loaded in place of existing definitions.
   Without *METH_COEXIST*, the default is to skip repeated
   definitions.  Since slot wrappers are loaded before the method
   table, the existence of a *sq_contains* slot, for example, would
   generate a wrapped method named ``__contains__()`` and preclude the
   loading of a corresponding PyCFunction with the same name.  With
   the flag defined, the PyCFunction will be loaded in place of the
   wrapper object and will co-exist with the slot.  This is helpful
   because calls to PyCFunctions are optimized more than wrapper
   object calls.

PyMemberDef

   Structure which describes an attribute of a type which corresponds
   to a C struct member.  Its fields are:

   +--------------------+---------------+---------------------------------+
   | Field              | C Type        | Meaning                         |
   +====================+===============+=================================+
   | ``name``           | char *        | name of the member              |
   +--------------------+---------------+---------------------------------+
   | ``type``           | int           | the type of the member in the C |
   |                    |               | struct                          |
   +--------------------+---------------+---------------------------------+
   | ``offset``         | Py_ssize_t    | the offset in bytes that the    |
   |                    |               | member is located on the type's |
   |                    |               | object struct                   |
   +--------------------+---------------+---------------------------------+
   | ``flags``          | int           | flag bits indicating if the     |
   |                    |               | field should be read-only or    |
   |                    |               | writable                        |
   +--------------------+---------------+---------------------------------+
   | ``doc``            | char *        | points to the contents of the   |
   |                    |               | docstring                       |
   +--------------------+---------------+---------------------------------+

   ``type`` can be one of many ``T_`` macros corresponding to various
   C types.  When the member is accessed in Python, it will be
   converted to the equivalent Python type.

   +-----------------+--------------------+
   | Macro name      | C type             |
   +=================+====================+
   | T_SHORT         | short              |
   +-----------------+--------------------+
   | T_INT           | int                |
   +-----------------+--------------------+
   | T_LONG          | long               |
   +-----------------+--------------------+
   | T_FLOAT         | float              |
   +-----------------+--------------------+
   | T_DOUBLE        | double             |
   +-----------------+--------------------+
   | T_STRING        | char *             |
   +-----------------+--------------------+
   | T_OBJECT        | PyObject *         |
   +-----------------+--------------------+
   | T_OBJECT_EX     | PyObject *         |
   +-----------------+--------------------+
   | T_CHAR          | char               |
   +-----------------+--------------------+
   | T_BYTE          | char               |
   +-----------------+--------------------+
   | T_UBYTE         | unsigned char      |
   +-----------------+--------------------+
   | T_UINT          | unsigned int       |
   +-----------------+--------------------+
   | T_USHORT        | unsigned short     |
   +-----------------+--------------------+
   | T_ULONG         | unsigned long      |
   +-----------------+--------------------+
   | T_BOOL          | char               |
   +-----------------+--------------------+
   | T_LONGLONG      | long long          |
   +-----------------+--------------------+
   | T_ULONGLONG     | unsigned long long |
   +-----------------+--------------------+
   | T_PYSSIZET      | Py_ssize_t         |
   +-----------------+--------------------+

   ``T_OBJECT`` and ``T_OBJECT_EX`` differ in that ``T_OBJECT``
   returns ``None`` if the member is *NULL* and ``T_OBJECT_EX`` raises
   an ``AttributeError``.  Try to use ``T_OBJECT_EX`` over
   ``T_OBJECT`` because ``T_OBJECT_EX`` handles use of the ``del``
   statement on that attribute more correctly than ``T_OBJECT``.

   ``flags`` can be 0 for write and read access or ``READONLY`` for
   read-only access.  Using ``T_STRING`` for ``type`` implies
   ``READONLY``.  Only ``T_OBJECT`` and ``T_OBJECT_EX`` members can be
   deleted.  (They are set to *NULL*).
