.. 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`:

.. 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`.

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:: 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.

.. data:: METH_OLDARGS

   This calling convention is deprecated.  The method must be of type

   :ctype:`PyCFunction`.  The second argument is *NULL* if no arguments are given,

   a single object if exactly one argument is given, and a tuple of objects if more

   than one argument is given.  There is no way for a function using this

   convention to distinguish between a call with multiple arguments and a call with

   a tuple as the only 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.

   .. versionadded:: 2.3

.. 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.

   .. versionadded:: 2.3

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.

   .. versionadded:: 2.4

.. cfunction:: PyObject* Py_FindMethod(PyMethodDef table[], PyObject *ob, char *name)

   Return a bound method object for an extension type implemented in C.  This can

   be useful in the implementation of a :attr:`tp_getattro` or :attr:`tp_getattr`

   handler that does not use the :cfunc:`PyObject_GenericGetAttr` function.