.. highlightlang:: c

.. _common-structs:

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
:ctype:`PyObject` and :ctype:`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.


.. ctype:: 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.


.. ctype:: PyVarObject

   This is an extension of :ctype:`PyObject` that adds the :attr:`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 :ctype:`PyObject` and
:ctype:`PyVarObject`:

.. XXX need to document PEP 3123 changes here

.. cmacro:: PyObject_HEAD

   This is a macro which expands to the declarations of the fields of the
   :ctype:`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 :cmacro:`Py_TRACE_REFS`.  By default, that macro is not
   defined, and :cmacro:`PyObject_HEAD` expands to::

      Py_ssize_t ob_refcnt;
      PyTypeObject *ob_type;

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

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


.. cmacro:: PyObject_VAR_HEAD

   This is a macro which expands to the declarations of the fields of the
   :ctype:`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 :cmacro:`PyObject_HEAD` is part of the expansion, and that its own
   expansion varies depending on the definition of :cmacro:`Py_TRACE_REFS`.

.. cmacro:: PyObject_HEAD_INIT


.. ctype:: PyCFunction

   Type of the functions used to implement most Python callables in C. Functions of
   this type take two :ctype:`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.


.. ctype:: PyCFunctionWithKeywords

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


.. ctype:: PyMethodDef

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

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

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

The :attr:`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 :const:`METH_VARARGS` and
:const:`METH_KEYWORDS` can be combined (but note that :const:`METH_KEYWORDS`
alone is equivalent to ``METH_VARARGS | METH_KEYWORDS``). Any of the calling
convention flags can be combined with a binding flag.


.. data:: METH_VARARGS

   This is the typical calling convention, where the methods have the type
   :ctype:`PyCFunction`. The function expects two :ctype:`PyObject\*` values.  The
   first one is the *self* object for methods; for module functions, it has the
   value given to :cfunc:`Py_InitModule4` (or *NULL* if :cfunc:`Py_InitModule` was
   used).  The second parameter (often called *args*) is a tuple object
   representing all arguments. This parameter is typically processed using
   :cfunc:`PyArg_ParseTuple` or :cfunc:`PyArg_UnpackTuple`.


.. data:: METH_KEYWORDS

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


.. data:: METH_NOARGS

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


.. data:: METH_O

   Methods with a single object argument can be listed with the :const:`METH_O`
   flag, instead of invoking :cfunc:`PyArg_ParseTuple` with a ``"O"`` argument.
   They have the type :ctype:`PyCFunction`, with the *self* parameter, and a
   :ctype:`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.


.. data:: METH_CLASS

   .. index:: builtin: classmethod

   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 :func:`classmethod` built-in function.


.. data:: METH_STATIC

   .. index:: builtin: staticmethod

   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 :func:`staticmethod` built-in function.

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


.. data:: 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 :meth:`__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.

